MicroBiology MCQs with Answer and Explnation | Chapter 46

Answer: All of the above tests

There is no single test definitive for diagnosing gonorrhea. Different tests identify different aspects of the infection, and accurate diagnosis often requires a combination of approaches. Here’s why each option can be used:

• Complement fixation tests (CFTs): Detect antibodies against the bacteria, indicating past or present infection. Useful for chronic or asymptomatic cases.
• Pili agglutination tests: Agglutinate (clump) the bacteria based on their pili (hair-like structures). Quick and simple, but less sensitive than other tests.
• Haemagglutination tests: Similar to pili agglutination, but use red blood cells to attach to the bacteria. Moderately sensitive and specific.

Answer: Streptococcus viridans

Streptococcus viridans is the most common bacteria responsible for subacute bacterial endocarditis, accounting for approximately 50% to 80% of cases. These are low-virulence bacteria typically found in the mouth and upper respiratory tract. They enter the bloodstream during routine activities like brushing teeth or flossing, and can adhere to damaged heart valves, causing infection.

The other options are incorrect:

• Haemolytic streptococci: While some strains of haemolytic streptococci (e.g., Group A Streptococcus) can cause invasive infections, they are not typically associated with subacute bacterial endocarditis.
• Staphylococcus aureus: This type of bacteria usually leads to acute bacterial endocarditis, which progresses rapidly and has a different set of symptoms compared to the slower course of subacute endocarditis.
• Nisseria: This genus includes bacteria like Neisseria meningitidis and Neisseria gonorrhoeae, which cause meningitis and gonorrhea, respectively. They are not involved in subacute bacterial endocarditis.

Answer: Niacin

While Salmonella can synthesize most of the nutrients it needs, some strains lack the ability to produce niacin (vitamin B3) and require it for growth. Niacin is essential for various metabolic processes within the bacteria, and its absence hinders their ability to multiply.

The other options are incorrect:

• Tryptophan: Some Salmonella strains can convert tryptophan into niacin, making it not essential for all.
• B-12: Not directly required for most Salmonella growth, although some specific enzymes might utilize it.
• Citrate: Primarily an energy source for Salmonella, not directly related to growth factor requirements.

Answer: Escherichia coli

Escherichia coli (E. coli) is the most common culprit behind urinary tract infections (UTIs), accounting for roughly 80-85% of cases. This bacterium lives naturally in the gut but can occasionally migrate to the urethra and ascend into the bladder, causing inflammation and infection.

The other options are incorrect:

• Neisseria gonorrhea: Primarily causes sexually transmitted infections like gonorrhea and chlamydia, though it can rarely reach the bladder and cause UTIs.
• T-strain mycoplasma: While this organism can be associated with UTIs, it’s significantly less frequent than E. coli and often misdiagnosed due to challenging detection methods.
• Streptococcus fecalis: This bacteria can cause UTIs but is less common than E. coli and typically originates from the gut or vagina.

Answer: Spinal card

The tetanus neurotoxin, also known as tetanospasmin, specifically targets inhibitory neurons in the spinal cord. These neurons release neurotransmitters like glycine and gamma-aminobutyric acid (GABA) that prevent overactivation of motor neurons. The toxin blocks the release of these inhibitory neurotransmitters, resulting in uncontrolled muscle contractions and the characteristic spastic paralysis seen in tetanus.

The other options are incorrect:

• Synapse: While tetanospasmin binds to nerve terminals at the neuromuscular junction, its primary action is in the spinal cord.
• Muscle: The toxin itself doesn’t directly affect muscle tissue. Its action on the spinal cord leads to uncontrolled muscle contractions.
• Brain: Tetanospasmin rarely reaches the brain and its main site of action is the spinal cord.

Answer: Prevent their phagocytosis

Encapsulation in bacteria primarily serves as a defense mechanism that helps prevent their phagocytosis by white blood cells (phagocytes) of the immune system. The capsule, composed of polysaccharides, glycoproteins, or other polymers, acts as a physical barrier, masking the bacterium’s surface antigens from recognition by phagocytes. This hinders engulfment and destruction of the bacteria, allowing them to survive and potentially cause infection.

The other options are incorrect:

• Spore formation: While some bacteria form spores as a dormant, resistant stage for survival in harsh environments, this is not an aspect of “encapsulation” as it involves internal restructuring and dehydration, not a surrounding capsule.
• Decreased virulence: While encapsulation can indirectly contribute to decreased virulence by limiting bacterial interactions with host cells, it is not the primary purpose of the capsule. Some encapsulated bacteria can still be highly virulent.
• Oxygen effect: The presence of a capsule can have various effects on bacterial interaction with oxygen depending on the specific bacterium and environmental conditions. However, it’s not the main function of encapsulation.

Answer: Pili

Pili are hair-like appendages made of protein, found on the surface of many bacteria. They are specifically designed for adhesion to host cell surfaces, tissues, and other surfaces. Pili function like tiny grappling hooks, anchoring the bacteria and facilitating colonization or infection.

The other options are incorrect:

• Capsule: While capsules can offer some indirect attachment by forming a sticky layer around the bacteria, their primary function is protection against phagocytosis by immune cells, not active attachment.
• Flagella: Flagella are whip-like structures responsible for bacterial movement, not attachment. Although flagella can influence bacterial positioning, they don’t directly bind to other surfaces.
• Mesosomes: Mesosomes are internal folds in the bacterial membrane, involved in various cellular processes like respiration and cell division. They have no role in external attachment.

Answer: Decease in virulence

A capsule is a polysaccharide layer surrounding some bacterial cells. It acts as a shield, protecting the bacteria from phagocytosis by the host’s immune system and desiccation. Therefore, loss of the capsule usually weakens the bacteria’s ability to cause disease, resulting in a decrease in virulence.

The other options are incorrect:

• Loss of infectivity: While decreased virulence may indirectly impact infectivity (fewer successful infections), bacteria lacking a capsule can still adhere to host cells and initiate infection.
• Inability to spread through tissue: Capsular polysaccharides can facilitate bacterial movement through tissues, but their absence doesn’t completely prevent it. Other factors like flagella and adhesins can still contribute to bacterial spread.
• Increase in invasiveness: The opposite is true. Loss of the capsule weakens the bacteria’s defenses against the host’s immune system, making them less invasive and more susceptible to phagocytosis.

Answer: Gas gangrene

The Nagler reaction is a specific test used to differentiate the bacterium Clostridium perfringens, a major causative agent of gas gangrene, from other Clostridium species. It relies on the ability of C. perfringens to produce a specific enzyme called lecithinase C, which degrades egg yolk in a characteristic way.

The other options are incorrect:

• Malaria: Diagnosed by identifying the malaria parasite Plasmodium in blood smears or using specific blood tests.
• Whooping cough: Diagnosed by detecting the Bordetella pertussis bacteria through PCR tests or analyzing nasal swabs.
• Wool sorters disease: Diagnosed by identifying the Bacillus anthracis bacteria in blood, skin lesions, or lymph nodes.

Answer: Auramine and Rhodamine

Auramine and Rhodamine are the most commonly used fluorescent dyes for detecting Mycobacterium tuberculosis, the bacteria that cause tuberculosis. They bind specifically to the waxy cell walls of Mycobacteria, making them glow bright yellow or orange-red under a fluorescence microscope. This allows for rapid and easy identification of the bacteria in sputum, tissue samples, and other clinical specimens.

The other options are incorrect::

• Acridine orange: While Acridine orange is also a fluorescent dye, it is not specific to Mycobacterium tuberculosis. It can stain a variety of bacteria and other cells, making it less suitable for TB diagnosis.
• Thioglavin: Thioglavin is another fluorescent dye used in microscopy but is not commonly used for TB diagnosis. Its primary use is in the detection of fungi and yeasts.
• Congo red: Congo red is a non-fluorescent dye used in histology to stain amyloid deposits and other specific tissues. It has no role in TB diagnosis.

Answer: Anaerobic, gram positive, non-acid fast bacteria.

Actinomycosis is a condition caused by bacteria belonging to the Actinomyces genus. These bacteria are anaerobic, gram-positive, and filamentous. Actinomyces species are normal inhabitants of the oral and gastrointestinal tract, and infection occurs when they penetrate tissues, leading to the formation of abscesses, sinus tracts, and characteristic sulfur granules.

The other options are incorrect:

• Fungus: Actinomycosis is not caused by a fungus. It is caused by bacteria of the Actinomyces genus.
• Acid-fast, non-motile bacillus: Actinomycosis is not caused by an acid-fast bacillus. Acid-fast bacilli are more commonly associated with mycobacteria, such as Mycobacterium tuberculosis. Actinomycosis is caused by anaerobic, gram-positive bacteria.
• Retrovirus: Actinomycosis is not caused by a retrovirus. Retroviruses, such as HIV, belong to a different group of infectious agents. Actinomycosis is caused by bacteria, specifically Actinomyces species.

Answer: Able to grow in the presence of absence of oxygen

Facultative anaerobic bacteria are versatile organisms that can adapt their metabolism to survive in environments with or without oxygen. They possess the biochemical machinery for both aerobic respiration (using oxygen) and anaerobic respiration or fermentation (not using oxygen) for generating energy.

The other options are incorrect:

• Unable to grow in the presence of oxygen: These would be obligate anaerobes, which are poisoned by oxygen and thrive only in its absence.
• Unable to grow in the absence of oxygen: These would be obligate aerobes, relying solely on oxygen for survival.
• Able to grow in the presence of carbon dioxide: While carbon dioxide can be involved in some metabolic pathways, its presence or absence is not the defining characteristic of facultative anaerobes.

Answer: Anthrax

Anthrax is a potentially fatal disease caused by the bacterium Bacillus anthracis. It can be transmitted from infected animals to humans through direct contact with contaminated skin, fur, or hair, or by inhaling or ingesting spores. This zoonotic characteristic sets it apart from the other options.

The other options are incorrect:

• Typhoid: Caused by the bacterium Salmonella Typhi, primarily transmitted through contaminated food and water, not directly from animals.
• Bacillary dysentery: Caused by Shigella bacteria, also spread through contaminated food and water, not zoonotic.
• Cholera: Caused by the bacterium Vibrio cholerae, spread through contaminated water and food, not a zoonotic disease.

Answer: Alpha hymolytic streptococci (S. viridans

Alpha-hemolytic streptococci (specifically Streptococcus viridans) are the most common inhabitants of the throat and tonsils in healthy individuals. They are part of the normal flora of the upper respiratory tract and play a role in maintaining a healthy balance of microorganisms in this region.

The other options are incorrect:

• Staphylococci: While Staphylococcus aureus can be found in the throat and tonsils, it’s not as prevalent as alpha-hemolytic streptococci and is not considered part of the normal flora.
• Beta hemolytic streptococci: These, including the notorious Group A Streptococcus pyogenes, are more commonly associated with infections like strep throat and tonsillitis rather than being regular inhabitants.
• Pneumococci: Streptococcus pneumoniae (pneumococci) primarily colonize the nasopharynx and are more associated with respiratory infections like pneumonia than being normal inhabitants of the throat and tonsils.

Answer: Elaboration of an enzyme that destroy penicillin

Staphylococcus aureus resistance to penicillin arises primarily from the acquisition of genes encoding beta-lactamase enzymes. These enzymes break down the beta-lactam ring, the crucial structural element of penicillin, rendering the antibiotic ineffective.

The other options are incorrect:

• Pleomorphism: While S. aureus can change its shape slightly, this trait isn’t directly related to antibiotic resistance.
• Penicillin analogue production: This is not a known mechanism of resistance in S. aureus.
• Lack of nucleic acid in the cell wall: The cell wall of S. aureus, like most bacteria, does contain nucleic acid (DNA). Its absence would be incompatible with life.

Answer: Cutaneous anthrax

Malignant pustule is a term specifically used to describe the characteristic skin lesion associated with cutaneous anthrax, a type of anthrax infection that occurs on the skin. It is a dark, raised pimple surrounded by swelling and can progress to a central area of dead tissue (eschar).

The other options are incorrect:

• Facio-cervical actinomycosis: This refers to a specific form of actinomycosis infection involving the face and neck, caused by a different type of bacteria. While it can present with abscesses, the features differ from the malignant pustule of anthrax.
• Infected squamous cell carcinoma: This describes a cancerous lesion that has become secondarily infected, not a characteristic of anthrax. The appearance and progression differ from a malignant pustule.
• None of the above: While facio-cervical actinomycosis and infected squamous cell carcinoma are not accurate descriptions of a malignant pustule, the term itself implies a specific infectious nature, making “None of the above” not the best answer.

Answer: Salmonella

Salmonella species are the primary example of major intestinal pathogens that are non-lactose fermenters. They lack the enzyme responsible for lactose fermentation, resulting in colonies appearing colorless on lactose-containing agar plates like MacConkey agar. This characteristic helps differentiate them from other enteric bacteria, many of which are lactose fermenters and appear pink or red on such media.

The other options are incorrect:

• Klebsiella: While some Klebsiella species can be lactose fermenters, others are non-fermenters. However, Klebsiella are not considered major intestinal pathogens and are more commonly associated with respiratory and urinary tract infections.
• Escherichia coli: Most, but not all, E. coli strains are lactose fermenters. Typically, pathogenic E. coli strains associated with intestinal infections do ferment lactose.
• Paracolons: This term refers to a group of bacteria closely related to Escherichia coli, and most are lactose fermenters. They are also not as significant intestinal pathogens as Salmonella.

Answer: Salk vaccine

The Salk vaccine, also known as the inactivated poliovirus vaccine (IPV), is produced using inactivated (killed) poliovirus. This vaccine is named after Dr. Jonas Salk, who developed it. The inactivated poliovirus cannot cause the disease but stimulates the immune system to produce an immune response, providing protection against polio.

The other options are incorrect:

• Tetanus toxoid: The tetanus toxoid vaccine is not made from inactivated microorganisms. It is produced by inactivating the toxin (toxoid) produced by the bacterium Clostridium tetani. It is used to protect against tetanus.
• Sabin’s oral vaccine: Sabin’s oral vaccine, or the oral poliovirus vaccine (OPV), is not made from inactivated microorganisms. It contains live, attenuated (weakened) forms of the poliovirus. It was developed by Dr. Albert Sabin and is an alternative to the inactivated poliovirus vaccine (Salk vaccine).
• All of the above: This option is incorrect because only the Salk vaccine among the listed options is made from inactivated microorganisms. Tetanus toxoid and Sabin’s oral vaccine have different manufacturing processes involving inactivated toxins or live, attenuated viruses, respectively.

Answer: Bacteroides

Bacteroides are the only anaerobic bacteria among the listed options. They are strict anaerobes, meaning they cannot survive in the presence of oxygen. They obtain energy through fermentation processes that occur in the absence of oxygen.

The other options are incorrect:

• E. coli: Escherichia coli is a facultative anaerobe, meaning it can grow both with and without oxygen. It can switch between aerobic respiration (using oxygen) and anaerobic fermentation depending on the available oxygen levels.
• Pseudomonas: Pseudomonas is another facultative anaerobe with similar adaptability to E. coli.
• Klebsiella: Klebsiella is also a facultative anaerobe capable of both aerobic and anaerobic metabolism.

Answer: Staphylococcus

Staphylococcus bacteria, particularly certain coagulase-positive species like Staphylococcus aureus, commonly grow in grape-bunch shaped colonies on agar plates. This characteristic appearance arises from the irregular arrangement of cells clinging to each other in tightly packed clusters, resembling a bunch of grapes.

The other options are incorrect:

• Streptococcus: While streptococci can also form clustered colonies, their typical patterns include chains or pairs, not the grape-bunch arrangement characteristic of Staphylococcus.
• E. coli: Escherichia coli typically forms round, flat colonies with smooth edges on agar plates, not the distinctive grape-bunch clusters.
• Gonococci: These diplococci (bacteria occurring in pairs) usually form small, convex, and translucent colonies, not grape-bunch clusters.

Answer: Coagulase

Coagulase is the key enzyme produced by pathogenic staphylococci (primarily Staphylococcus aureus) that induces the formation of a plasma clot. It converts fibrinogen, a soluble protein in plasma, into fibrin, which forms a mesh-like network that traps red blood cells and other components to create a clot. This clotting process helps the bacteria evade the immune system and establish infection.

The other options are incorrect:

• Lysozyme: This enzyme is actually destructive to bacteria and plays a role in the immune system’s defense against them. It breaks down peptidoglycan, a component of bacterial cell walls, causing lysis (destruction) of the bacteria.
• Fibrinolysin: This enzyme dissolves fibrin clots and is produced by some bacteria to counteract host clotting mechanisms. It wouldn’t be involved in clot formation by staphylococci.
• S protein: While S protein is a staphylococcal surface protein that interacts with fibrinogen and other clotting factors, it doesn’t have the enzymatic activity to directly convert fibrinogen to fibrin like coagulase does.

Answer: Clostridium tetani

Clostridium tetani produces a potent exotoxin called tetanospasmin, which binds to nerve cells and causes severe muscle contractions, a hallmark symptom of tetanus. Pseudomonas, Shigella, and Klebsiella primarily produce endotoxins, which are less potent and have different effects on the body.

The other options are incorrect:

• Pseudomonas: While some Pseudomonas species produce exotoxins, they are not as potent as tetanospasmin and typically cause different symptoms such as respiratory infections or wound infections.
• Shigella: Shigella bacteria primarily cause foodborne illness via endotoxins that trigger an inflammatory response in the intestines.
• Klebsiella: Similar to Shigella, Klebsiella primarily causes infections through endotoxins that affect the respiratory system or urinary tract.

Answer: Syphilis

The Wassermann test, developed in 1906, was a groundbreaking non-treponemal test for detecting syphilis by identifying antibodies developed in response to the infection. While newer, more specific tests have replaced it, the Wassermann test played a crucial role in diagnosing and controlling syphilis for decades.

The other options are incorrect:

• Gonorrhea: Gonorrhea is diagnosed through different methods like gram staining of urethral discharge or cervical swabs, nucleic acid amplification tests (NAATs), and culture tests. The Wassermann test is not specific for gonorrhea.
• TB: Tuberculosis (TB) diagnosis involves various methods like chest X-rays, tuberculin skin tests, sputum smear microscopy, and culture tests. The Wassermann test is not specific for TB and can produce false-positive results in some cases of TB.
• Typhoid: Typhoid fever is diagnosed through blood, stool, or bone marrow cultures, specific serological tests like the Widal test, and rapid diagnostic tests. The Wassermann test is not specific for typhoid and can cross-react with other infections.

Answer: Diptheria

Phage conversion refers to the phenomenon where a bacteriophage (virus that infects bacteria) introduces new genetic material into a bacterial cell, leading to changes in the characteristics of the bacterium. In the case of diphtheria, the bacterium Corynebacterium diphtheriae is lysogenized by a bacteriophage carrying the tox gene. This phage conversion results in the production of diphtheria toxin, which is a key virulence factor in causing diphtheria.

The other options are incorrect:

• Tularemia: Phage conversion is not required for tularemia. Tularemia is caused by the bacterium Francisella tularensis, and its pathogenicity is associated with factors other than phage conversion.
• Gonorrhoea: Phage conversion is not required for gonorrhea. Gonorrhea is caused by the bacterium Neisseria gonorrhoeae, and its pathogenicity is not linked to phage conversion.
• All of the above: This option is incorrect because phage conversion is specifically associated with diphtheria. It is not required for tularemia or gonorrhea.

Answer: in the absence of oxygen

Anaerobic bacteria are microorganisms that thrive in environments without oxygen (anaerobic conditions). They rely on alternative metabolic pathways for energy production, utilizing molecules other than oxygen as electron acceptors. Their growth would be inhibited or even fatal in the presence of oxygen.

The other options are incorrect:

• in the presence of oxygen: This goes against the definition of anaerobic, as they are specifically unable to grow in oxygenated environments.
• in the presence of nitrogen: While nitrogen gas is often present in anaerobic environments, it isn’t directly relevant to anaerobic bacterial growth. They don’t utilize nitrogen as an electron acceptor in their metabolism.
• differential media: This refers to a type of culture medium used in microbiology to distinguish between different types of bacteria based on their specific growth requirements or reactions. It isn’t a description of the environment needed for anaerobic bacteria to grow.

Answer: It is a unique flexible plastic structure

While the bacterial cell wall does possess many of the listed properties, it’s not a flexible plastic structure. It’s a rigid cell envelope that gives bacteria their shape and protects them from mechanical and osmotic pressure.

The other options are incorrect:

• It consists of a mixed polymer called peptidoglycan: This is true. Peptidoglycan is a mesh-like polymer of sugars and amino acids that provides structural strength to the cell wall.
• It is the structure principally responsible for the reaction of gram staining: This is true. The composition of the cell wall determines the outcome of Gram staining, where Gram-positive bacteria retain the crystal violet stain due to their thick peptidoglycan layer, while Gram-negative bacteria lose the stain due to their thinner peptidoglycan layer and outer membrane.
• It contains D-isomers of amino acids: This is true. While most proteins in nature are made up of L-amino acids, the amino acids in peptidoglycan are primarily D-isomers, contributing to its unique structure and resistance to certain enzymes.

Answer: Cl. difficile

While all the listed options are bacteria capable of causing foodborne illness, Clostridium difficile (Cl. difficile) is primarily associated with healthcare settings and antibiotic use. It rarely causes food poisoning through contaminated food.

The other options are incorrect:

• Staphylococcus: Several Staphylococcus species, like Staphylococcus aureus (Staph aureus), can cause foodborne illness through toxins produced in contaminated food. Staph food poisoning is known for its rapid onset of symptoms like nausea, vomiting, and abdominal cramps.
• Cl. welchii (Clostridium perfringens): This bacterium is a common cause of foodborne illness, particularly associated with cooked meats and poultry that are not properly cooled or reheated. It produces toxins that cause abdominal cramps, diarrhea, and sometimes vomiting.
• Cl. botulinum: This bacterium is infamous for its potent neurotoxin, botulinum toxin, which causes botulism, a rare but potentially fatal paralytic illness. Spores of Cl. botulinum can be present in improperly canned or preserved foods.

Answer: Its DNA has both introns and extrons

Bacterial genomes typically do not have introns and exons. In prokaryotic organisms like bacteria, the genetic material is organized into a circular DNA molecule, and the genes are generally continuous, lacking the introns and exons characteristic of eukaryotic genomes. In eukaryotes, introns are non-coding regions that are transcribed but are spliced out during RNA processing, while exons are coding regions.

The other options are incorrect:

• It is composed of DNA: This is a correct statement. Bacterial genomes are composed of DNA, as are the genomes of all living organisms.
• It does not contain histones: This is a correct statement. Bacterial genomes lack histones, which are proteins associated with the packaging of DNA in eukaryotic cells. Bacterial DNA is typically organized by other proteins.
• It is circular: This is a correct statement. Bacterial genomes are often organized into a circular DNA molecule, in contrast to the linear DNA of eukaryotic genomes.

Answer: Spirochetes

Necrotizing ulcerative gingivitis (NUG) is a severe form of gingivitis characterized by inflammation, ulceration, and necrosis of the gingival tissues. Electron microscopic examination has revealed the presence of spirochetes, particularly Treponema species, within non-necrotic tissues. Spirochetes are spiral-shaped bacteria known for their motility, and their involvement in NUG suggests a role in the pathogenesis of the condition.

The other options are incorrect:

• Cocci: Cocci are spherical-shaped bacteria. While various bacteria, including cocci, can be associated with oral flora, the specific involvement of spirochetes is highlighted in the context of necrotizing ulcerative gingivitis.
• Bacteriophages: Bacteriophages are viruses that infect bacteria. Their presence is not typically associated with the bacterial flora observed in necrotizing ulcerative gingivitis.
• Filamentous rods: Filamentous rods refer to elongated, rod-shaped bacteria. The involvement of spirochetes, not filamentous rods, is emphasized in the electron microscopic examination of necrotizing ulcerative gingivitis.

Answer: Fimbria

Fimbria, also known as pili, are hair-like appendages on the surface of bacteria that play a crucial role in attachment to host cells or surfaces. They are composed of protein subunits called pilin and can vary in length and number depending on the bacterial species.

The other options are incorrect:

• Capsule: While capsules are a polysaccharide layer surrounding some bacteria, their primary function is protection against environmental stress and phagocytosis by immune cells. They do not directly participate in bacterial attachment.
• Flagella: Flagella are whip-like structures used for bacterial motility, allowing them to move and navigate their environment. They are not involved in adhesion to surfaces.

Answer: All of the above

• Typhoid fever is a serious bacterial infection caused by Salmonella Typhi bacteria. It primarily affects the digestive system and can cause a range of severe complications, including:
• Haemorrhage: Internal bleeding can occur in the intestines, leading to bloody stools or vomit.
• Perforation: The weakened intestinal wall can rupture, spilling bacteria and intestinal contents into the abdominal cavity and causing peritonitis, a life-threatening inflammation.
• Osteomyelitis: The bacteria can spread to bones, particularly the long bones, causing inflammation and bone destruction.

Answer: Blood

In the initial stages of typhoid fever, detecting Salmonella Typhi in blood cultures is the most reliable and sensitive method. This is because the bacteria initially invade the bloodstream, multiplying within white blood cells and causing the characteristic fever.

The other options are incorrect:

• Feaces: While Salmonella can be present in stool during later stages of typhoid fever, it may not be detectable in the early phases due to low bacterial numbers. Additionally, stool cultures can be less sensitive and have a higher risk of contamination.
• Urine: Salmonella Typhi typically does not colonize the urinary tract, so urine cultures are not effective for early detection of typhoid fever.

Answer: Drum stick

Clostridium tetani is a bacterium that causes tetanus. On a stained slide, its characteristic appearance is that of a drumstick or tennis racket. This distinctive morphology is due to the presence of terminal spores, which give the bacterium its characteristic shape.

The other options are incorrect:

• Bunch of grapes: This option is not accurate. The characteristic appearance of Clostridium tetani is more like a drumstick, and it does not resemble a bunch of grapes.
• Chain of beads: Clostridium tetani does not have a chain of beads appearance on a stained slide. This description is more commonly associated with Streptococcus pneumoniae, where the bacterial cells appear in chains resembling a string of beads.
• Safety pin: Clostridium tetani does not resemble a safety pin on a stained slide. The characteristic appearance is that of a drumstick due to the presence of terminal spores. The safety pin appearance is associated with Yersinia pestis, the causative agent of bubonic plague.

Answer: Dark ground microscope

Leptospira bacteria are thin, motile spirochetes that are difficult to visualize under a conventional brightfield microscope. Therefore, the most appropriate tool for screening urine samples for Leptospira is a dark ground microscope.

The other options are incorrect:

• Scanning microscope: Scanning electron microscopes provide high-resolution images of the surface of cells but are not used for routine screening of urine samples and require special preparation.
• Inverted microscope: Inverted microscopes are primarily used for studying cells and tissues in culture and are not ideal for examining urine samples.
• Electron microscope: Electron microscopes offer even higher resolution than scanning electron microscopes but are even more specialized and expensive, making them unsuitable for routine screening purposes.

Answer: None of the above

Koch’s postulates are a set of four criteria that must be met to establish a specific bacterium as the cause of a particular disease.

• The bacterium must be present in every case of the disease.
• The bacterium must be isolated from the diseased host and grown in pure culture.
• The bacterium from the pure culture must cause the disease when inoculated into a healthy susceptible host.
• The bacterium must be reisolated from the inoculated host and shown to be identical to the original bacterium.

The other options are incorrect:

• Treponema pallidum: This bacterium is the cause of syphilis, but it cannot be grown in pure culture, making it impossible to fulfill the third postulate.
• Mycobacterium leprae: This bacterium is the cause of leprosy, but it is very slow-growing and requires special conditions to grow in culture, making it difficult to fulfill the second and third postulates.
• N. gonorrhoeae: This bacterium is the cause of gonorrhea, but it can be grown in pure culture, which violates the fourth postulate.

Answer: Typhoid fever

Typhoid fever is the only disease listed that is primarily transmitted through contaminated food and water. It is caused by the bacterium Salmonella Typhi, which can be found in improperly cooked or stored meat, poultry, eggs, and dairy products, as well as contaminated fruits and vegetables.

The other options are incorrect:

• Viral stomatitis: This is a viral infection that typically causes mouth sores and is not transmitted through food.
• Diptheria: This is a bacterial infection that spreads through respiratory droplets and not through food.
• Tuberculosis: This is a bacterial infection that primarily affects the lungs and is most commonly spread through coughing and sneezing, not food.

Answer: Buccae

Black pigmented anaerobes are a group of bacteria known for their dark pigmentation and ability to thrive in oxygen-deprived environments. While the other listed options (Tannerella, Porphyromonas, Prevotella) are all members of this group, Buccae is not.

The other options are incorrect:

• Tannerella: This bacterium is a black-pigmented anaerobe associated with periodontitis, a gum infection.
• Porphyromonas: Another black-pigmented anaerobe, Porphyromonas is also linked to periodontitis and other oral infections.
• Prevotella: Several Prevotella species are black-pigmented anaerobes found in the mouth and gut, some associated with gum disease and others with digestive issues.

Answer: 10

A complete primary course of tetanus immunization typically includes three doses of tetanus toxoid vaccine spread over several months. This complete course provides immunity for about 10 years.

The other options are incorrect:

• 5 years: This is too short of a duration. Immunity from a complete tetanus vaccination series usually wanes after around 10 years.
• 15 years: This is longer than the typical duration of immunity after a full course of tetanus vaccination. Booster shots are recommended every 10 years to maintain protection.
• 20 years: This is significantly longer than the recommended interval for booster doses. While some individuals may have protection for longer, relying on such extended immunity without boosters is not advisable.

Answer:

111

Answer: Infection is mostly due to endogenous source

Pseudomonas is a gram-negative, aerobic bacterium that is commonly found in soil, water, and moist environments. While it can cause infections in various settings, such as hospitals and in patients with compromised immune systems, the primary source of Pseudomonas infections is often exogenous, originating from the external environment rather than from the patient’s own flora.

The other options are incorrect:

• Strict aerobes: This is incorrect. Pseudomonas is known as an obligate aerobe, meaning it requires oxygen for growth.
• Can grow in disinfectants in hospitals: This is correct. Pseudomonas has inherent resistance to many disinfectants, making it a concern in healthcare settings.
• Most common organism in burn patients: This is correct. Pseudomonas aeruginosa is often associated with infections in burn patients due to its ability to thrive in moist environments and its resistance to many antibiotics.

Answer: Diagnosis

The lepromin test, also known as the leprosy skin test, is primarily used for the diagnosis of leprosy. It involves injecting a small amount of inactivated Mycobacterium leprae, the bacterium that causes leprosy, into the skin and observing the patient’s immune response. There are two types of lepromin tests: the lepromin A test (for tuberculoid leprosy) and the lepromin B test (for lepromatous leprosy). The reaction to the test helps in diagnosing the form of leprosy and assessing the individual’s immune response to the infection.

The other options are incorrect:

• Prognosis: The lepromin test is primarily a diagnostic tool and is not typically used for determining the prognosis of leprosy.
• Treatment plan: The lepromin test does not guide the treatment plan directly. Treatment decisions for leprosy are generally based on clinical and histopathological findings, as well as the specific form of leprosy diagnosed.
• Epidemiology: While lepromin tests can provide information about the prevalence of leprosy in a population, they are not the primary tool for epidemiological studies. Epidemiological studies would involve broader surveys, case tracking, and analysis of various factors influencing disease spread.

Answer: O antibodiest last longer and hence is not indicative of recent infection

The Widal test is a serological test used for the diagnosis of typhoid fever. It detects antibodies against specific antigens of Salmonella typhi, the bacterium that causes typhoid fever. However, O antibodies are generally not specific and can persist for a long time even after a person has recovered from a previous infection or has been vaccinated. Therefore, elevated O antibody levels do not necessarily indicate a recent or current infection.

The other options are incorrect:

• Baseline differs depending on the endemicity of the disease: This is true. The baseline titers can vary depending on the prevalence of typhoid fever in a particular region.
• H antigen cannot differentiate between subtypes: This is true. The H antigen in the Widal test does not distinguish between different subtypes of Salmonella typhi.
• High titre value in a single Widal test is not confirmative: This is true. A single elevated titer in the Widal test is not considered confirmatory for typhoid fever. Diagnosis usually involves a combination of clinical symptoms, blood cultures, and multiple serological tests.

Answer: Onset of IgM in 2 days

In leptospirosis, the onset of IgM antibodies typically occurs around 7 to 10 days after the onset of symptoms, not within 2 days. IgM antibodies are indicative of an acute or recent infection. Leptospirosis is a bacterial infection caused by the spirochetes of the genus Leptospira. It is commonly transmitted through the urine of infected animals, especially rodents.

The other options are incorrect:

• Faeces & urine of rodents is the cause: This is true. Leptospirosis is often transmitted to humans through contact with water, soil, or food contaminated with the urine of infected animals, particularly rodents.
• Incubation period is 10 days: This is true. The incubation period for leptospirosis is typically 5 to 14 days, with an average of around 10 days.
• It is sensitive to penicillin: This is true. Leptospirosis is generally sensitive to antibiotics such as penicillin and doxycycline, especially when administered in the early stages of the disease.

Answer: Exotoxin

Diphtheria’s severe clinical manifestations are primarily caused by a potent exotoxin produced by the bacterium Corynebacterium diphtheriae. This exotoxin interferes with protein synthesis in cells, leading to tissue damage and widespread dysfunction.

The other options are incorrect:

• Invasion: While C. diphtheriae can invade the epithelium of the respiratory tract, this initial colonization itself isn’t responsible for the severe symptoms. It’s the subsequent toxin production and dissemination that cause major harm.
• Endotoxin: Endotoxins are released from the cell wall of certain bacteria upon cell death. However, C. diphtheriae does not produce significant amounts of endotoxin, and its pathogenesis is mainly driven by the exotoxin.
• Capsule: The polysaccharide capsule surrounding C. diphtheriae can contribute to its ability to evade the immune system and persist in the throat, but it isn’t directly responsible for the harmful effects associated with diphtheria.

Answer: Caused by leptospira interrogans

Vincent’s angina is a specific bacterial infection caused by a combination of Fusiform bacteria and spirochetes (primarily Borrelia vincentii), not leptospira interrogans. Leptospira interrogans is the causative agent of leptospirosis, a distinct disease with different symptoms and pathogenesis.

The other options are incorrect:

• Ulcerative gingivostomatitis: This term accurately describes a key characteristic of Vincent’s angina, which involves painful ulcers on the gums and inner cheeks.
• Caused due to malnutrition: Malnutrition can increase susceptibility to Vincent’s angina by weakening the immune system, but it is not the direct cause of the infection.
• A symbiotic infection: This is a valid description of Vincent’s angina, as the two main bacterial types cooperate to cause the disease. They create a more conducive environment for each other’s growth, enhancing their combined pathogenic effect.

Answer: Rheumatie fever

The ASO (Antistreptolysin O) test measures the level of antibodies your body produces in response to a group A streptococcal infection. While elevated ASO levels can be associated with various conditions, the primary use of the test is to diagnose or predict the risk of rheumatic fever, a serious inflammatory complication that can follow untreated or recurrent streptococcal infections.

The other options are incorrect:

• Rheumatoid arthritis: This is an autoimmune disease with different causes and diagnostic methods. ASO levels are not typically elevated in rheumatoid arthritis.
• Typhoid fever: This bacterial infection caused by Salmonella typhi has its own specific diagnostic tests, and ASO levels are not generally used for its diagnosis.
• Rickettsial fever: This group of tick-borne illnesses, caused by Rickettsia bacteria, also has distinct diagnostic tests, and ASO levels are not relevant.

Answer: Cardiac valves

The group A carbohydrate (Group A streptococcal carbohydrate, GAS CHO) of Streptococcus pyogenes exhibits a phenomenon called molecular mimicry with specific components of human cardiac valves. This means that the carbohydrate shares structural similarities with certain molecules found in the heart valves, leading antibodies produced against GAS CHO to mistakenly target and attack the heart tissue. This cross-reactivity plays a crucial role in the development of autoimmune complications like rheumatic fever and rheumatic heart disease.

The other options are incorrect:

• Synovial fluid: While GAS CHO does have some minor cross-reactivity with components of synovial fluid, it’s not considered a significant contributor to joint inflammation in rheumatic fever.
• Myocardium: Although GAS CHO may have minimal interactions with the heart muscle (myocardium), the primary target of the cross-reactive antibodies is the cardiac valves.
• Vascular intima: The inner lining of blood vessels (vascular intima) is not a primary target of the cross-reactive antibodies generated against GAS CHO.

Answer: Clostridium welchii

The most frequently encountered clostridial species in gas gangrene is Clostridium welchii, which is now known as Clostridium perfringens. Gas gangrene is a severe and potentially life-threatening condition characterized by the rapid destruction of soft tissue, often caused by infection with Clostridium perfringens. This bacterium is anaerobic and produces toxins that contribute to tissue necrosis and the production of gas within the affected tissues.

The other options are incorrect:

• Clostridium botulinum: Clostridium botulinum is not associated with gas gangrene. It is known for causing botulism, a condition characterized by muscle paralysis and neurotoxic symptoms.
• Clostridium tetani: Clostridium tetani is not the causative agent for gas gangrene. It is responsible for tetanus, a condition characterized by muscle stiffness and spasms.
• Clostridium difficile: Clostridium difficile is not the primary cause of gas gangrene. It is associated with antibiotic-associated diarrhea and colitis.

Answer: ADP – ribosylation of the G regulatory protein

Cholera toxin’s primary mode of action in the small intestine involves irreversibly modifying a G protein subunit through ADP-ribosylation. This modification disrupts the normal regulation of adenylate cyclase (AC), leading to uncontrolled production of cyclic AMP (cAMP). Elevated cAMP levels subsequently trigger increased secretion of chloride and bicarbonate ions into the intestinal lumen, along with impaired absorption of sodium and water. This cascade of events results in the profuse watery diarrhea characteristic of cholera.

The other options are incorrect:

• Inhibition of adenyl cyclase: Though cholera toxin ultimately causes decreased activity of AC through its downstream effects, its direct action is a potentiation, not inhibition.
• Activation of GTPase: Cholera toxin does not directly activate GTPases. Instead, it interacts with a G protein that requires GTP binding for its activity.
• Active absorption of NaCI: Cholera toxin disrupts normal electrolyte and fluid balance in the small intestine, actually leading to impaired absorption of sodium and chloride, contributing to the dehydrating diarrhea.

Answer: Cardiolipin

The reagin test, once widely used for syphilis diagnosis, employed cardiolipin as its key antigen. This phospholipid, extracted from beef heart, resembles certain components of treponema pallidum, the bacterium that causes syphilis. When present in an infected individual’s serum, specific anti-phospholipid antibodies (reagins) bind to the cardiolipin in the test, leading to visible clumping (agglutination) – a positive test result.

The other options are incorrect:

• Plasminogen: This inactive precursor enzyme involved in blood clotting holds no significance in the reagin test or syphilis diagnosis.
• Palmityl lecithin: Another phospholipid, but not used in the reagin test. While cardiolipin is a negatively charged phospholipid, lecithin lacks this specific characteristic, rendering it unsuitable for the reagin test’s mechanism.
• Serine: An amino acid, not a lipid, and plays no role in the reagin test or syphilis pathogenesis.