Free ASCP MLS Exam Practice Questions Mock Test: Part 25 – Microbiology – Gram-Negative Cocci Identification

• Pseudomonas aeruginosa is a Gram-negative bacillus known for its ability to contaminate disinfectants, hospital equipment, and water sources (e.g., sinks, respiratory therapy equipment). It is a classic opportunist in burn wounds, where it can cause severe infections, sepsis, and characteristic blue-green pus due to pyocyanin pigment.

• a) Escherichia coli: A common gut bacterium but not typically a contaminant of disinfectants or specifically linked to burn infections.

• c) Klebsiella pneumoniae: Associated with healthcare settings but less commonly with disinfectant contamination or burns.

• d) Shigella flexneri: A cause of dysentery, transmitted fecal-orally, not linked to disinfectants or burns.

• Escherichia coli is typically motile (due to peritrichous flagella) and indole-positive (produces indole from tryptophan).

• Klebsiella pneumoniae is non-motile and indole-negative.

• This combination (motility and indole test) is a reliable way to differentiate the two.

Why the other options are incorrect:

• b) Lactose fermentation: Both ferment lactose (though K. pneumoniae may do so more slowly), so this does not distinguish them.

• c) Beta-hemolysis on blood agar: Most strains of E. coli are beta-hemolytic, but K. pneumoniae is usually non-hemolytic. However, hemolysis is not a consistent differential feature (as some E. coli are non-hemolytic, and some Klebsiella may show alpha-hemolysis).

• d) Oxidase positivity: Both are oxidase-negative, so this test is useless for differentiation.

• Pseudomonas aeruginosa is a common cause of hospital-acquired pneumonia, especially in ventilated patients (ventilator-associated pneumonia, VAP). It is known for producing a distinctive grape-like or fruity odor due to the production of volatile compounds like 2-aminoacetophenone.

• a) Escherichia coli: Can cause pneumonia but is not typically associated with a grape-like odor.

• c) Klebsiella pneumoniae: Causes pneumonia (often with “red currant jelly” sputum) but has a mucoid appearance, not a grape-like odor.

• d) Proteus mirabilis: Associated with UTIs and a “burnt chocolate” odor, not grape-like.

P. aeruginosa thrives in moist environments (e.g., ventilators, respiratory equipment) and is resistant to many antibiotics. Its odor, oxidase positivity, and pigment production (e.g., pyocyanin) are key identifying features in the lab.

• Gram-negative diplococci, oxidase-positive: Classic for Neisseria species.

• Grows on chocolate agar but not on MacConkey agar: This is typical of fastidious Neisseria species that require enriched media.

• Acid from glucose but not from maltose: This is the fundamental carbohydrate utilization pattern that differentiates N. gonorrhoeae (glucose only) from N. meningitidis (glucose and maltose).

• b) N. meningitidis would produce acid from both glucose and maltose.

• c) Moraxella catarrhalis does not produce acid from any carbohydrates (asaccharolytic).

• d) Kingella kingae is a Gram-negative rod or coccobacillus (not a diplococcus) and is typically oxidase-positive but produces acid from glucose and maltose.

• The indole test is a key differential test:

  • Escherichia coli is indole-positive (it produces indole from tryptophan).

  • Enterobacter cloacae is indole-negative.

• This test reliably distinguishes between the two.

Why the other options are incorrect:

• b) Oxidase: Both E. coli and E. cloacae are oxidase-negative.

• c) Lactose fermentation: Both ferment lactose (though timing and intensity may vary, it is not a reliable differentiator).

• d) Citrate: E. coli is citrate-negative, while E. cloacae is citrate-positive. This is a useful test, but indole is more specific for distinguishing E. coli (indole-positive) from Enterobacter (indole-negative).

• Pseudomonas aeruginosa is a non-fermentative (it oxidizes carbohydrates instead of fermenting them), oxidase-positive, Gram-negative bacillus. It is known for producing a distinct grapelike or fruity odor due to the production of compounds like 2-aminoacetophenone.

• a) Escherichia coli is a fermentative, oxidase-negative bacterium and does not produce a fruity odor.

• c) Proteus mirabilis is fermentative and oxidase-negative; it produces a “burnt chocolate” odor, not fruity.

• d) Klebsiella pneumoniae is fermentative and oxidase-negative; it may have a mucoid appearance but no characteristic fruity odor.

• MacConkey agar is both selective (inhibits Gram-positive bacteria) and differential (distinguishes lactose fermenters from non-fermenters).

  • Lactose fermenters (e.g., E. coli, Klebsiella) produce pink/red colonies due to acid production, which causes the pH indicator (neutral red) to change color.

  • Non-lactose fermenters (e.g., Pseudomonas, Proteus, Salmonella) produce colorless or transparent colonies.

• a) Escherichia coli is a common Gram-negative rod that ferments lactose and typically appears as pink colonies on MacConkey agar. It is also the most frequent cause of urinary tract infections (UTIs).

• b) Pseudomonas aeruginosa does not ferment lactose and produces colorless colonies.

• c) Proteus mirabilis does not ferment lactose (initially; it may slowly ferment it but does not produce pink colonies) and is known for swarming on agar.

• d) Salmonella enterica does not ferment lactose and produces colorless colonies.

• All medically significant Neisseria species (including N. gonorrhoeae and N. meningitidis) are oxidase-positive. This is a fundamental biochemical test used to identify and distinguish them from other bacteria.

• a) Beta-lactamase production is not universal; some strains produce it (e.g., penicillin-resistant N. gonorrhoeae), but many do not.

• c) DNase production is not a characteristic of Neisseria; it is associated with other genera like Staphylococcus and Serratia.

• d) Hippurate hydrolysis is not a trait of Neisseria; it is used to identify organisms like Campylobacter jejuni and Group B Streptococcus.

• CIN agar (Cefsulodin-Irgasan-Novobiocin agar) is a selective medium used to isolate Yersinia enterocolitica from stool samples.

• Yersinia enterocolitica produces characteristic deep red “bull’s eye” colonies with a transparent border on CIN agar. This appearance is due to the fermentation of mannitol (which lowers the pH and turns the red dye inward) while the outer region remains less acidic.

• b) Shigella sonnei may appear as colorless or pale colonies on CIN agar.

• c) Salmonella Typhi does not grow well on CIN agar and does not produce a bull’s eye pattern.

• d) Klebsiella oxytoca may grow but appears as pink or mucoid colonies without the bull’s eye morphology.

• Salmonella Typhi (now classified as Salmonella enterica serotype Typhi) uniquely possesses a Vi capsule (virulence antigen), which is absent in most other Salmonella serotypes (e.g., S. Typhimurium, S. Enteritidis). This antigen helps it evade host immune responses and is a key diagnostic marker.

• a) Produces abundant H₂S: While S. Typhi does produce H₂S, many other Salmonella species (e.g., S. Typhimurium) also produce H₂S, so this is not a distinguishing feature.

• c) Is indole positive: S. Typhi is indole-negative, like most Salmonella species.

• d) Ferments lactose: S. Typhi does not ferment lactose (like all Salmonella), appearing as colorless colonies on MacConkey agar.

The Vi antigen is used in serological tests (e.g., agglutination) for identification and is the basis for some typhoid vaccines. Biochemically, S. Typhi also differs from other Salmonella by producing minimal gas from glucose fermentation and being citrate-negative, but the Vi antigen is the most specific marker.

• A triple sugar iron (TSI) agar test result with:

  • Alkaline (red) slant: Indicates no fermentation of lactose/sucrose (only glucose was fermented, and its byproducts were oxidized on the slant, raising the pH).

  • Acid (yellow) butt: Indicates glucose fermentation in the anaerobic environment of the butt.

  • H₂S production (black precipitate): Indicates hydrogen sulfide gas production from thiosulfate reduction.

• This pattern (K/A, H₂S+) is classic for Salmonella species (e.g., Salmonella enterica).

• a) Escherichia coli: Typically ferments lactose/sucrose, producing acid throughout (yellow slant/yellow butt, A/A) and no H₂S.

• c) Shigella species: Ferments glucose but not lactose/sucrose (red slant/yellow butt, K/A) but does not produce H₂S.

• d) Pseudomonas aeruginosa: Oxidizes glucose (may cause weak alkalinity or no change) and is H₂S-negative; it often produces a blue-green pigment

• Source: Cerebrospinal fluid (CSF) is the classic source for N. meningitidis, a leading cause of bacterial meningitis in adolescents and young adults.

• Morphology and Biochemistry: A Gram-negative diplococcus that is oxidase-positive fits the genus Neisseria.

• Carbohydrate Utilization: The ability to produce acid from both glucose and maltose is the definitive test result that identifies N. meningitidis. N. gonorrhoeae only oxidizes glucose, not maltose.

• a) N. gonorrhoeae does not ferment maltose and is an uncommon cause of meningitis.

• c) Moraxella catarrhalis does not ferment any carbohydrates (asaccharolytic) and is not a typical cause of meningitis.

• d) Haemophilus influenzae is a small Gram-negative coccobacillus, not a diplococcus, and requires factors V and X for growth.

The key to differentiating the pathogenic Neisseria and Moraxella lies in their carbohydrate utilization patterns:

• Neisseria gonorrhoeae ferments glucose (produces acid) but not maltose, lactose, or sucrose. This matches the description perfectly.

• a) Neisseria meningitidis ferments both glucose and maltose.

• b) Neisseria lactamica ferments glucose, maltose, and lactose.

• d) Moraxella catarrhalis does not ferment any carbohydrates (it is asaccharolytic).

• Moraxella osloensis is a Gram-negative diplococcus that can be part of the normal genital flora. Its size, shape, and staining characteristics are very similar to those of Neisseria gonorrhoeae, making it a known potential pitfall in the microscopic diagnosis from endocervical specimens.

• a) Lactobacillus species are Gram-positive rods.

• b) Streptococcus agalactiae (Group B Strep) is a Gram-positive coccus, often in chains.

• d) Pseudomonas aeruginosa is a Gram-negative rod.

• Gonorrhea and chlamydia are frequently co-transmitted (concurrent infection occurs in a significant proportion of cases). A patient with recurrent symptoms shortly after treatment for gonorrhea likely had an undiagnosed co-infection with Chlamydia trachomatis that was not treated initially.

• Standard treatment for gonorrhea (e.g., ceftriaxone) does not cover Chlamydia trachomatis. Therefore, guidelines recommend empiric dual therapy (e.g., ceftriaxone plus doxycycline or azithromycin) to cover both pathogens simultaneously.

• a) Mycoplasma hominis and d) Ureaplasma urealyticum are associated with genitourinary infections but are less commonly co-transmitted with gonorrhea and are not the primary concern in this scenario.

• c) Trichomonas vaginalis causes vaginitis/urethritis but is less frequently co-infected with gonorrhea compared to chlamydia.

• Yersinia enterocolitica is psychrotrophic, meaning it can grow at refrigeration temperatures (4°C).

• This property helps differentiate it from most other Enterobacteriaceae, which generally do not grow at such low temperatures.

• Clinically, this allows Yersinia to multiply in refrigerated foods, contributing to foodborne outbreaks.

Other options:

• Ferment lactose → Yersinia is usually non-lactose fermenting, similar to some Enterobacteriaceae.

• Produce pyocyanin → characteristic of Pseudomonas aeruginosa, not Yersinia.

• Swarm on blood agar → characteristic of Proteus species, not Yersinia.

• Neisseria gonorrhoeae → Gram-negative diplococcus, oxidase-positive, does not grow on MacConkey agar.

• Acinetobacter lwoffii → Gram-negative coccobacillus, oxidase-negative, and can grow on MacConkey agar (colonies often appear colorless since it’s non-lactose fermenting).

Other options:

• Gram stain reaction (a): Both are Gram-negative.

• Oxidase production (c): True, N. gonorrhoeae is oxidase-positive, Acinetobacter is oxidase-negative → also a differentiator, but the more distinctive key difference used in labs is MacConkey growth.

• Cell shape (d): Both can appear as small cocci/coccobacilli; morphology alone is not reliable.

The satellite growth phenomenon is a classic diagnostic test for Haemophilus influenzae. This bacterium requires both X factor (hemin) and V factor (NAD) for growth. Staphylococcus aureus produces NAD (V factor) as a metabolic byproduct during growth. When H. influenzae is plated near S. aureus, the NAD diffuses into the surrounding agar, allowing the H. influenzae to form colonies only in the area “satelliting” around the staphylococcal streak. This specific test is not used to identify the other listed organisms.

• a) Neisseria meningitidis does not require V factor; it grows on chocolate agar (where NAD is released from RBCs) but does not exhibit satelliteing.

• c) Moraxella catarrhalis does not require X or V factors.

• d) Streptococcus pneumoniae may be alpha-hemolytic and require cysteine but does not require V factor for growth.

• The cryptococcal antigen test (often performed on serum or cerebrospinal fluid) is the most sensitive test for the initial diagnosis of cryptococcal disease, particularly meningitis. It detects capsular polysaccharide antigens of Cryptococcus neoformans and C. gattii and is highly sensitive (>95%) and specific.

• a) India ink prep of CSF is a rapid test that visualizes the encapsulated yeast but has low sensitivity (approximately 50% in AIDS-related cryptococcal meningitis and even lower in non-HIV patients). It is not reliable for ruling out infection.

• b) Gram stain is insensitive for detecting Cryptococcus in CSF because the yeast may be present in low numbers and can be mistaken for other cells or artifacts.

• d) Culture on Sabouraud dextrose agar is the gold standard for confirmation and allows for species identification and antifungal susceptibility testing. However, it can take several days to become positive and is less sensitive than antigen testing for rapid diagnosis.

• ONPG Test: This test detects the enzyme β-galactosidase. Neisseria lactamica is positive for ONPG hydrolysis (it produces β-galactosidase), while Neisseria meningitidis is negative. This is the most reliable and specific test to distinguish these two species.

• a) Both N. meningitidis and N. lactamica grow on modified Thayer-Martin agar.

• b) Both are oxidase-positive.

• d) Both produce acid from glucose.

• MacConkey agar contains lactose and the pH indicator neutral red.

• When bacteria ferment lactose, they produce acidic byproducts (e.g., lactic acid).

• The drop in pH causes the neutral red indicator to turn pink/red, identifying the colony as a lactose fermenter.

• b) Alkaline reaction: Would not cause a color change to pink (neutral red turns yellow in alkaline conditions).

• c) Gas production: May occur with fermentation but is not directly responsible for the pink color (it is detected by cracks or bubbles in the agar).

• d) H₂S release: Produces a black precipitate (e.g., in Salmonella or Proteus) but does not affect the pink color of lactose fermentation.

• Swarming growth is a unique phenomenon where bacteria move across the surface of solid media, forming thin, spreading waves or a film. This is highly characteristic of Proteus species, especially Proteus mirabilis and Proteus vulgaris.

• a) Escherichia coli: Forms discrete, smooth colonies and does not swarm.

• c) Salmonella enterica: May be motile but does not exhibit the concentric swarming pattern seen in Proteus.

• d) Klebsiella oxytoca: Is non-motile and forms mucoid colonies without swarming.

• Kingella kingae is a fastidious Gram-negative coccobacillus that often appears as diplococci or short chains, resembling Neisseria.

• Its key biochemical characteristics are that it is oxidase-positive and, crucially, catalase-negative. This helps distinguish it from many other similar organisms.

• a) This describes Haemophilus influenzae (requires X and V factors).

• c) This describes bacteria like Listeria or Corynebacterium, but Kingella is Gram-negative.

• d) Non-fermentative bacilli include organisms like Pseudomonas aeruginosa, but Kingella kingae is a fermenter.

• Nucleic Acid Amplification Tests (NAATs) are the gold standard for diagnosing Neisseria gonorrhoeae in adults, especially in females. They are highly sensitive and specific, can be performed on non-invasive samples like urine or vaginal swabs, and are more effective than culture for detecting asymptomatic infections.

• a) Gram stain of cervical secretions has low sensitivity (around 50-70%) in females because the cervix has a complex normal flora where it’s difficult to distinguish the Gram-negative diplococci. It is highly specific but insensitive in this context (it is excellent for symptomatic males).

• b) Latex agglutination is not a standard or recommended test for gonorrhea diagnosis.

• d) The catalase test is a biochemical test used on bacterial colonies grown in culture, not a direct diagnostic method for a patient sample.

• Ophthalmia neonatorum is conjunctivitis that occurs in the first month of life. The most classic and severe bacterial causes are acquired during passage through the birth canal.

• Neisseria gonorrhoeae is a common and classic cause of a hyperacute, purulent form of ophthalmia neonatorum that can lead to corneal perforation and blindness if not treated promptly.

• While other organisms like Chlamydia trachomatis (not listed) are also very common causes, N. gonorrhoeae remains a primary concern.

• a) Streptococcus agalactiae (Group B Strep) can cause neonatal sepsis but is not a typical cause of conjunctivitis.

• c) Haemophilus influenzae is a cause of conjunctivitis in children and adults, but not the classic cause in newborns.

• d) Staphylococcus aureus is associated with various infections but is not the most common cause of this specific neonatal condition.

The definitive identification of Neisseria gonorrhoeae is based on its pattern of carbohydrate utilization. N. gonorrhoeae oxidizes glucose but not maltose, sucrose, or lactose. This test (often done in cystine trypticase agar or commercial rapid tests) is the standard for distinguishing it from other non-pathogenic Neisseria species (e.g., N. meningitidis, which oxidizes both glucose and maltose).

• a) Detects antibiotic resistance, not species identity.

• c) Colony morphology can be suggestive but is not definitive, as other species can appear similar.

• d) A positive catalase test is a characteristic of the genus Neisseria but does not differentiate between species within the genus.

• The oxidase test detects the presence of cytochrome c oxidase, an enzyme in the electron transport chain of certain bacteria.

• For quality control, a known oxidase-positive organism and a known oxidase-negative organism must be used to verify the test reagents are working properly.

  • Pseudomonas aeruginosa is a reliable oxidase-positive control.

  • Escherichia coli is a reliable oxidase-negative control.

• b) Staphylococcus aureus and Staphylococcus epidermidis are both oxidase-negative; this pair would not validate a positive result.

• c) Streptococcus pyogenes and Enterococcus faecalis are both oxidase-negative.

• d) Klebsiella pneumoniae and Proteus mirabilis are both oxidase-negative.

• Pseudomonas aeruginosa is a non-fermentative, Gram-negative bacillus that is a common cause of ventilator-associated pneumonia (VAP) and other healthcare-associated infections. It thrives in moist environments (e.g., ventilators, respiratory equipment) and is notorious for its resistance to many antibiotics.

• b) Klebsiella pneumoniae: This is a fermentative bacterium (not non-fermentative) and can cause pneumonia, but it is less specifically linked to VAP compared to P. aeruginosa.

• c) Escherichia coli: Also fermentative and a cause of hospital-acquired infections, but not primarily associated with VAP.

• d) Proteus vulgaris: Fermentative and more commonly associated with urinary tract infections than respiratory infections.

Pseudomonas aeruginosa produces virulence factors like exotoxin A, elastase, and a polysaccharide capsule, contributing to severe pneumonia in immunocompromised or critically ill patients. Its non-fermentative metabolism (it oxidizes rather than ferments carbohydrates) and oxidase positivity help distinguish it from Enterobacteriaceae.

• Thayer-Martin agar and Martin-Lewis agar are selective media designed to isolate pathogenic Neisseria species (N. gonorrhoeae and N. meningitidis) from specimens containing normal flora (e.g., genital, respiratory).

• Their selectivity comes from a combination of antibiotics:

  • Vancomycin: Inhibits Gram-positive bacteria.

  • Colistin: Inhibits Gram-negative bacteria (except resistant species like Neisseria).

  • Nystatin (or Anisomycin in Martin-Lewis): Inhibits fungi.

  • Trimethoprim (sometimes added): Inhibits Proteus species and other swarming organisms.

• a) Colistin and nalidixic acid are used in CNA agar (selective for Gram-positive bacteria).

• c) Cycloheximide is used in fungal cultures to inhibit saprophytic molds.

• d) Sodium chloride is used in media like mannitol salt agar to select for staphylococci.

• Regan-Lowe agar is a selective medium specifically designed for the isolation of Bordetella pertussis. It contains charcoal, horse blood, and antibiotics to suppress the growth of other nasopharyngeal flora.

• a) Thayer-Martin agar is a selective medium used for Neisseria gonorrhoeae and N. meningitidis.

• c) MacConkey agar is used to isolate and differentiate Gram-negative enteric bacteria.

• d) Buffered Charcoal Yeast Extract (BCYE) agar is used for the culture of Legionella species.

• Vibrio cholerae cannot grow in media containing 6% NaCl, while Aeromonas species can tolerate and grow in this high salt concentration. This is a key test to differentiate them.

• a) Oxidase: Both V. cholerae and Aeromonas are oxidase-positive, so this test does not differentiate them.

• c) Urease: Both are typically urease-negative (V. cholerae is urease-negative; most Aeromonas strains are also urease-negative, though some may be variable).

• d) Citrate: Both can be citrate-positive, so it is not discriminatory.

Salt tolerance (specifically the inability of V. cholerae to grow in 6% NaCl) is a standard criterion in microbiological protocols to distinguish it from Aeromonas, which is salt-tolerant. Additional tests like sensitivity to the compound O/129 (vibriostatic agent) can also be used.

• Coagulase is a virulence factor primarily associated with Staphylococcus aureus. It causes plasma to clot by converting fibrinogen to fibrin, helping the bacteria evade the immune system. Neisseria meningitidis does not produce coagulase.

• The other options are all well-established virulence factors for N. meningitidis:

  • a) Polysaccharide capsule: Essential for evading phagocytosis and complement-mediated killing. It is the basis for serogrouping (e.g., A, B, C, W, Y) and is a key target for vaccines.

  • b) Endotoxin (LOS): The lipooligosaccharide (LOS) in the outer membrane triggers a massive inflammatory response, contributing to septic shock and vascular collapse in meningococcemia.

  • c) IgA protease: Degrades secretory IgA antibodies on mucosal surfaces, allowing the bacterium to colonize and invade the nasopharynx.

• Pseudomonas aeruginosa is oxidase-positive and can grow at 42°C, a combination that helps distinguish it from many other non-fermentative Gram-negative bacilli (e.g., Acinetobacter species, which are oxidase-negative, or some Burkholderia species, which may not grow well at 42°C).

• a) Lactose fermentation: P. aeruginosa is non-fermentative (it oxidizes carbohydrates) and does not ferment lactose. This is a trait of Enterobacteriaceae, not non-fermenters.

• c) Production of indole: P. aeruginosa is indole-negative. Indole production is associated with bacteria like E. coli.

• d) Absence of pigment production: P. aeruginosa often produces pigments (e.g., pyocyanin [blue-green] or pyoverdine [yellow-green]), which are characteristic and aid identification. Lack of pigment is not typical.

• Vibrio cholerae, the causative agent of cholera, is infamous for producing “rice water stools” – a characteristic watery, pale, milky diarrhea that resembles water used to wash rice. This results from the action of cholera toxin, which causes massive secretion of fluid and electrolytes into the intestinal lumen.

• a) Salmonella Typhi causes typhoid fever, characterized by rose spots, constipation, or “pea soup” diarrhea (not rice water stools).

• c) Escherichia coli can cause traveler’s diarrhea or hemorrhagic colitis (e.g., O157:H7), but not rice water stools.

• d) Klebsiella pneumoniae is associated with pneumonia and UTIs, not gastrointestinal symptoms like rice water stools.

The rice water stool is a classic clinical sign of cholera, a severe dehydrating illness often linked to contaminated water sources.

• Moraxella catarrhalis colonies on blood agar are smooth, opaque, gray-white, and after 24–48 hours they become hard and can be moved intact across the agar surface when pushed with a loop → this is the classic “hockey puck” sign.

• b) Beta-hemolytic on sheep blood agar → seen in some streptococci (e.g., Streptococcus pyogenes), not Moraxella.

• c) Produce a green pigment → characteristic of Pseudomonas aeruginosa.

• d) Require cysteine for growth → seen in Francisella tularensis and Legionella, not Moraxella.

• For a symptomatic male with urethritis (e.g., purulent discharge), the urethral swab is the specimen of choice because it samples the site of infection directly.

• This swab can be used for:

  • Gram stain, which is highly sensitive and specific (>95%) in symptomatic males (showing intracellular Gram-negative diplococci).

  • Culture or NAAT (Nucleic Acid Amplification Testing).

• a) First-catch urine is an excellent non-invasive specimen for NAAT testing and is often used for screening, but it is not the best specimen for culture recovery. For culture, a swab is superior.

• c) Clean-catch midstream urine is used to culture the bladder/kidneys (e.g., for UTIs) and would not adequately sample the urethral organisms.

• d) Rectal swab is used for screening at rectal sites but is not the primary specimen for a symptomatic male with urethritis.

• Hektoen enteric (HE) agar is a selective and differential medium used to isolate Salmonella and Shigella from stool samples.

  • Non-lactose fermenters (like Shigella and Salmonella) appear as blue-green to green colonies on HE agar.

  • Shigella species are non-H₂S producers, so they do not form black centers. This distinguishes them from Salmonella, which typically produces H₂S (black centers).

• b) Black-centered, H₂S producing: This describes Salmonella, not Shigella.

• c) Pink, lactose-fermenting: This describes lactose fermenters like E. coli or Klebsiella.

• d) Yellow, sucrose-fermenting: Sucrose fermentation would cause yellow colonies, but Shigella does not ferment sucrose.

A Gram stain is unreliable for diagnosing gonorrhea from a vaginal discharge because the normal vaginal flora contains other Gram-negative and Gram-variable bacteria (e.g., Acinetobacter spp., some Bacteroides) and diplococci (e.g., Moraxella osloensis) that can be easily mistaken for N. gonorrhoeae, leading to false-positive results. Culture or nucleic acid amplification tests (NAATs) are required for accurate diagnosis in females.

• a) Male urethral exudate: Gram stain is highly reliable and specific here due to the classic finding of intracellular Gram-negative diplococci in neutrophils.

• b) Cerebrospinal fluid (CSF): Gram stain is a critical and reliable test for diagnosing bacterial meningitis (e.g., from N. meningitidis), as CSF is normally sterile and lacks confounding flora.

• d) Synovial fluid: This is a normally sterile fluid, so any organisms seen on Gram stain are significant and not confused with normal flora.

In a symptomatic male with urethritis (e.g., presenting with discharge), the observation of intracellular Gram-negative diplococci in a urethral exudate is considered highly specific and diagnostic for gonorrhea. The combination of symptoms and the classic microscopic finding is sufficient to begin treatment.

• a) & d) In asymptomatic females or post-menopausal females, the cervical flora can include other non-pathogenic Neisseria species that may look similar. Therefore, this finding is not considered definitive, and culture or NAAT is required for diagnosis.

• c) For a newborn infant (e.g., with ophthalmia neonatorum), the standard of care is confirmation by culture or NAAT due to the severe consequences and the need for a definitive diagnosis.

• Vibrio vulnificus is a Gram-negative bacterium naturally found in warm seawater and marine environments. Infections are most commonly linked to:

  • Consumption of raw or undercooked seafood (especially oysters) harvested from contaminated waters.

  • Exposure of open wounds to seawater.

• a) Undercooked poultry: Associated with Salmonella or Campylobacter, not V. vulnificus.

• c) Contaminated soil: Linked to pathogens like Clostridium tetani or Burkholderia pseudomallei.

• d) Respiratory droplets: Associated with airborne pathogens like Mycobacterium tuberculosis or Streptococcus pneumoniae.

V. vulnificus causes severe wound infections, septicemia, and necrotizing fasciitis, particularly in immunocompromised individuals or those with liver disease. Rapid treatment with antibiotics (e.g., doxycycline and ceftazidime) and wound debridement is crucial. Public health advisories often warn against consuming raw oysters from high-risk areas.

The HACEK group (Haemophilus spp., Aggregatibacter spp., Cardiobacterium spp., Eikenella corrodens, Kingella spp.) consists of fastidious Gram-negative bacteria that are part of the normal oropharyngeal flora. A key common characteristic is their requirement for an increased carbon dioxide concentration (5-10% CO₂) for optimal growth, which often leads to delayed growth (several days) on standard culture media.

• a) Incubation at 42°C is not a standard requirement for HACEK organisms.

• b) They are capnophiles (require CO₂), not strict anaerobes.

• d) Only some members (e.g., certain Haemophilus species) require X and V factors; this is not a universal trait of the entire HACEK group

• The string test is a rapid, bedside test used to identify Vibrio cholerae. When a loop is used to mix a colony of V. cholerae with a drop of sodium deoxycholate solution, the bacteria lyse and release DNA, forming a viscous mucoid string that stretches >5 mm if pulled upward. This is due to the bacterium’s thick capsular polysaccharide and biofilm matrix.

• a) Shigella sonnei: Does not produce a viscous string; it forms compact colonies.

• c) Yersinia pestis: Exhibits bipolar “safety-pin” staining but does not produce a positive string test.

• d) Salmonella Paratyphi: May be motile but does not form a mucoid string in this test.

The string test is highly specific for V. cholerae and is often used in resource-limited settings during cholera outbreaks for presumptive diagnosis. Confirmatory tests (e.g., serotyping, PCR) are still recommended.

• Neisseria gonorrhoeae develops plasmid-mediated resistance to penicillin primarily through production of a β-lactamase (penicillinase), which hydrolyzes the β-lactam ring.

• Alteration of PBPs (chromosomal-mediated resistance) can also occur but is less common compared to β-lactamase production.

• Efflux pumps contribute to resistance against tetracyclines and macrolides, not the main mechanism for penicillin.

• Lack of a cell wall → describes Mycoplasma, not Neisseria.

So the most common mechanism = β-lactamase production.

Do you want me to also explain the difference between plasmid-mediated vs chromosomal-mediated resistance in N. gonorrhoeae?

• Escherichia coli O157:H7 is a Shiga toxin-producing E. coli (STEC) strain that is a common cause of bloody diarrhea (hemorrhagic colitis) and hemolytic uremic syndrome (HUS) in children. HUS is characterized by hemolytic anemia, thrombocytopenia, and acute renal failure.

• b) Shigella sonnei: Causes bacillary dysentery (bloody diarrhea) but is less commonly associated with HUS compared to STEC.

• c) Salmonella Typhi: Causes typhoid fever, which involves systemic symptoms like fever and abdominal pain but not typically bloody diarrhea or HUS.

• d) Klebsiella oxytoca: Not a typical cause of bloody diarrhea or HUS; it is more associated with healthcare-related infections

• The string test is a rapid, simple test used to identify Vibrio cholerae. When a loop is used to touch a colony of V. cholerae and then pulled away, it forms a viscous string (like mucus) that stretches several millimeters. This is due to the bacterium’s production of a thick capsular polysaccharide or biofilm matrix.

• b) Shigella dysenteriae does not produce viscous colonies or a positive string test.

• c) Escherichia coli may form mucoid colonies in some cases (e.g., E. coli K1), but it is not typically identified by the string test.

• d) Proteus vulgaris exhibits swarming motility, not viscous colonies.

• Klebsiella pneumoniae is renowned for producing very large, mucoid, glistening colonies due to its abundant polysaccharide capsule. This capsule is a major virulence factor that protects the bacterium from phagocytosis and desiccation.

• a) Escherichia coli: Some strains (e.g., those expressing the K1 antigen) can be slightly mucoid, but it is not as characteristically pronounced as in K. pneumoniae.

• c) Proteus mirabilis: Does not produce a capsule; instead, it exhibits swarming motility on agar surfaces.

• d) Shigella flexneri: Does not produce a capsule and forms non-mucoid colonies.

The mucoid appearance of K. pneumoniae is a key visual clue in microbiology labs, especially when isolated from respiratory specimens (e.g., in pneumonia) or urinary tract infections. The capsule also contributes to its resistance to host immune defenses.

• Vibrio parahaemolyticus is halophilic (salt-loving) and can grow in media containing 6–8% NaCl.

• Vibrio cholerae, in contrast, cannot grow in high salt concentrations (e.g., 6% NaCl).

• This difference in salt tolerance is a key test to differentiate the two species.

Why the other options are incorrect:

• a) Indole: Both V. cholerae and V. parahaemolyticus are typically indole-positive, so this test does not differentiate them.

• b) Oxidase: Both are oxidase-positive.

• d) Citrate: Both can utilize citrate (citrate-positive), though variations exist.

Vibrio parahaemolyticus is a common cause of seafood-associated gastroenteritis, while V. cholerae causes cholera. Salt tolerance testing is a standard method in clinical laboratories to distinguish between these pathogens. Additional tests (e.g., sucrose fermentation: V. cholerae ferments sucrose, while V. parahaemolyticus does not) can also be used.

• The key characteristics described are:

  • Non-lactose fermenting (appears as colorless colonies on MacConkey agar).

  • No H₂S production (no black centers on media like Hektoen enteric agar).

  • Non-motile (lacks flagella).

• Shigella species (including S. dysenteriae) fit this profile perfectly:

  • They do not ferment lactose.

  • They do not produce H₂S.

  • They are non-motile (a major distinction from Salmonella).

• b) Salmonella Typhi: Produces H₂S (weakly) and is motile.

• c) Escherichia coli: Most strains ferment lactose (pink colonies on MacConkey agar) and are motile.

• d) Klebsiella pneumoniae: Ferments lactose (pink colonies) and is non-motile, but it produces H₂S only rarely and is not typically a primary stool pathogen like Shigella.

The combination of non-lactose fermentation, lack of H₂S, and non-motility is classic for Shigella, a common cause of bacillary dysentery. Shigella is also oxidase-negative and urease-negative, further supporting its identification.

• Swarming growth is a unique phenomenon where bacteria move across the surface of solid media, forming a thin, spreading film that often appears as concentric waves. This is highly characteristic of Proteus species (especially P. mirabilis and P. vulgaris).

• a) Escherichia coli does not swarm; it forms discrete, smooth colonies.

• c) Serratia marcescens may exhibit motility but does not swarm like Proteus; it is known for producing red pigment (prodigiosin) at room temperature.

• d) Klebsiella pneumoniae is non-motile and forms large, mucoid colonies due to its capsule.

• Serratia marcescens is a Gram-negative bacillus known for producing a distinctive red pigment, called prodigiosin, when cultured at room temperature (25-30°C). This pigment is often less intense or absent at higher temperatures (e.g., 37°C).

• b) Klebsiella pneumoniae produces large, mucoid colonies but no red pigment.

• c) Proteus vulgaris exhibits swarming motility and a characteristic “burnt chocolate” odor but does not produce pigment.

• d) Enterobacter aerogenes is non-pigmented and may appear similar to other Enterobacteriaceae.

• Ceftriaxone (a third-generation cephalosporin) is the empiric drug of choice for suspected Neisseria meningitidis infections (meningitis and meningococcemia).

• The primary reason is the global emergence of penicillin-resistant strains of N. meningitidis. While many strains remain susceptible, resistance due to altered penicillin-binding proteins (PBPs) is sufficiently common that penicillin G (answer b) can no longer be relied upon for empiric therapy.

• a) Vancomycin is used for empiric coverage of Gram-positive meningitis (like Streptococcus pneumoniae) but is not active against the Gram-negative N. meningitidis.

• d) Azithromycin is not a first-line treatment for bacterial meningitis; its coverage is not broad or potent enough for this life-threatening infection.

• Hektoen enteric (HE) agar is a selective and differential medium used to isolate enteric pathogens, particularly Salmonella and Shigella.

  • Non-lactose fermenters appear as blue-green colonies on HE agar.

  • H₂S production is visualized as black centers in the colonies due to the reaction of H₂S with ferric ammonium citrate in the medium.

• Salmonella species (e.g., S. enterica) are typically non-lactose fermenters and produce H₂S, leading to blue-green colonies with black centers on HE agar.

• a) Shigella flexneri is a non-lactose fermenter but does not produce H₂S (colonies appear blue-green without black centers).

• c) Escherichia coli ferments lactose and appears as orange to salmon-colored colonies on HE agar.

• d) Klebsiella oxytoca is a lactose fermenter and also produces orange colonies.

• Serratia marcescens is a Gram-negative rod that ferments lactose, but it does so slowly and delayed (typically taking 2–3 days to produce pink colonies on MacConkey agar). This contrasts with rapid fermenters like E. coli, which ferment lactose within 24 hours.

• b) Escherichia coli: Ferments lactose rapidly (within 18–24 hours), producing bright pink colonies.

• c) Salmonella Typhi: Does not ferment lactose; it appears as colorless colonies on MacConkey agar.

• d) Shigella sonnei: Generally does not ferment lactose (except some strains may show slow fermentation after prolonged incubation, but it is not characteristic).

• The indole test is a key biochemical test used to distinguish between Proteus vulgaris and Proteus mirabilis:

  • Proteus vulgaris is indole-positive (it produces indole from tryptophan).

  • Proteus mirabilis is indole-negative.

• Both species share many other characteristics (e.g., they are urease-positive, oxidase-negative, and produce H₂S), but the indole test reliably differentiates them.

• b) Citrate: Both P. vulgaris and P. mirabilis can utilize citrate (citrate-positive), so this test does not differentiate them.

• c) Oxidase: Both are oxidase-negative.

• d) Lactose fermentation: Neither ferments lactose.

Thus, the indole test is the most straightforward and commonly used method to tell these two Proteus species apart in the microbiology lab.

• Pseudomonas aeruginosa is:

  • Oxidase-positive (a key test for identification).

  • Non-fermentative (it uses oxidative metabolism instead of fermentation to process carbohydrates).

• The other options are all oxidase-negative and fermentative:

  • a) Escherichia coli: Ferments sugars (e.g., lactose), oxidase-negative.

  • b) Salmonella enterica: Ferments sugars (e.g., glucose), oxidase-negative.

  • d) Shigella sonnei: Ferments sugars (though slowly), oxidase-negative.

Pseudomonas aeruginosa is a common opportunistic pathogen that thrives in moist environments and is known for its resistance to many antibiotics. The oxidase test is critical to differentiate it from Enterobacteriaceae family members (like E. coli, Salmonella, and Shigella), which are oxidase-negative.

• Salmonella Typhi (now classified as Salmonella enterica serotype Typhi) possesses a unique Vi capsule (virulence antigen), which is absent in most other Salmonella serotypes (e.g., S. Enteritidis, S. Typhimurium). Detection of the Vi antigen is a key test for identifying S. Typhi.

• a) Urease: S. Typhi is urease-negative, but many other Salmonella species are also urease-negative (e.g., S. Paratyphi A), so this test does not differentiate them.

• b) Citrate utilization: S. Typhi is citrate-negative, while some other Salmonella species (e.g., S. Enteritidis) are citrate-positive. However, citrate negativity is not unique to S. Typhi (e.g., S. Paratyphi A is also citrate-negative).

• d) Indole: S. Typhi is indole-negative, but most Salmonella species are indole-negative, so this test is not discriminatory.

The Vi antigen is a critical marker for S. Typhi and is used in serological tests (e.g., agglutination with anti-Vi antisera) and for epidemiological tracking. It also contributes to the bacterium’s virulence by inhibiting phagocytosis. Other tests, such as biochemical patterns (e.g., S. Typhi produces minimal H₂S and no gas from glucose), also aid identification, but Vi antigen detection is highly specific

• Klebsiella pneumoniae → non-motile.

• Enterobacter cloacae → motile (peritrichous flagella).

Other options:

• Indole → helps differentiate Klebsiella pneumoniae (indole–) from Klebsiella oxytoca (indole+), but not from Enterobacter.

• Citrate → both Klebsiella and Enterobacter are citrate-positive.

• Lactose fermentation → both ferment lactose (pink colonies on MacConkey).

The Gram stain finding of Gram-negative diplococci in a joint aspirate is highly suggestive of Neisseria gonorrhoeae, a common cause of septic arthritis.

The chemistry and cell count of an infected (septic) joint fluid typically shows:

• Decreased glucose: Bacteria and white blood cells (WBCs) consume glucose during their metabolic activity.

• Increased protein: Inflammation increases vascular permeability, allowing proteins to leak into the joint space.

• Increased white blood cells (WBCs): A massive influx of neutrophils is the body’s primary response to a bacterial infection. Counts are often dramatically elevated (>50,000 cells/µL).

• Serratia marcescens is a Gram-negative bacillus known for producing a distinctive red pigment, called prodigiosin, when cultured at room temperature (25–30°C). This pigment often fades or is absent at higher temperatures (e.g., 37°C).

• b) Klebsiella pneumoniae: Produces large, mucoid colonies due to its capsule but no red pigment.

• c) Proteus vulgaris: Exhibits swarming motility and a “burnt chocolate” odor but does not produce pigment.

• d) Enterobacter cloacae: Typically non-pigmented and may appear similar to other Enterobacteriaceae.

The production of prodigiosin is a key identifying feature of S. marcescens and has been historically noted in environmental contamination (e.g., on bread, in showers) and in clinical settings as an opportunistic pathogen

• Proteus mirabilis is a non-lactose fermenter (appears as colorless colonies on MacConkey agar) and is urease-positive (rapidly hydrolyzes urea, producing ammonia and a pH change). This combination is highly characteristic of Proteus species.

• a) Klebsiella pneumoniae is a lactose fermenter (pink colonies on MacConkey agar) and urease-positive, but it does not fit the “non-lactose fermenting” criterion.

• c) Escherichia coli is typically a lactose fermenter (pink colonies) and is urease-negative.

• d) Shigella flexneri is a non-lactose fermenter but is urease-negative.

• Proteus mirabilis is a Gram-negative bacillus that is strongly urease-positive. It hydrolyzes urea into ammonia and carbon dioxide, which alkalinizes the urine.

• This alkalinization promotes the formation of struvite stones (magnesium ammonium phosphate crystals) in the urinary tract. These stones are often associated with chronic UTIs caused by Proteus.

• b) Escherichia coli: A common cause of UTIs but is urease-negative and does not typically cause struvite stones.

• c) Salmonella Typhi: Causes typhoid fever, is urease-negative, and is not linked to UTIs or stones.

• d) Shigella dysenteriae: Causes bacillary dysentery, is urease-negative, and is not a uropathogen.

Proteus mirabilis is also known for its swarming motility on agar and is frequently isolated in complicated UTIs, especially in catheterized patients. The urease activity is a key virulence factor driving stone formation and persistent infections.

• Escherichia coli is typically motile (possesses peritrichous flagella).

• Klebsiella pneumoniae is non-motile (lacks flagella).

• This difference in motility is a key test to differentiate the two.

Why the other options are incorrect:

• b) Urease: Both can be urease-positive or variable (E. coli is usually urease-negative, but some strains are positive; K. pneumoniae is often urease-positive). It is not reliable for differentiation.

• c) Oxidase: Both are oxidase-negative.

• d) Citrate: E. coli is citrate-negative, while K. pneumoniae is citrate-positive. This is a useful test, but motility is more consistent and commonly used for initial differentiation.

• Klebsiella pneumoniae is a Gram-negative bacillus known for causing a severe form of pneumonia, particularly in alcoholics, diabetics, or immunocompromised individuals.

• A classic clinical feature is “red currant jelly sputum” – thick, tenacious, and blood-tinged sputum that resembles red currant jelly. This results from the bacterium’s ability to cause extensive tissue destruction, hemorrhage, and necrosis in the lungs.

• a) Escherichia coli: Can cause pneumonia but is not associated with this characteristic sputum.

• c) Pseudomonas aeruginosa: Causes pneumonia in cystic fibrosis or hospitalized patients, often producing greenish sputum due to pyocyanin pigment.

• d) Serratia marcescens: Can cause pneumonia but is known for red pigment (prodigiosin) in cultures, not typically in sputum.

• Oxidase-positive, Gram-negative diplococcus: This describes the genera Neisseria and Moraxella.

• Grows on blood/chocolate agar, but not on modified Thayer-Martin (MTM) agar: MTM contains antibiotics (vancomycin, colistin, nystatin, trimethoprim) that inhibit many non-pathogenic Neisseria species and other flora. Moraxella catarrhalis is resistant to these antibiotics and will grow on MTM. Therefore, the fact that it does NOT grow on MTM is a major red flag that it is not a typical Neisseria and points strongly to M. catarrhalis. This is the most critical clue.

• No carbohydrate fermentation: M. catarrhalis is asaccharolytic (does not ferment sugars), which matches the result. In contrast, N. meningitidis ferments glucose and maltose, and N. lactamica ferments glucose, maltose, and lactose.

• a) N. meningitidis ferments glucose and maltose and would grow on MTM agar.

• b) N. lactamica ferments glucose, maltose, and lactose and would grow on MTM agar.

• d) Acinetobacter lwoffii is oxidase-negative, which rules it out immediately.

• Neisseria lactamica is unique among Neisseria species because it ferments lactose (hence the name “lactamica”). This is detected by a positive ONPG test (ortho-Nitrophenyl-β-galactoside), which indicates the presence of β-galactosidase, the enzyme needed to break down lactose.

• Neisseria meningitidis does not ferment lactose and gives a negative ONPG test.

• a) Positive oxidase reaction: Both N. meningitidis and N. lactamica are oxidase-positive, so this test cannot differentiate them.

• b) Ability to grow on selective media: Both can grow on selective media like Thayer-Martin agar.

• d) Gram stain morphology: Both appear as Gram-negative diplococci and cannot be reliably distinguished by microscopy alone.

• Yersinia pestis, the causative agent of plague, exhibits a distinctive “safety-pin” or bipolar staining appearance on Gram stain (or with Wright-Giemsa stain). This is due to the concentration of dye at the poles of the bacterium, creating a swollen, safety pin-like shape.

• b) Shigella dysenteriae: Appears as typical Gram-negative rods without bipolar staining.

• c) Escherichia coli: Also appears as uniform Gram-negative rods.

• d) Vibrio cholerae: May show curved (comma-shaped) rods but not safety-pin bipolar staining.

• H₂S (hydrogen sulfide) production is a key test to differentiate Salmonella from Shigella:

  • Salmonella species typically produce H₂S (black precipitate in media like triple sugar iron agar or Hektoen enteric agar).

  • Shigella species do not produce H₂S.

• a) Indole: While some Shigella species (e.g., S. sonnei) may be indole-positive and Salmonella is usually indole-negative, this is not consistent across all species and is less reliable.

• c) Oxidase: Both Salmonella and Shigella are oxidase-negative, so this test does not differentiate them.

• d) Urease: Both genera are generally urease-negative (though some Salmonella species may rarely be positive), so it is not useful for differentiation.

H₂S production is a fundamental criterion in initial biochemical screening for enteric pathogens. Salmonella’s ability to generate H₂S (due to cysteine desulfurase activity) contrasts sharply with Shigella’s inability, making it a primary diagnostic tool.

• The butyrate esterase test is a rapid, specific test used to identify Moraxella catarrhalis. A positive result (typically a blue color change within 30 minutes) is a key characteristic for confirming this organism.

• a) Neisseria meningitidis is identified by its carbohydrate fermentation pattern (ferments glucose and maltose).

• b) Haemophilus influenzae is identified by its requirement for X and V factors.

• d) Streptococcus pneumoniae is identified by its sensitivity to optochin and bile solubility.

• Vibrio vulnificus is a Gram-negative rod that is naturally found in warm seawater and marine environments. It is notorious for causing severe wound infections (e.g., cellulitis, necrotizing fasciitis) after exposure to seawater, especially in individuals with open wounds or compromised immune systems. It can also lead to life-threatening sepsis.

• a) Escherichia coli: Commonly associated with UTIs or gastrointestinal infections, not specifically seawater exposure.

• c) Salmonella enterica: Linked to foodborne illnesses (e.g., contaminated poultry or eggs), not seawater wounds.

• d) Shigella sonnei: Causes bacillary dysentery via fecal-oral transmission, not seawater exposure.

Vibrio vulnificus infections are particularly dangerous for people with liver disease or iron overload conditions (e.g., hemochromatosis). Rapid treatment with antibiotics (e.g., doxycycline and ceftazidime) and wound debridement is critical.