MicroBiology MCQs with Answer and Explnation | Chapter 56

Answer: Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common cause of urinary tract infections, especially in elderly patients or those with compromised immune systems. It is characterized by gram-negative rods that do not ferment lactose, as seen on EMB agar. Additionally, the absence of swarming motility further supports the likelihood of Pseudomonas aeruginosa, as it typically lacks this characteristic.

The other options are incorrect:

• Enterococcus faecalis: Enterococcus faecalis is a gram-positive bacterium that can cause urinary tract infections, but it does not fit the description provided in the scenario, which specifically mentions gram-negative rods.
• Escherichia coli: Escherichia coli is the most common cause of urinary tract infections, particularly in otherwise healthy individuals. However, it typically ferments lactose, which would result in pink colonies on EMB agar, unlike the lactose-negative colonies described in the scenario.
• Proteus vulgaris: Proteus vulgaris is known for its swarming motility, which results in a characteristic spreading growth pattern on agar plates. While it can cause urinary tract infections and exhibits lactose-negative colonies, the absence of swarming motility in the scenario makes it less likely to be the causative organism compared to Pseudomonas aeruginosa.

Answer: M proteins

M proteins are a key virulence factor of Streptococcus pyogenes, the bacteria most commonly associated with poststreptococcal glomerulonephritis (PSGN). Serotyping of M proteins helps identify the specific strain of Streptococcus involved.

The other options are incorrect:

• Capsule: While some Streptococcus strains have capsules, they are not typically used for serotyping in PSGN diagnosis.
• Teichoic acids: These are components of the bacterial cell wall but are not specific enough for serotyping purposes in PSGN.
• Outer membrane proteins: Streptococcus pyogenes doesn’t have an outer membrane, so this option is irrelevant for PSGN diagnosis.

Answer: Red blood cells and albumin in her urine

Red blood cells and albumin in the urine are indicative of post-streptococcal glomerulonephritis (PSGN), a potential complication of impetigo caused by Streptococcus pyogenes (the bacteria identified by the Gram stain and culture).

The other options are incorrect:

• Bloody diarrhea: While some strains of Streptococcus can cause diarrhea, it’s not a common sequelae of impetigo.
• Paralysis of the facial nerve (Bell’s palsy): Bell’s palsy is not a typical complication of impetigo.
• Blurred vision: Blurred vision is not a specific sequelae of impetigo.

Answer: Legionella pneumophila

Legionella pneumophila is a bacterium that causes Legionnaires’ disease, a type of pneumonia. It typically enters the body through inhalation of contaminated aerosols. While hospital-acquired pneumonia can occur, prostate surgery wouldn’t directly expose the patient to the environment where Legionella is commonly found (e.g., stagnant water sources).

The other options are incorrect:

• Escherichia coli (E. coli): E. coli is a common bacteria found in the intestines and can cause urinary tract infections (UTIs). Urinary tract contamination is a possible source of infection after prostate surgery.
• Enterococcus faecalis: This is another bacterium that can cause UTIs and is a potential cause of post-surgical infections, including those following prostate surgery.
• Klebsiella pneumoniae: Klebsiella is a bacterium that can cause various infections, including hospital-acquired pneumonia and UTIs. It’s a possible culprit for post-surgical infections.

Answer: Streptococcus pyogenes

Streptococcus pyogenes, also known as Group A Streptococcus, is the most common bacterial cause of acute poststreptococcal glomerulonephritis (PSGN). PSGN develops after a Streptococcus pyogenes infection, typically a week to two weeks later.

The other options are incorrect:

• Enterococcus faecalis: While Enterococcus faecalis can cause UTIs and other infections, it is not typically associated with PSGN.
• Streptococcus agalactiae (Group B Streptococcus): Streptococcus agalactiae is associated with complications in newborns and pregnant women, not typically PSGN.
• Streptococcus pneumoniae: Streptococcus pneumoniae is a common cause of pneumonia but is not a primary cause of PSGN.

Answer: Enterococcus faecalis

Enterococcus faecalis is a gram-positive, catalase-negative bacterium capable of growing in 6.5% sodium chloride. It is commonly associated with healthcare-associated infections, particularly urinary tract infections in patients with indwelling catheters. Additionally, it is a known cause of bacteremia in hospitalized patients, especially those receiving broad-spectrum antibiotics like cephalosporins.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is catalase-positive and typically doesn’t grow in high concentrations of sodium chloride.
• Viridans streptococci: While gram-positive cocci, viridans streptococci are usually catalase-negative but wouldn’t grow in 6.5% sodium chloride.
• Streptococcus pyogenes: Streptococcus pyogenes is also catalase-negative but wouldn’t grow in such high salt concentrations.

Answer: A gram-negative bacterium capable of reducing nitrates to nitrites

The symptoms described suggest a urinary tract infection (UTI), and the presence of grossly bloody urine indicates a potentially severe infection. The most likely causal agent for this scenario is a gram-negative bacterium capable of reducing nitrates to nitrites. This ability is commonly observed in Enterobacteriaceae, such as Escherichia coli, which is the most common cause of UTIs.

The other options are incorrect:

• A gram-positive bacillus grown on a low oxidation-reduction medium: This description does not match typical characteristics of organisms associated with urinary tract infections. Gram-positive bacilli are less commonly implicated in UTIs compared to gram-negative bacteria.
• An optochin-resistant, catalase-negative, gram-positive coccus: These characteristics are suggestive of Streptococcus pneumoniae, a common cause of respiratory infections, but it is not typically associated with urinary tract infections or gross hematuria.
• A gram-negative diplococcus, which is oxidase positive but does not ferment maltose: This description fits Neisseria gonorrhoeae, the causative agent of gonorrhea. However, gonorrhea typically presents with urethritis rather than bloody urine, making it less likely to be the causal agent in this scenario. Additionally, it is not commonly associated with urinary tract infections in women.

Answer: Tissue sections from a patient with mucormycosis show budding yeasts.

Mucormycosis is caused by fungi belonging to the order Mucorales, which are characterized by hyphae invading blood vessels and causing tissue necrosis. It is commonly transmitted through airborne asexual spores, and diabetic patients with ketoacidosis are indeed at a higher risk for mucormycosis due to compromised immune function. However, mucormycosis does not involve budding yeasts; instead, it is associated with hyphal forms.

The other options are incorrect:

• Hyphae typically invade blood vessels and cause necrosis of tissue. – This is correct. Mucormycosis is characterized by fungal invasion of blood vessels, leading to tissue death (necrosis).
• The fungi that cause mucormycosis are transmitted by airborne asexual spores. – This is also correct. Asexual spores from mucormycosis fungi are inhaled and can cause infection.
• Ketoacidosis in diabetic patients is a predisposing factor to mucormycosis. – This is correct as well. Uncontrolled diabetes with ketoacidosis weakens the

Answer: Malassezia furfur

Malassezia furfur is a fungus that normally lives on human skin and typically doesn’t cause serious illness in immunocompromised patients. While it can cause dandruff or seborrheic dermatitis, these are not life-threatening conditions.

The other options are incorrect:

• Mucor species: Mucor is a common fungus associated with mucormycosis, a severe infection affecting immunocompromised individuals.
• Aspergillus fumigatus: This is another fungus frequently linked to invasive aspergillosis, a serious lung infection in immunocompromised patients.
• Cryptococcus neoformans: Cryptococcus is a fungus known to cause cryptococcal meningitis, a potentially life-threatening fungal infection of the brain and meninges in immunocompromised individuals.

Answer: It is an encapsulated non dimorphic yeast found worldwide

Which is an encapsulated yeast commonly found worldwide. It typically exists in the yeast form and is not dimorphic. Cryptococcus neoformans is known for causing meningitis, especially in immunocompromised individuals such as HIV-positive patients. The presence of encapsulated organisms visible by India ink in the cerebrospinal fluid (CSF) is indicative of cryptococcal meningitis.

The other options are incorrect:

• It exists as a mycelial form at room temperature and a yeast at 37°C (Dimorphic): This describes fungi like histoplasma, which can exist as a mold at room temperature and convert to yeast form at body temperature.
• It consists of branching septate hyphae: Hyphae are thread-like structures characteristic of molds. Cryptococcus doesn’t form these structures.
• It is a nonencapsulated dimorphic yeast that reproduces by budding: While Cryptococcus reproduces by budding, it has a capsule and remains in yeast form at all temperatures.

Answer: Cryptococcus neoformans

This encapsulated yeast is a leading cause of fungal meningitis, especially in immunocompromised individuals. The positive latex agglutination test for capsular polysaccharide antigen strongly points towards Cryptococcus neoformans.

The other options are incorrect:

• Candida albicans: While Candida can cause meningitis, it’s less common than Cryptococcus in immunocompromised patients. A latex agglutination test wouldn’t typically be used for Candida diagnosis.
• Aspergillus fumigatus: This fungus is more commonly associated with invasive aspergillosis, a lung infection, rather than meningitis.
• Histoplasma capsulatum: This dimorphic fungus can cause meningitis, but it’s less frequent than Cryptococcus, and a different diagnostic test would likely be used.

Answer: C. albicans is transmitted primarily by respiratory aerosol.

Candida albicans is a commensal fungus, meaning it lives on the skin and mucous membranes of healthy individuals without causing harm. Transmission typically occurs through direct contact with infected mucous membranes or skin.

The other options are incorrect:

• C. albicans causes thrush: This is correct. Candida albicans is a major cause of thrush, a fungal infection of the mouth or throat.
• Impaired cell-mediated immunity is an important predisposing factor to disease: This is also correct. A weakened immune system, particularly cell-mediated immunity, increases the risk of Candida overgrowth and infection.
• C. albicans is a budding yeast that forms pseudohyphae when it invades tissue: This is correct. Candida albicans exists as a yeast form and can transition to form elongated cells called pseudohyphae during tissue invasion.

Answer: The finding of encapsulated budding cells in spinal fluid

Cryptococcus neoformans is a encapsulated yeast that commonly causes meningitis, particularly in immunocompromised individuals. The most useful finding in establishing the diagnosis of cryptococcal meningitis would be the identification of encapsulated budding cells in the patient’s spinal fluid, typically observed using India ink staining or other microscopy techniques.

The other options are incorrect:

• Positive heterophil agglutination test: This test is typically used for diagnosing Epstein-Barr virus (EBV) and some other viral infections, not fungal infections like cryptococcal meningitis.
• Recovery of an acid-fast organism: Acid-fast organisms refer to bacteria like Mycobacterium tuberculosis, not fungi like Cryptococcus neoformans. Sputum culture wouldn’t be the primary diagnostic tool for cryptococcal meningitis.
• History of recent travel: While Cryptococcus is found worldwide, travel history isn’t the most useful factor for diagnosis. The presence of the fungus in the CSF directly confirms the infection.

Answer: Aspergillus fumigatus

The clinical presentation of a heart transplant recipient with pleuritic chest pain, hemoptysis, fever, chills, and subsequent myocardial infarction, along with the autopsy findings of septate, acutely branching hyphae in various tissues, strongly suggests invasive aspergillosis. Aspergillus fumigatus is a mold commonly associated with invasive fungal infections in immunocompromised individuals, such as transplant recipients, and it presents with septate, acutely branching hyphae on histopathological examination.

The other options are incorrect:

• Mucor species: While Mucor can also cause invasive fungal infections, it’s less frequent than Aspergillus fumigatus in immunocompromised patients.
• Histoplasma capsulatum: This fungus typically causes chronic infections and wouldn’t be the most likely culprit in this scenario with rapid progression.
• Cryptococcus neoformans: This fungus primarily affects the central nervous system, not causing the lung and chest symptoms described.

Answer: Culture of the discharge on Sabouraud’s agar should produce a white mycelium with aerial conidia.

While Candida albicans can grow on Sabouraud’s agar and form white colonies, it doesn’t necessarily produce aerial conidia (spores) on this media in all cases. Diagnosis of Candida albicans is usually done through other methods like germ tube formation or microscopic examination.

The other options are incorrect:

• The clinical laboratory can use germ tube formation to identify the isolate as C. albicans. – This statement is accurate. Germ tube formation is a reliable test to identify Candida albicans.
• Antibiotics predispose to Candida vaginitis by killing the normal flora lactobacilli that keep the vaginal pH low. – This statement is accurate. Antibiotics can disrupt the natural balance of vaginal flora, allowing Candida to overgrow.
• A Gram stain of the discharge should reveal budding yeasts. – This statement is also accurate. A Gram stain showing budding yeasts can be indicative of Candida albicans, although it’s not definitive for diagnosis and can be confused with other yeasts.

Answer: Thrush

Thrush is a fungal infection caused by Candida albicans, which primarily affects the mucous membranes in the mouth and throat. Aspergillus fumigatus, on the other hand, is a mold that typically doesn’t cause oral thrush.

The other options are incorrect:

• Allergy following inhalation of airborne particles of the fungus: This is a well-documented condition called allergic bronchopulmonary aspergillosis (ABPA).
• Colonization of tuberculous cavities in the lung: Aspergillus fumigatus can sometimes grow in pre-existing lung cavities, including those caused by tuberculosis.
• Tissue invasion in immunocompromised host: Invasive aspergillosis is a serious condition that can affect various organs in immunocompromised individuals.

Answer: Pneumocystis jiroveci

The clinical presentation of progressively increasing dyspnea, dry, painful cough, fatigue, low-grade fever, and bilateral symmetrical interstitial and alveolar infiltration on chest x-ray is highly suggestive of Pneumocystis jiroveci pneumonia (PCP), especially in a patient with AIDS. PCP is a common opportunistic infection in individuals with compromised immune systems, particularly those with HIV/AIDS.

The other options are incorrect:

• Toxoplasma gondii: While this parasite can cause infections in immunocompromised individuals, it typically presents with central nervous system involvement, not respiratory symptoms.
• Histoplasma capsulatum: This fungus can cause pneumonia, but it’s less common in AIDS patients compared to PCP, and symptoms may develop more gradually.
• Cryptococcus neoformans: This fungus primarily affects the central nervous system, not causing the respiratory illness described in the scenario.

Answer: Clostridium perfringens

Nagler’s reaction is a specific test used to identify Clostridium perfringens. This test relies on the alpha toxin produced by C. perfringens, which can break down egg yolk lecithin in the culture medium. A positive Nagler reaction shows an opaque halo surrounding colonies grown on media containing egg yolk.

The other options are incorrect:

• Clostridium botulinum: While a toxin is produced by C. botulinum, Nagler’s reaction wouldn’t be used for its identification. Specific tests for botulinum toxin are employed.
• Clostridium tetani: Tetanus is caused by C. tetani, but diagnosis involves detecting the toxin in the nervous system, not using Nagler’s reaction.
• Clostridium difficile: C. difficile is identified through different methods like toxin detection or cytotoxicity assays. Nagler’s reaction wouldn’t be suitable for this organism.

Answer: Enterococcus faecalis

The clinical presentation described, with sepsis and the development of a necrotic lesion with a black center (eschar) and an erythematous margin, is highly suggestive of cutaneous anthrax, caused by Bacillus anthracis. Cutaneous anthrax typically occurs following contact with contaminated animal products and presents with a painless papule that progresses to a vesicle and then a depressed black eschar surrounded by edema.

The other options are incorrect:

• Enterococcus faecalis: Enterococcus faecalis is a gram-positive bacterium that can cause various infections, but it is not typically associated with the clinical presentation described, especially the characteristic necrotic lesion seen in cutaneous anthrax.
• Pseudomonas aeruginosa: Pseudomonas aeruginosa can cause a variety of infections, including skin and soft tissue infections. However, the clinical presentation described, with the characteristic black eschar, is not typical of infections caused by Pseudomonas aeruginosa.
• Clostridium perfringens: Clostridium perfringens is a gram-positive bacterium that can cause gas gangrene (clostridial myonecrosis) and other soft tissue infections. However, the clinical presentation described, with a necrotic lesion and black eschar, is more characteristic of cutaneous anthrax caused by Bacillus anthracis.

Answer: Single stranded positive sense RNA virus

Influenza A virus is a negative-sense, single-stranded RNA virus. This means its genetic material is RNA, but it’s in a negative strand format that needs to be converted to a positive strand for protein production.

The other options are incorrect:

• Viral transcription occurs in the nucleus: This statement is correct. Influenza A virus utilizes the host cell’s nucleus for viral RNA transcription.
• Mature virion are released by budding: This statement is correct. Influenza A virus particles bud out of the infected cell membrane.
• Attaches to cell-surface sialic acid: This statement is correct. Influenza A virus uses hemagglutinin (HA) proteins to bind to sialic acid receptors on host cells.

Answer: It is a gram-negative rod that does not ferment lactose.

Clostridium perfringens is a gram-positive, spore-forming rod-shaped bacterium. It does not ferment lactose, but it is gram-positive, not gram-negative. This statement is incorrect as Clostridium perfringens is a gram-positive bacterium.

The other options are incorrect:

• It causes gas gangrene: This statement is correct. Clostridium perfringens is a major cause of gas gangrene.
• It produces an exotoxin that degrades lecithin and causes necrosis and hemolysis: This statement is correct. One of the toxins produced by C. perfringens is alpha toxin, which has these effects.
• It causes food poisoning: This statement is correct. C. perfringens is a common cause of foodborne illness.

Answer: Red blood cells and albumin in her urine

Impetigo, while a common bacterial skin infection, can rarely lead to a complication called post-streptococcal glomerulonephritis (PSGN). This condition involves inflammation of the kidneys due to an immune reaction triggered by the bacteria (usually Group A Streptococcus) responsible for impetigo. PSGN can cause symptoms like blood and protein in the urine.

The other options are incorrect:

• Blurred vision: This isn’t a typical complication of impetigo or PSGN.
• Bloody diarrhea: This can be caused by various bacterial infections, but it’s not a common consequence of impetigo.
• Paralysis of the facial nerve (Bell’s palsy): Bell’s palsy can have various causes, but it’s not directly linked to impetigo or PSGN.

Answer: Anaerobic conditions at the wound site are not required to cause tetanus, because spores will form in the presence of oxygen.

Tetanus is caused by the neurotoxin produced by Clostridium tetani bacteria, which typically thrives in anaerobic (low oxygen) conditions. The correct statement is incorrect because anaerobic conditions are indeed necessary for C. tetani spores to germinate and produce toxin. Oxygen inhibits spore germination.

The other options are incorrect:

• Pathogenic clostridia are found both in the soil and in the normal flora of the colon. – This statement is correct. Certain species of Clostridia are part of the gut microbiome and can also be found in soil.
• Botulism, which is caused by ingesting preformed toxin, can be prevented by boiling food prior to eating. – This statement is correct. Boiling food inactivates the preformed botulinum toxin produced by Clostridium botulinum, preventing foodborne botulism.
• Antibiotic-associated (pseudomembranous) colitis is due to a toxin produced by Clostridium difficile. – This statement is correct. C. difficile takes advantage of antibiotic disruption of gut flora to overgrow and produce toxins that cause colitis.

Answer: Clostridium tetani

Clostridium tetani is the causative agent of tetanus. Tetanus spores can enter the body through a wound, including an umbilical cord stump if not properly cared for and becomes contaminated. The anaerobic environment within the wound can be suitable for spore germination and bacterial growth, leading to toxin production and the characteristic symptoms of tetanus.

The other options are incorrect:

• Clostridium difficile: This bacterium is primarily associated with antibiotic-induced disruption of gut flora and is not a common cause of umbilical cord infection.
• Clostridium perfringens: While C. perfringens can cause gas gangrene through wound contamination, it’s less frequent than tetanus in umbilical cord infections.
• Clostridium botulinum: Botulism is caused by ingesting preformed toxin, not by wound contamination.

Answer: Clostridium perfringens

The stormy clot reaction is a characteristic test used to identify Clostridium perfringens. In this test, when C. perfringens is grown in litmus milk medium, it ferments the lactose sugar, producing acid and gas. The gas rapidly accumulates, causing the casein clot to break up into shreds along the sides of the test tube, resembling a stormy appearance.

The other options are incorrect:

• Clostridium tetani: Tetanus diagnosis relies on clinical symptoms and detection of the toxin in the nervous system, not a growth medium test like the stormy clot reaction.
• Clostridium difficile: C. difficile is identified through different methods like toxin detection or cytotoxicity assays. The stormy clot reaction wouldn’t be suitable for this organism.
• Clostridium botulinum: Similar to C. tetani, botulism diagnosis involves detecting the preformed toxin, not using a growth medium test.

Answer: Endotoxin is an important pathogenetic factor.

Staphylococcus aureus is a gram-positive bacterium and possesses a cell wall component called peptidoglycan, a key virulence factor. Endotoxin is a lipopolysaccharide associated with the outer membrane of gram-negative bacteria. S. aureus doesn’t have an outer membrane and therefore lacks endotoxin.

The other options are incorrect:

• The coagulase test is positive: This statement is correct. S. aureus can convert fibrinogen to fibrin, a positive coagulase test.
• Treatment should include a β-lactamase-resistant penicillin: This can be true depending on the strain of S. aureus. Some strains are resistant to β-lactamase antibiotics, and these resistant strains may require specific treatment with drugs like methicillin-resistant Staphylococcus aureus (MRSA) antibiotics.
• Gram-positive cocci in grapelike clusters are seen on Gram-stained smear: This statement is correct. S. aureus appears as gram-positive cocci arranged in clusters resembling grapes on a Gram stain.

Answer: Eikenella corrodens

The most likely cause of the infection described in the scenario is Eikenella corrodens. This bacterium is commonly associated with human bite wounds and can cause infections characterized by a bleach-like odor. It is a facultative anaerobic, gram-negative rod that typically grows on blood agar and exhibits a characteristic pitting or “corrosive” appearance on the agar surface.

The other options are incorrect:

• Actinobacillus actinomycetemcomitans: While this bacterium can also be found in the mouth, it’s less commonly associated with human bite wounds compared to Eikenella corrodens.
• Pseudomonas aeruginosa: This bacterium can cause wound infections, but it’s not typically associated with human bites and wouldn’t have the characteristic bleach-like odor.
• Kingella kingae: This bacterium is more commonly associated with osteoarthristis and bacteremia in children, not human bite wounds.

Answer: Fusobacterium nucleatum and Peptostreptococcus intermedius

Lung abscesses are often polymicrobial infections involving a mixture of anaerobic and aerobic bacteria. Fusobacterium nucleatum and Peptostreptococcus intermedius are common anaerobic bacteria found in lung abscesses, especially in individuals with predisposing factors such as alcoholism and poor dental hygiene. The green, foul-smelling sputum is characteristic of anaerobic infections.

The other options are incorrect:

• Clostridium perfringens and Chlamydia psittaci: Clostridium perfringens is an anaerobic bacterium associated with gas gangrene but is less commonly associated with lung abscesses. Chlamydia psittaci is associated with psittacosis, a zoonotic respiratory infection, but it is not typically associated with lung abscesses.
• Listeria monocytogenes and Legionella pneumophila: Listeria monocytogenes is a facultative intracellular bacterium associated with foodborne infections such as listeriosis, but it is not commonly associated with lung abscesses. Legionella pneumophila is associated with Legionnaires’ disease, a severe form of pneumonia, but it is not typically associated with lung abscesses.
• Nocardia asteroides and Mycoplasma pneumoniae: Nocardia asteroides is an aerobic, filamentous bacterium that can cause pulmonary infections, including lung abscesses, but it is less commonly associated with lung abscesses than anaerobic bacteria. Mycoplasma pneumoniae is a common cause of atypical pneumonia, but it is not typically associated with lung abscesses.

Answer: Mycoplasma pneumoniae

Mycoplasma pneumoniae is a common cause of walking pneumonia, a milder form of pneumonia that often affects young adults with a normal immune system. It’s a bacterium that lacks a cell wall and presents with symptoms like fever, cough, and malaise.

The other options are incorrect:

• Nocardia asteroides: This fungus can cause pneumonia, but it’s more frequent in immunocompromised individuals or those with underlying lung conditions.
• Serratia marcescens: This gram-negative bacterium is an opportunistic pathogen and more likely to cause hospital-acquired pneumonia in patients with compromised respiratory systems.
• Legionella pneumophila: This bacterium can cause Legionnaires’ disease, a type of pneumonia that can affect both immunocompetent and immunocompromised individuals. However, it’s less common than Mycoplasma pneumoniae in young healthy adults.

Answer: Diphtheria toxoid should not be given to children younger than 3 years because the incidence of complications is too high.

Diphtheria toxoid is a vaccine used to prevent diphtheria. It’s recommended for children according to the immunization schedule, typically starting at a younger age than 3 years old. The benefits of vaccination far outweigh any potential complications, which are very rare.

The other options are incorrect:

• Toxin production is dependent on the organism’s being lysogenized by a bacteriophage. – This statement is correct. Corynebacterium diphtheriae needs a specific bacteriophage to carry the gene for toxin production.
• Antitoxin should be used to treat patients with diphtheria. – This statement is correct. Diphtheria antitoxin can help neutralize circulating toxin in diphtheria patients.
• C. diphtheriae is a gram-positive rod that does not form spores. – This statement is correct. C. diphtheriae is a gram-positive bacterium and doesn’t produce spores.

Answer: To retard phagocytosis by polymorphonuclear leukocytes

The capsule of Streptococcus pneumoniae is the most important virulence factor contributing to its pathogenicity. It helps the bacteria evade the host immune system by retarding phagocytosis, particularly by polymorphonuclear leukocytes (PMNs). The presence of the capsule inhibits opsonization and makes the bacteria less susceptible to engulfment and destruction by phagocytes.

The other options are incorrect:

• To accelerate tissue invasion by its collagenase-like activity: S. pneumoniae doesn’t possess a potent collagenase enzyme. While it can invade tissues, the capsule’s primary role isn’t tissue destruction.
• To inhibit polymorphonuclear leukocyte chemotaxis: Chemotaxis refers to the movement of immune cells towards a source of infection. The capsule might indirectly affect chemotaxis by hindering PMN access to the bacteria, but it’s not the primary mechanism.
• To prevent dehydration of the organisms on mucosal surfaces: The capsule might offer some protection from dehydration, but this isn’t its main function in causing disease.

Answer: An integrated temperate phage

Pathogenic Corynebacterium diphtheriae strains harbor a specific bacteriophage that carries the tox gene for diphtheria toxin production. This bacteriophage can integrate into the bacterial chromosome, becoming an integrated temperate phage. When lysogenized by this phage, C. diphtheriae gains the ability to produce the toxin and become virulent.

The other options are incorrect:

• An F factor: F factors are fertility factors or episomes found in some bacteria that facilitate conjugation (genetic exchange) between bacterial cells. They are not typically associated with toxin production in C. diphtheriae.
• A diphthamide on eEF-2: This refers to a specific modification of a protein (elongation factor 2) caused by the diphtheria toxin. It’s a consequence of toxin action, not a genetic element carried by the bacteria.
• An episome: An episome is a small, circular DNA molecule that can replicate independently or integrate into the bacterial chromosome. While the tox gene can be carried on an episomal phage, the key element is the integrated phage, not just the presence of an episome in general.

Answer: The antigen in the tuberculin skin test is a protein extracted from the organism.

The antigen used in the tuberculin skin test (TST) is not a protein directly extracted from M. tuberculosis. It’s a mixture of purified protein derivatives (PPDs) from the bacteria, not a single protein.

The other options are incorrect:

• Some strains of M. tuberculosis isolated from patients exhibit multiple drug resistance (i.e., they are resistant to both isoniazid and rifampin). This is a growing concern, and multidrug-resistant tuberculosis (MDR-TB) is a significant public health threat.
• M. tuberculosis grows slowly, often requiring 3 to 6 weeks before colonies appear. This is true. M. tuberculosis has a slow growth rate, making diagnosis through culture techniques time-consuming.
• M. tuberculosis contains a small amount of lipid in its cell wall and therefore stains poorly with the Gram stain. This statement is correct. The waxy cell wall of M. tuberculosis is rich in lipids and makes it difficult to stain with the standard Gram stain. It requires special staining techniques like Ziehl-Neelsen to be visualized.

Answer: Anaerobic streptococci

Anaerobic streptococci, particularly those from the normal oral flora, are commonly implicated in brain abscesses following dental procedures due to their ability to form biofilms and penetrate deep tissue when introduced into the bloodstream during dental procedures.

The other options are incorrect:

• Mycobacterium smegmatis: This is a slow-growing, nonpathogenic bacterium rarely associated with brain abscesses.
• Lactobacillus acidophilus: This bacterium is also part of the normal oral flora and is not typically linked to brain abscess development.
• Mycoplasma pneumoniae: This bacterium is a common cause of respiratory infections but wouldn’t be the primary suspect for a brain abscess following dental work.

Answer: P. carinii symptomatic infections can be prevented by administering penicillin orally.

Pneumocystis carinii (now known as Pneumocystis jirovecii) is a fungus that primarily infects the respiratory tract, causing pneumonia. It typically affects immunocompromised individuals, such as those with HIV/AIDS or undergoing immunosuppressive therapy. While various treatments are available for Pneumocystis pneumonia, such as trimethoprim-sulfamethoxazole (TMP-SMX), penicillin is not effective for preventing or treating Pneumocystis pneumonia.

The other options are incorrect:

• P. carinii infections primarily involve the respiratory tract: This statement is correct. Pneumocystis carinii infections primarily affect the respiratory tract, causing pneumonia.
• P. carinii can be diagnosed by seeing cysts in tissue: This statement is correct. Pneumocystis carinii can be diagnosed by visualizing cysts or trophozoites in tissue samples obtained through bronchoalveolar lavage or lung biopsy.
• P. carinii infections are symptomatic primarily in immunocompromised patients: This statement is correct. Pneumocystis carinii infections typically cause symptomatic pneumonia primarily in immunocompromised individuals, such as those with HIV/AIDS or undergoing immunosuppressive therapy.

Answer: Fusobacterium nucleatum and Peptostreptococcus intermedius

Lung abscesses, especially in individuals with a history of alcoholism and poor dental hygiene, are commonly caused by anaerobic bacteria from the oral cavity. Fusobacterium nucleatum and Peptostreptococcus intermedius are two of the most common anaerobic bacteria implicated in lung abscesses.

The other options are incorrect:

• Listeria monocytogenes and Legionella pneumophila: Listeria monocytogenes is a bacterium typically associated with foodborne infections, while Legionella pneumophila is associated with Legionnaires’ disease. These bacteria are not commonly associated with lung abscesses.
• Clostridium perfringens and Chlamydia psittaci: Clostridium perfringens is an anaerobic bacterium associated with gas gangrene, but it is less commonly associated with lung abscesses. Chlamydia psittaci is associated with psittacosis, a zoonotic respiratory infection, but it is not typically associated with lung abscesses.
• Nocardia asteroides and Mycoplasma pneumoniae: Nocardia asteroides is an aerobic bacterium associated with nocardiosis, including pulmonary nocardiosis, but it is less commonly implicated in lung abscesses. Mycoplasma pneumoniae is a bacterium responsible for causing atypical pneumonia but is not typically associated with lung abscesses.

Answer: Epstein–Barr virus

The clinical presentation of a severe sore throat, fatigue, anorexia, and febrile illness in a young adult, along with the growth of alpha-hemolytic, optochin-resistant colonies on blood agar, suggests infectious mononucleosis, which is most commonly caused by Epstein–Barr virus (EBV). EBV is known to cause pharyngitis with exudate, tender cervical lymphadenopathy, and fatigue, often referred to as the “kissing disease” due to its association with transmission through saliva.

The other options are incorrect:

• Streptococcus pneumoniae: Streptococcus pneumoniae can cause respiratory infections, including pneumonia and otitis media, but it is not a common cause of sore throat with the described clinical presentation. Furthermore, it typically appears as alpha-hemolytic, optochin-sensitive colonies on blood agar.
• Streptococcus pyogenes: Streptococcus pyogenes, also known as group A Streptococcus (GAS), is a common cause of pharyngitis, but it typically presents with fever, sore throat, and tonsillar exudates. However, the absence of white lesions on the tongue or pharynx makes GAS less likely in this scenario.
• Parvovirus B19: Parvovirus B19 can cause various clinical manifestations, including erythema infectiosum (fifth disease) and arthropathy. While it can cause mild respiratory symptoms, it is not a common cause of severe sore throat and febrile illness resembling infectious mononucleosis.

Answer: IV drug user

The presentation of gelatinous and bloody sputum along with gram-negative rods on Gram stain, particularly in the context of shortness of breath and a productive cough, suggests a severe pulmonary infection, most likely caused by Pseudomonas aeruginosa. IV drug users are at increased risk of acquiring Pseudomonas infections due to contaminated needles and drug paraphernalia.

The other options are incorrect:

• Hiker: While hikers may encounter environmental pathogens, such as fungi or mycobacteria, leading to respiratory infections, the presentation described, especially with gelatinous and bloody sputum and gram-negative rods on Gram stain, is less likely to be associated with hiking-related exposures.
• Homeless: Homeless individuals may face various health risks due to living conditions, including respiratory infections. However, the presentation described, particularly with the presence of gram-negative rods and gelatinous, bloody sputum, is not specific to homelessness and suggests a more severe infection.
• Alcoholic: Chronic alcoholism can predispose individuals to respiratory infections, including aspiration pneumonia. However, the presentation of gelatinous and bloody sputum along with gram-negative rods on Gram stain is not typically associated with alcoholism alone.

Answer: Mycoplasma pneumoniae

Mycoplasma pneumoniae is a common cause of community-acquired pneumonia in immunocompetent young adults. It is an atypical bacterium that lacks a cell wall and typically causes mild to moderate respiratory symptoms, such as a persistent cough, sore throat, and low-grade fever.

The other options are incorrect:

• Nocardia asteroides: Nocardia asteroides is an aerobic, filamentous bacterium that can cause pulmonary infections, particularly in immunocompromised individuals or those with underlying lung disease. It is less commonly associated with pneumonia in immunocompetent young adults.
• Serratia marcescens: Serratia marcescens is a gram-negative bacterium that can cause respiratory infections, but it is more commonly associated with hospital-acquired pneumonia, particularly in individuals with compromised immune systems or those on mechanical ventilation. It is less frequently implicated in community-acquired pneumonia in immunocompetent young adults.
• Legionella pneumophila: Legionella pneumophila is a gram-negative bacterium responsible for causing Legionnaires’ disease, a severe form of pneumonia. While Legionella pneumonia can occur in immunocompetent individuals, it is often associated with environmental exposures, such as contaminated water sources, and is less commonly implicated in community-acquired pneumonia in young adults without significant risk factors.

Answer: Anticapsular antibody

The pneumococcal polysaccharide capsule is a major virulence factor of Streptococcus pneumoniae, providing protection against phagocytosis by host immune cells, particularly macrophages. The most important host defense mechanism against the capsule is the production of anticapsular antibodies. These antibodies promote opsonization of the encapsulated bacteria, facilitating their recognition and engulfment by phagocytes, such as macrophages and neutrophils.

The other options are incorrect:

• Activated macrophages: While activated macrophages play a crucial role in host defense against bacterial infections by engulfing and destroying pathogens, they are not specifically targeted against the pneumococcal polysaccharide capsule.
• Polysaccharide-degrading enzymes: Polysaccharide-degrading enzymes can potentially degrade bacterial capsules, but they are not the primary mechanism by which the host counters the function of the pneumococcal polysaccharide capsule.
• T lymphocytes sensitized to polysaccharide antigens: T lymphocytes primarily recognize peptide antigens presented by major histocompatibility complex (MHC) molecules on antigen-presenting cells. While T lymphocytes can be involved in the immune response against bacterial infections, they are less directly involved in countering the function of polysaccharide capsules compared to the production of anticapsular antibodies by B lymphocytes.

Answer: Requires iron and cysteine for growth

The organism described, which grows on buffered charcoal yeast extract agar and stains weakly gram-negative, is consistent with Legionella pneumophila, the causative agent of Legionnaires’ disease. Legionella pneumophila is an aerobic, intracellular bacterium that requires iron and cysteine for growth. It commonly infects individuals who have been exposed to contaminated water sources, such as those found in cooling towers, air conditioning systems, and hot tubs.

The other options are incorrect:

• Optochin sensitive: Optochin sensitivity is a characteristic of Streptococcus pneumoniae, a bacterium commonly associated with respiratory infections such as pneumonia. Legionella pneumophila is not tested for optochin sensitivity.
• Capsule: Capsules are typically associated with bacteria such as Streptococcus pneumoniae, which have a thick polysaccharide capsule. Legionella pneumophila does not possess a capsule.
• No cell wall: Bacteria such as Mycoplasma pneumoniae lack a cell wall and are classified as “cell wall-deficient” or “wall-less” bacteria. Legionella pneumophila, however, does have a cell wall.

Answer: It causes atypical pneumonia, especially in those with reduced cell-mediated immunity.

Legionella pneumophila is a bacterium known to cause Legionnaires’ disease, a type of atypical pneumonia. People with weakened immune systems, particularly cell-mediated immunity, are more susceptible to Legionella infections.

The other options are incorrect:

• It is part of the normal flora of the colon: Legionella pneumophila is an environmental bacterium and not typically found in the healthy human gut microbiome.
• It does not have a cell wall: Like all bacteria, Legionella pneumophila has a cell wall, which is essential for its structure and survival.
• It cannot be grown on laboratory media: While Legionella can be slightly fastidious (meaning it has specific growth requirements), it can be cultured on specialized media in a laboratory setting.

Answer: Peptidoglycan

Peptidoglycan (also called murein) is the major component of the cell wall in bacteria, including gram-positive cocci like Streptococcus pyogenes (Group A Streptococcus), the most likely culprit in this case. Peptidoglycan provides structural rigidity and protects the bacterium from bursting due to osmotic pressure differences between the inside and outside of the cell.

The other options are incorrect:

• Polysaccharide: While bacteria can have polysaccharides in their capsules, these are not the primary structural component protecting against osmotic pressure.
• Teichoic acid: Teichoic acid is another component found in the cell wall of gram-positive bacteria, but it plays a secondary role compared to peptidoglycan in maintaining cell shape and osmotic stability.
• Lipopolysaccharide (LPS): Lipopolysaccharide is a major component of the outer membrane in gram-negative bacteria, not gram-positive ones like Streptococcus pyogenes. LPS does contribute to osmotic pressure resistance in gram-negative bacteria.

Answer:

Sulfatides are lipids found in the cell wall of Mycobacterium tuberculosis, the causative agent of tuberculosis. These sulfatide molecules are believed to play a crucial role in the bacterium’s virulence by interfering with the phagosome-lysosome fusion process within macrophages. This allows M. tuberculosis to survive and replicate inside the host cells.

The other options are incorrect:

• Calcium dipicolinate: This compound is found in the spores of some bacteria, including some Bacillus species. While it contributes to spore dormancy and resistance to harsh environments, it’s not a virulence factor specific to M. tuberculosis or related to phagosome-lysosome fusion.
• Peptidoglycan: Peptidoglycan, as mentioned previously, is a major structural component of the bacterial cell wall and offers rigidity and osmotic protection. It doesn’t directly contribute to inhibiting phagosome-lysosome fusion.
• Tuberculin: Tuberculin is a purified protein derivative (PPD) used in the tuberculin skin test (TST) to diagnose exposure to M. tuberculosis. It’s not a virulence factor of the bacteria itself.

Answer: M. tuberculosis contains a small amount of lipid in its cell wall and therefore stains poorly with the Gram stain.

Mycobacterium tuberculosis has a thick cell wall rich in lipids, including mycolic acids. This unique cell wall structure is why M. tuberculosis is acid-fast, meaning it retains a pink stain after a specific staining procedure. Therefore, M. tuberculosis stains well with specific stains but not with the routine Gram stain.

The other options are incorrect:

• Some strains of M. tuberculosis isolated from patients exhibit multiple drug resistance (i.e., they are resistant to both isoniazid and rifampin): This is a growing concern in tuberculosis treatment, and multidrug-resistant (MDR) TB strains are a significant public health threat.
• The antigen in the tuberculin skin test is a protein extracted from the organism: The tuberculin skin test (TST) uses a purified protein derivative (PPD) called tuberculin, which is a mixture of various M. tuberculosis antigens.
• M. tuberculosis grows slowly, often requiring 3 to 6 weeks before colonies appear: This is a characteristic feature of M. tuberculosis due to its complex cell wall and slow metabolic rate.

Answer: Gram-positive coccus, beta hemolytic, catalase positive

The description matches Streptococcus pyogenes, which causes streptococcal pharyngitis (strep throat) and can lead to complications such as scarlet fever. S. pyogenes is a gram-positive coccus that exhibits beta-hemolysis on blood agar, indicating complete lysis of red blood cells surrounding the colonies. It is catalase positive, meaning it produces catalase enzyme. Scarlet fever, characterized by an erythematous rash (scarlatiniform rash) and a “strawberry” tongue, is a complication of streptococcal pharyngitis caused by certain strains of S. pyogenes.

The other options are incorrect:

• Gram-positive coccus, alpha hemolytic, catalase negative: This description could be consistent with Streptococcus pneumoniae, which can cause pneumonia but wouldn’t typically cause the characteristic rash of Scarlet Fever.
• Gram-positive coccus, gamma hemolytic, catalase negative: Gamma hemolytic refers to no hemolysis on a blood agar plate. While some Streptococci are gamma hemolytic, they wouldn’t be associated with the clinical picture of Scarlet Fever.
• Gram-positive coccus, alpha hemolytic, catalase positive: This description is less likely for Scarlet Fever. Catalase-positive gram-positive cocci are more commonly associated with Staphylococcus species, not Streptococcus pyogenes.

Answer: The organism appears purple in Gram-stained smears of sputum.

Chlamydia psittaci is an obligate intracellular bacterium, meaning it can only replicate inside host cells. Due to this characteristic, it cannot be visualized directly on a Gram stain or any other routine stain used for bacteria in clinical specimens like sputum.

The other options are incorrect:

• The infection is more commonly acquired from a nonhuman source than from another human: Chlamydia psittaci is primarily transmitted through inhalation of dried fecal dust from infected birds, particularly parrots and pigeons.
• C. psittaci can be isolated by growth in cell culture and will not grow in blood agar: This is true. Chlamydia requires a special cell culture system to grow because it’s an obligate intracellular parasite. Blood agar is not suitable for its growth.
• The infection is more readily diagnosed by serologic tests than by isolation of the organism: While culturing C. psittaci can be challenging, serologic tests detecting antibodies against the organism are often the preferred diagnostic method due to their higher sensitivity.

Answer: An acid-fast rod that forms colonies within 7 days

The clinical presentation, including the persistent nonproductive cough, fever, rales on lung examination, and patchy infiltrate on chest X-ray, suggests a respiratory infection. The fact that her serum agglutinates red blood cells at 4°C but not at 37°C indicates the presence of cold agglutinins, which are often associated with Mycoplasma pneumoniae infections. Mycoplasma pneumoniae is a very small bacterium that lacks a cell wall and is known to cause atypical pneumonia. It can be cultured on specialized media and typically forms colonies within 7 days.

The other options are incorrect:

• A filamentous gram-positive rod that is weakly acid-fast: This description does not match Mycoplasma pneumoniae. Mycoplasma pneumoniae is not filamentous, nor is it acid-fast. Acid-fast staining is characteristic of Mycobacterium species, not Mycoplasma.
• A spirochete that has never been grown on blood agar: This description does not match Mycoplasma pneumoniae. Mycoplasma pneumoniae is not a spirochete. Additionally, Mycoplasma pneumoniae can be grown on specialized media, such as Eaton’s agar or PPLO agar.
• A very small bacterium that has no cell wall: While this description partially fits Mycoplasma pneumoniae (a very small bacterium that lacks a cell wall), the critical aspect of forming colonies within 7 days is missing. Mycoplasma pneumoniae typically requires longer incubation periods to form colonies on agar media compared to the timeframe described.

Answer: Protein A

Streptococcus pyogenes, also known as Group A Streptococcus (GAS), possesses several virulence factors, but Protein A is not one of them. Protein A is a virulence factor associated with Staphylococcus aureus, not Streptococcus pyogenes.

The other options are incorrect:

• Beta-hemolysin: Streptococcus pyogenes produces beta-hemolysin, which causes complete lysis of red blood cells on blood agar plates, resulting in a clear zone around the colonies.
• Polysaccharide group-specific substance: Streptococcus pyogenes produces polysaccharide group-specific substances, which are antigens used to classify streptococci into different Lancefield groups. Group A streptococci are characterized by their possession of a specific polysaccharide antigen.
• M protein: Streptococcus pyogenes expresses M protein, a major virulence factor that plays a role in evasion of the host immune response. M protein helps the bacterium resist phagocytosis and promotes adherence to host cells, contributing to the pathogenesis of streptococcal infections.

Answer: An integrated temperate phage

An integrated temperate phage: This is the genetic material that encodes diphtheria toxin in pathogenic Corynebacterium diphtheriae. The phage integrates its DNA into the bacterial chromosome, lysogenizing the bacterium. The diphtheria toxin gene (tox) is carried on this bacteriophage.

The other options are incorrect:

• A diphthamide on eEF-2: Diphthamide is a modified amino acid found on a host cell protein, elongation factor-2 (EF-2). Diphtheria toxin ADP-ribosylates this diphthamide, inhibiting protein synthesis in the host cell.
• An episome: An episome is a circular piece of extrachromosomal DNA that can replicate independently of the chromosome. While some bacteria may harbor plasmids with virulence factors, the diphtheria toxin gene in C. diphtheriae is typically located on an integrated phage, not a free-floating episome.
• Highly repetitive bacterial DNA: Repetitive DNA sequences are found throughout bacterial genomes, but they are not specific to pathogenic C. diphtheriae.