Bacteriology — MCQs
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What is the most dominant colonic bacteria?
Which of the following organisms does NOT show bipolar staining?
Toxic shock syndrome is caused by which bacterium?
How can Mycobacterium TB be differentiated from other mycobacteria?
An oxidase-positive, gram-negative rod which produces a bluish-green pigment has been grown in culture from a swab obtained from an infected burn wound. This organism is susceptible to gentamicin, ticarcillin and tobramicin, but resistant to all the other antibiotics. Which of the following organisms is it likely to be?
An infant presents with hemolytic uremic syndrome and thrombocytopenia. Stool culture reveals non-lactose fermenting colonies on Sorbitol MacConkey agar that also ferment glucose with gas. Which of the following bacteria is most likely isolated from the stool specimen?
What is the drug of choice for meningococcal chemoprophylaxis?
What is the most likely candidate responsible for the production of overwhelming septicemic shock complicating bacteremia with Neisseria meningitidis?
A 12-year-old boy presents with sudden onset of fever, headache, and stiff neck. Two days prior, he swam in a lake potentially contaminated with dog excreta. Leptospirosis is suspected. Which of the following laboratory tests is most appropriate for diagnosing leptospiral infection?
Which organism is strongly urease positive?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1091: What is the most dominant colonic bacteria?
- A. Escherichia coli
- B. Bacteroides (Correct Answer)
- C. Clostridium
- D. Veillonella
Explanation: ### Explanation The human colon contains a massive and complex microbial ecosystem, with concentrations reaching up to $10^{11}$ to $10^{12}$ organisms per gram of feces. **Why Bacteroides is the correct answer:** While many students mistakenly believe *E. coli* is the most numerous, **anaerobes** actually outnumber facultative anaerobes by a ratio of **1000:1**. Among these, the genus **Bacteroides** (specifically *Bacteroides fragilis* group) is the most dominant and numerically significant component of the normal colonic flora. They are Gram-negative, non-spore-forming obligate anaerobes that play a crucial role in fermenting dietary carbohydrates and maintaining gut homeostasis. **Analysis of Incorrect Options:** * **A. Escherichia coli:** Although it is the most common **facultative anaerobe** and the most frequent cause of urinary tract infections, it represents less than 1% of the total intestinal microbial population. * **C. Clostridium:** While present in the colon (e.g., *C. perfringens*), they are found in significantly lower concentrations compared to *Bacteroides* and *Bifidobacterium*. * **D. Veillonella:** These are Gram-negative anaerobic cocci. While they are part of the normal flora of the mouth and GI tract, they are not the dominant species in the colon. **High-Yield Clinical Pearls for NEET-PG:** * **Bacteroides fragilis** is the most common anaerobe isolated from intra-abdominal abscesses and peritonitis following bowel perforation. * **Bifidobacterium** is another major anaerobic genus, particularly dominant in the stools of breastfed infants. * The most common organism in the **vagina** is *Lactobacillus*. * The most common organism on the **skin** is *Staphylococcus epidermidis*.
Question 1092: Which of the following organisms does NOT show bipolar staining?
- A. Calymmatobacter granulomatis
- B. Yersinia pestis
- C. Pseudomonas mallei
- D. Haemophilus influenzae (Correct Answer)
Explanation: **Explanation:** Bipolar staining (also known as the "safety-pin appearance") is a characteristic feature of certain Gram-negative bacteria where the ends of the bacilli stain more intensely than the center. This occurs due to the accumulation of storage granules or specific capsular material at the poles. **Why Haemophilus influenzae is the correct answer:** *Haemophilus influenzae* is a small, pleomorphic, Gram-negative coccobacillus. While it is Gram-negative, it **does not** exhibit bipolar staining. It typically shows uniform staining throughout the cell. **Analysis of Incorrect Options:** * **Calymmatobacter granulomatis (Klebsiella granulomatis):** The causative agent of Granuloma Inguinale (Donovanosis). It shows prominent bipolar staining within macrophages, appearing as **Donovan bodies**. * **Yersinia pestis:** The classic example of bipolar staining. When stained with Wayson, Giemsa, or methylene blue, it exhibits a distinct **"safety-pin" appearance**, which is a high-yield diagnostic feature for Plague. * **Pseudomonas mallei (Burkholderia mallei):** The causative agent of Glanders. It is known to show irregular or bipolar staining. **NEET-PG High-Yield Pearls:** To remember organisms showing bipolar staining, use the mnemonic **"Way to Go, My Past Yersinia"**: 1. **W**hitmore’s bacillus (*Burkholderia pseudomallei*) 2. **G**ranuloma inguinale (*Klebsiella granulomatis*) 3. **M**allei (*Burkholderia mallei*) 4. **Past**eurella multocida 5. **Yersinia** pestis (and *Y. enterocolitica*) 6. **Vibrio** parahemolyticus (occasionally) *Note:* Bipolar staining is best demonstrated using **Wayson stain**, Methylene blue, or Giemsa stain rather than a standard Gram stain.
Question 1093: Toxic shock syndrome is caused by which bacterium?
- A. Streptococcus pyogenes
- B. Staphylococcus aureus (Correct Answer)
- C. Streptococcus albicans
- D. Enterococcus durans
Explanation: **Explanation:** **Toxic Shock Syndrome (TSS)** is a life-threatening multisystem illness characterized by high fever, hypotension, a diffuse erythematous rash (resembling a sunburn), and multi-organ failure. **Why Staphylococcus aureus is correct:** The classic form of TSS is caused by **Staphylococcus aureus** strains that produce the **TSST-1 (Toxic Shock Syndrome Toxin-1)**. This toxin acts as a **superantigen**, which bypasses normal antigen processing. It binds directly to the MHC class II molecules on antigen-presenting cells and the Vβ region of T-cell receptors. This leads to the non-specific activation of up to 20% of the body's T-cells, causing a massive "cytokine storm" (release of IL-1, IL-2, TNF-α, and IFN-γ), which results in capillary leak and shock. Historically, it is strongly associated with the use of highly absorbent tampons. **Analysis of Incorrect Options:** * **Streptococcus pyogenes:** While it causes a similar condition called *Streptococcal Toxic Shock-like Syndrome (TSLS)*, the question specifically refers to the classic "Toxic Shock Syndrome." TSLS is usually associated with necrotizing fasciitis or bacteremia. * **Streptococcus albicans:** This is a distracter; *Candida albicans* is a fungus, not a Streptococcus species. * **Enterococcus durans:** This is a commensal organism of the intestinal tract and is not associated with superantigen-mediated toxic shock. **High-Yield NEET-PG Pearls:** * **TSST-1** is a chromosomal-mediated toxin in some strains but often carried by a pathogenicity island (SaPI). * **Clinical Hallmark:** Desquamation of the skin (especially palms and soles) occurs 1–2 weeks after the onset of illness. * **Criteria:** Diagnosis requires involvement of ≥3 organ systems (GI, Muscular, Renal, Hepatic, Hematologic, or CNS). * **Management:** Requires aggressive fluid resuscitation and clindamycin (which inhibits toxin production).
Question 1094: How can Mycobacterium TB be differentiated from other mycobacteria?
- A. Production of niacin (Correct Answer)
- B. Arylsulphatase test
- C. Coagulase test
- D. Bile solubility test
Explanation: **Explanation:** The differentiation of *Mycobacterium tuberculosis* (MTB) from Non-Tuberculous Mycobacteria (NTM) is a high-yield topic in bacteriology. **1. Why Niacin Production is Correct:** All mycobacteria produce niacin (nicotinic acid) as a metabolic byproduct. However, most species possess an enzyme that converts niacin into niacin ribonucleotide. **MTB lacks this enzyme**, leading to an accumulation of free niacin in the culture medium. When tested with cyanogen bromide and aniline (Konno’s test), a positive result (canary yellow color) is highly characteristic of MTB. **2. Analysis of Incorrect Options:** * **Arylsulphatase test:** This test is primarily used to identify **Rapid Growers** (Runyon Group IV), specifically *M. fortuitum* and *M. chelonae*, which produce the enzyme within 3 days. MTB is typically negative. * **Coagulase test:** This is a biochemical test used to differentiate *Staphylococcus aureus* (positive) from Coagulase-negative Staphylococci (CoNS). It has no application in mycobacteriology. * **Bile solubility test:** This is the gold standard for identifying ***Streptococcus pneumoniae***, which lyses in the presence of bile salts. **3. NEET-PG Clinical Pearls:** * **Nitrate Reduction Test:** MTB is also positive for nitrate reduction, which helps differentiate it from *M. bovis* (negative). * **Catalase Test:** MTB is **catalase-negative at 68°C** (heat-labile catalase). Most NTMs remain catalase-positive at this temperature. * **Culture Media:** While LJ (Lowenstein-Jensen) medium is the traditional solid medium, **MGIT (Mycobacteria Growth Indicator Tube)** is the current gold standard for rapid liquid culture. * **Morphology:** MTB shows "serpentine cords" in liquid culture due to the presence of **Cord Factor**, a virulence factor that inhibits neutrophil migration.
Question 1095: An oxidase-positive, gram-negative rod which produces a bluish-green pigment has been grown in culture from a swab obtained from an infected burn wound. This organism is susceptible to gentamicin, ticarcillin and tobramicin, but resistant to all the other antibiotics. Which of the following organisms is it likely to be?
- A. Escherichia coli
- B. Klebsiella pneumoniae
- C. Proteus mirabilis
- D. Pseudomonas aeruginosa (Correct Answer)
Explanation: ### Explanation The correct answer is **Pseudomonas aeruginosa**. This question highlights the classic clinical and microbiological presentation of a common opportunistic pathogen. **Why Pseudomonas aeruginosa is correct:** * **Microbiological Characteristics:** *P. aeruginosa* is a Gram-negative, aerobic, non-fermenting rod. It is characteristically **oxidase-positive**, which distinguishes it from the Enterobacteriaceae family. * **Pigment Production:** It produces unique water-soluble pigments, most notably **Pyocyanin** (bluish-green) and Pyoverdin (fluorescent yellow-green). * **Clinical Context:** It is the most common cause of **burn wound infections** (often leading to "blue-pus" or ecthyma gangrenosum in septicemia). * **Antibiotic Profile:** It is notorious for multi-drug resistance but typically remains sensitive to specific "anti-pseudomonal" agents like aminoglycosides (Gentamicin, Tobramycin) and extended-spectrum penicillins (Ticarcillin, Piperacillin). **Why the other options are incorrect:** * **Escherichia coli:** A Gram-negative rod that ferments lactose (pink colonies on MacConkey) and is **oxidase-negative**. It does not produce bluish pigments. * **Klebsiella pneumoniae:** A Gram-negative, non-motile, encapsulated rod. It produces mucoid colonies and is **oxidase-negative**. * **Proteus mirabilis:** Known for "swarming motility" on agar and being urease-positive. It is **oxidase-negative** and does not produce blue-green pigments. **High-Yield NEET-PG Pearls:** * **Smell:** *Pseudomonas* cultures have a characteristic **fruity or grape-like odor** (due to aminoacetophenone). * **Virulence:** Its primary exotoxin, **Exotoxin A**, acts by inhibiting Protein Synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2)—the same mechanism as *Diphtheria* toxin. * **Cystic Fibrosis:** It is the leading cause of chronic pulmonary infections in Cystic Fibrosis patients due to biofilm formation (alginate production).
Question 1096: An infant presents with hemolytic uremic syndrome and thrombocytopenia. Stool culture reveals non-lactose fermenting colonies on Sorbitol MacConkey agar that also ferment glucose with gas. Which of the following bacteria is most likely isolated from the stool specimen?
- A. Shigella
- B. Salmonella
- C. Aeromonas
- D. E. coli O157/H7 (Correct Answer)
Explanation: ### Explanation The clinical presentation of **Hemolytic Uremic Syndrome (HUS)**—characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—following a diarrheal illness is classic for **Enterohemorrhagic *E. coli* (EHEC)**, specifically the **O157:H7** serotype. **Why the correct answer is right:** * **Sorbitol MacConkey (SMAC) Agar:** While most *E. coli* strains ferment sorbitol, the O157:H7 strain is unique because it is a **non-sorbitol fermenter (NSF)**. It appears as colorless/pale colonies on SMAC agar, which is the gold standard for its identification. * **Biochemical Profile:** As a member of the *Enterobacteriaceae* family, *E. coli* is a **glucose fermenter** and typically produces **gas** (CO2 and H2) during fermentation. * **Pathogenesis:** It produces **Shiga-like toxins (Verotoxins)** which damage glomerular endothelial cells, leading to HUS. **Why the incorrect options are wrong:** * **Shigella:** While *S. dysenteriae* type 1 can cause HUS via Shiga toxin, it is **anaerogenic** (does not produce gas from glucose fermentation). * **Salmonella:** These are non-lactose fermenters and produce gas, but they are not typically associated with HUS or the specific NSF profile on SMAC agar used for screening. * **Aeromonas:** These are oxidase-positive (unlike *Enterobacteriaceae*) and are generally associated with watery diarrhea rather than the classic HUS presentation. **NEET-PG High-Yield Pearls:** * **EHEC/VTEC:** Most common cause of HUS in children. * **Transmission:** Often via undercooked ground beef (hamburger) or contaminated milk/water. * **Antibiotics:** Generally avoided in EHEC infections as they may increase toxin release and worsen the risk of HUS. * **Culture:** Look for "Colorless colonies on Sorbitol MacConkey" in the question stem to immediately point toward O157:H7.
Question 1097: What is the drug of choice for meningococcal chemoprophylaxis?
- A. Penicillin
- B. Gentamicin
- C. Rifampicin (Correct Answer)
- D. Chloramphenicol
Explanation: **Explanation:** The primary goal of chemoprophylaxis in meningococcal disease is to eradicate the nasopharyngeal carriage of *Neisseria meningitidis* in close contacts, thereby preventing the spread of the bacteria and the development of secondary cases. **Why Rifampicin is the Correct Answer:** **Rifampicin** is considered the traditional drug of choice for meningococcal chemoprophylaxis because it achieves high concentrations in salivary and nasopharyngeal secretions. It effectively eliminates the carrier state in approximately 90% of cases. The standard adult dose is 600 mg twice daily for 2 days. **Analysis of Incorrect Options:** * **A. Penicillin:** While Penicillin G is a treatment of choice for active meningococcal meningitis, it does not eliminate the nasopharyngeal carrier state because it does not reach sufficient concentrations in respiratory secretions. * **B. Gentamicin:** Aminoglycosides have poor penetration into the cerebrospinal fluid and respiratory secretions; they are not effective against *N. meningitidis*. * **C. Chloramphenicol:** This is used as an alternative for treating active meningitis in patients with severe penicillin allergy, but it is not used for prophylaxis due to its potential for serious bone marrow toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Alternative Agents:** While Rifampicin is the classic answer, **Ciprofloxacin** (500 mg single dose) and **Ceftriaxone** (250 mg IM single dose) are also highly effective and frequently used in clinical practice. * **Ceftriaxone** is the preferred prophylactic agent for **pregnant women**. * **Close Contacts:** Prophylaxis is indicated for household members, daycare contacts, and healthcare workers directly exposed to respiratory secretions (e.g., during intubation). * **Timing:** Prophylaxis should ideally be administered within 24 hours of identifying the index case.
Question 1098: What is the most likely candidate responsible for the production of overwhelming septicemic shock complicating bacteremia with Neisseria meningitidis?
- A. Capsular polysaccharide
- B. Pili
- C. Lipopolysaccharide and outer membrane (Correct Answer)
- D. Low-molecular-weight outer membrane proteins
Explanation: ### Explanation **Correct Option: C (Lipopolysaccharide and outer membrane)** The primary mediator of septic shock in *Neisseria meningitidis* (Meningococcus) is its **Lipooligosaccharide (LOS)**, which is the neisserial equivalent of Lipopolysaccharide (LPS) found in the outer membrane. Unlike most Gram-negative bacteria, Meningococci shed excessive amounts of their outer membrane in the form of "blebs" during rapid growth. These blebs release massive quantities of LOS into the bloodstream. LOS acts as a potent **endotoxin**, triggering the release of pro-inflammatory cytokines (TNF-α, IL-1, and IL-6). This leads to the classic triad of meningococcemia: disseminated intravascular coagulation (DIC), massive hemorrhage into the adrenal glands (**Waterhouse-Friderichsen Syndrome**), and profound septic shock. **Why other options are incorrect:** * **A. Capsular polysaccharide:** This is the primary **virulence factor** responsible for preventing phagocytosis and is used for serotyping and vaccine production, but it does not trigger the cytokine storm required for shock. * **B. Pili:** These are essential for **initial attachment** and colonization of the nasopharyngeal epithelium. * **D. Low-molecular-weight outer membrane proteins (OMPs):** Proteins like PorA and PorB function as porins and are involved in nutrient transport and intracellular survival, but they lack the endotoxic activity of LOS. --- ### High-Yield NEET-PG Pearls * **LOS vs. LPS:** *Neisseria* lacks the "O-antigen" side chain found in enteric bacteria, hence it is called Lipooligosaccharide (LOS). * **Waterhouse-Friderichsen Syndrome:** Characterized by adrenal insufficiency, petechial rash, and shock. * **DOC:** Ceftriaxone is the drug of choice for treatment; Rifampicin is used for prophylaxis of close contacts. * **Nasopharynx:** The natural reservoir and portal of entry for *N. meningitidis*.
Question 1099: A 12-year-old boy presents with sudden onset of fever, headache, and stiff neck. Two days prior, he swam in a lake potentially contaminated with dog excreta. Leptospirosis is suspected. Which of the following laboratory tests is most appropriate for diagnosing leptospiral infection?
- A. Agglutination test for leptospiral antigen
- B. Counter-immune electrophoresis of urine sample
- C. Gram stain of urine specimen
- D. Spinal fluid for dark-field microscopy and culture in Fletcher serum medium (Correct Answer)
Explanation: ### Explanation **1. Why the Correct Answer is Right:** Leptospirosis is a biphasic illness. During the **first week (Septicemic phase)**, the spirochetes are present in the **blood and Cerebrospinal Fluid (CSF)**. This patient presents with acute meningeal symptoms (fever, headache, stiff neck) only two days after exposure, placing him in this initial phase. * **Dark-field microscopy (DFM):** Used to visualize the characteristic thin, tightly coiled spirochetes with hooked ends ("shepherd’s crook"). * **Culture:** *Leptospira* are fastidious and require specialized media containing serum or albumin, such as **Fletcher’s, EMJH, or Korthof’s medium**. Since the organism is in the CSF during the first week, this is the most appropriate diagnostic step for this clinical presentation. **2. Why the Other Options are Wrong:** * **Option A & B:** While antigen detection and electrophoresis exist, they are not the gold standard or the primary diagnostic choice in the acute phase compared to culture and microscopy. * **Option C:** *Leptospira* are too thin to be seen on a standard **Gram stain**. They require DFM, silver impregnation stains (Fontana), or phase-contrast microscopy. Furthermore, leptospires typically appear in the **urine** only after the **second week** of infection (Leptospiruric phase). **3. High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Contact with water/soil contaminated by **rat or dog urine** (enters through abraded skin/mucosa). * **Gold Standard Test:** **Microscopic Agglutination Test (MAT)**—detects antibodies, but usually becomes positive after 7–10 days. * **Specimen Timing:** * **Week 1:** Blood and CSF. * **Week 2 onwards:** Urine. * **Weil’s Disease:** A severe triad of jaundice, renal failure, and hemorrhage. * **Morphology:** "C" or "S" shaped spirochetes with hooked ends.
Question 1100: Which organism is strongly urease positive?
- A. Proteus species (Correct Answer)
- B. Klebsiella species
- C. Helicobacter pylori
- D. Staphylococcus species
Explanation: **Explanation:** The **urease enzyme** catalyzes the hydrolysis of urea into ammonia and carbon dioxide. The production of ammonia increases the pH of the environment (alkalinization), which is a key diagnostic feature and virulence factor for several pathogens. **1. Why Proteus species is the correct answer:** *Proteus* (specifically *P. mirabilis* and *P. vulgaris*) is the classic example of a **strongly urease-positive** organism. In the laboratory, it produces a rapid positive result on Christensen’s Urea Agar (turning the medium pink within 1–2 hours). Clinically, this alkalinization of urine leads to the precipitation of magnesium ammonium phosphate, resulting in the formation of **Staghorn calculi (Struvite stones)**. **2. Analysis of Incorrect Options:** * **B. Klebsiella species:** While *Klebsiella* is urease-positive, it is generally considered a "weak" or "slow" producer compared to the rapid reaction seen in *Proteus*. * **C. Helicobacter pylori:** *H. pylori* is indeed strongly urease-positive (using the enzyme to survive gastric acid). However, in the context of standard bacteriology questions regarding rapid laboratory identification and urinary tract pathology, *Proteus* remains the primary answer choice. * **D. Staphylococcus species:** Only *Staphylococcus saprophyticus* and *Staphylococcus epidermidis* are typically urease-positive; *S. aureus* is generally negative. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Urease-Positive Organisms (PUNCH):** **P**roteus, **U**reaplasma, **N**ocardia, **C**ryptococcus, **H**elicobacter pylori (also includes *Klebsiella* and *S. saprophyticus*). * **Urine Microscopy:** *Proteus* infections are associated with "coffin-lid" shaped crystals (struvite). * **Culture Characteristic:** *Proteus* exhibits **swarming growth** on blood agar due to its peritrichous flagella.
Practice by Chapter
Staphylococci
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Streptococci and Enterococci
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Neisseria and Moraxella
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Corynebacterium and Listeria
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Bacillus and Clostridium
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Enterobacteriaceae
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Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
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Mycobacteria
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Spirochetes
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