Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 941: Enterotoxins are produced by all except?

A. Clostridium perfringens
B. Staphylococcus aureus
C. Streptococcus pyogenes (Correct Answer)
D. Bacillus cereus

Explanation: **Explanation:** The correct answer is **Streptococcus pyogenes** because it primarily produces **Pyrogenic Exotoxins** (SpeA, SpeB, SpeC), which act as superantigens causing Scarlet Fever and Streptococcal Toxic Shock Syndrome, rather than enterotoxins that target the gastrointestinal tract. **Analysis of Options:** * **Clostridium perfringens (Option A):** Produces *C. perfringens enterotoxin (CPE)*, typically associated with Type A strains. It is a common cause of food poisoning, where the toxin is released in the intestine during sporulation, leading to watery diarrhea. * **Staphylococcus aureus (Option B):** Produces heat-stable enterotoxins (A-E). These are pre-formed in contaminated food (like creamy salads or meats). They act as superantigens and trigger the vomiting center via the vagus nerve, causing rapid-onset food poisoning. * **Bacillus cereus (Option D):** Produces two types of enterotoxins: a heat-stable toxin (causing the emetic/vomiting form) and a heat-labile toxin (causing the diarrheal form). **Clinical Pearls for NEET-PG:** 1. **Staph. aureus vs. B. cereus:** If symptoms start within 1–6 hours of eating, suspect pre-formed toxins (Staph or Emetic B. cereus). 2. **Superantigens:** Both Staphylococcal enterotoxins and Streptococcal pyrogenic exotoxins are superantigens, but only the former is classified as an enterotoxin. 3. **C. perfringens:** It is unique because the toxin is produced *in vivo* (inside the gut) during the transition from vegetative cell to spore. 4. **Vibrio cholerae:** Produces the classic "Choleragen" enterotoxin, which increases cAMP, leading to rice-water stools.

Question 942: Transovarian transmission is a feature of which disease?

A. Scrub fever (Correct Answer)
B. Epidemic typhus
C. Endemic typhus
D. Trench fever

Explanation: **Explanation** **Transovarian transmission** refers to the passage of a pathogen from a female vector to its offspring via the eggs. This mechanism allows the pathogen to persist in the environment even in the absence of a vertebrate host. **Why Scrub Typhus (Scrub Fever) is correct:** Scrub typhus is caused by *Orientia tsutsugamushi* and is transmitted by the bite of the larval stage (chigger) of **Leptotrombidium mites**. Since only the larvae feed on mammals, the bacteria must be passed from the adult female mite to her eggs (transovarian transmission) and subsequently to the larvae to maintain the life cycle. The mite acts as both the vector and the primary reservoir. **Why the other options are incorrect:** * **Epidemic Typhus (*Rickettsia prowazekii*):** Transmitted by the human body louse. The louse dies from the infection and does not pass the bacteria to its offspring. * **Endemic Typhus (*Rickettsia typhi*):** Transmitted by the rat flea (*Xenopsylla cheopis*). While it can persist in flea populations, transovarian transmission is not the primary epidemiological feature compared to scrub typhus. * **Trench Fever (*Bartonella quintana*):** Also transmitted by the human body louse; no transovarian transmission occurs. **High-Yield Clinical Pearls for NEET-PG:** * **Vector for Scrub Typhus:** Larval mite (Chigger). * **Pathognomonic Sign:** An **eschar** (a black, necrotic scab) at the site of the chigger bite. * **Weil-Felix Reaction:** Scrub typhus shows a positive reaction with **OX-K** (negative for OX-2 and OX-19). * **Drug of Choice:** Doxycycline is the gold standard for all rickettsial diseases.

Question 943: Which of the following statements regarding Shigella dysenteriae type I is true?

A. It can lead to hemolytic uremic syndrome. (Correct Answer)
B. It produces an invasive enterotoxin.
C. It is a facultative aerobe.
D. It is MR negative.

Explanation: **Explanation:** **Shigella dysenteriae type I** (also known as Shiga bacillus) is the most virulent species of the Shigella genus. 1. **Why Option A is Correct:** *S. dysenteriae* type I produces the potent **Shiga toxin (Stx)**. This toxin enters the bloodstream and targets the glomerular endothelial cells. It leads to microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—the triad defining **Hemolytic Uremic Syndrome (HUS)**. This is a classic high-yield association for NEET-PG. 2. **Why Other Options are Incorrect:** * **Option B:** While Shigella is highly invasive, the Shiga toxin is an **exotoxin** (specifically a cytotoxin), not an "invasive enterotoxin." The invasion of the colonic mucosa is mediated by large virulence plasmids, not the toxin itself. * **Option C:** Shigella species are **facultative anaerobes**, not facultative aerobes. They can grow with or without oxygen but prefer anaerobic conditions in the gut. * **Option D:** Shigella is **Methyl Red (MR) positive**. In biochemical testing, it ferments glucose with acid production, maintaining a low pH that keeps the MR indicator red. **Clinical Pearls for NEET-PG:** * **Low Infectious Dose:** Only 10–100 organisms are required to cause infection (due to resistance to gastric acid), unlike *Vibrio cholerae* or *Salmonella*. * **Non-Motile:** Shigella lacks H-antigen (non-flagellated). * **Culture:** On DCA (Deoxycholate Citrate Agar) or MacConkey agar, it produces **pale, non-lactose fermenting (NLF)** colonies. * **Catalase Test:** *S. dysenteriae* type I is unique as it is **catalase negative**, whereas all other Shigella species are catalase positive.

Question 944: Which of the following is NOT a rapid grower among Mycobacteria?

A. M. fortuitum
B. M. chelonae
C. M. avium intracellulare (Correct Answer)
D. M. smegmatis

Explanation: ### Explanation The classification of Non-Tuberculous Mycobacteria (NTM) is primarily based on the **Runyon Classification**, which categorizes them into four groups based on growth rate and pigment production. **Why M. avium intracellulare (MAC) is the correct answer:** * **M. avium intracellulare** belongs to **Runyon Group III (Non-photochromogens)**. * These are **slow growers**, typically requiring **2 to 4 weeks** to form visible colonies on solid media (like Lowenstein-Jensen medium). * Clinically, MAC is the most common opportunistic bacterial infection in AIDS patients with low CD4 counts. **Why the other options are incorrect:** Options A, B, and D all belong to **Runyon Group IV (Rapid Growers)**. These organisms produce visible colonies within **7 days**. * **M. fortuitum & M. chelonae:** These are classic rapid growers often associated with skin and soft tissue infections, post-surgical wound infections, and infections following tattooing or cosmetic procedures. * **M. smegmatis:** A rapid grower usually considered a commensal (found in smegma), though it can rarely cause skin or soft tissue infections. **High-Yield NEET-PG Pearls:** 1. **Runyon Classification Summary:** * **Group I (Photochromogens):** Pigment in light (e.g., *M. kansasii, M. marinum*). * **Group II (Scotochromogens):** Pigment in light and dark (e.g., *M. scrofulaceum*). * **Group III (Non-photochromogens):** No pigment (e.g., *MAC*). * **Group IV (Rapid Growers):** Growth < 7 days (e.g., *M. fortuitum, M. chelonae, M. abscessus*). 2. **M. marinum:** Known as "Swimming pool granuloma" or "Fish tank granuloma." 3. **M. scrofulaceum:** Common cause of cervical lymphadenitis in children. 4. **Arylsulfatase Test:** Rapid growers (Group IV) are typically positive for this biochemical test.

Question 945: Exudative plaques in the form of pseudomembranes are due to:

A. Escherichia coli
B. Clostridium difficile (Correct Answer)
C. Salmonella
D. Campylobacter jejuni

Explanation: **Explanation:** The correct answer is **Clostridium difficile**. This organism is the primary causative agent of **Pseudomembranous Colitis (PMC)**, typically occurring after broad-spectrum antibiotic use (e.g., Clindamycin, Fluoroquinolones). **Why Clostridium difficile is correct:** The pathogenesis involves the release of two potent exotoxins: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. These toxins cause mucosal inflammation, epithelial cell necrosis, and the recruitment of inflammatory cells. This results in the formation of characteristic **"volcano lesions"**—exudative plaques composed of fibrin, mucus, and necrotic inflammatory cells that coalesce to form a yellowish-white **pseudomembrane** over the colonic mucosa. **Why the other options are incorrect:** * **Escherichia coli:** Most strains cause watery diarrhea (ETEC) or hemorrhagic colitis (EHEC) without pseudomembrane formation. * **Salmonella:** Typically causes enteric fever or gastroenteritis characterized by mucosal inflammation and potential ulceration (especially in Peyer’s patches), but not pseudomembranes. * **Campylobacter jejuni:** A common cause of inflammatory diarrhea/dysentery; while it causes mucosal ulceration and bloody stools, it does not produce the classic exudative pseudomembranes seen in PMC. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Oral **Vancomycin** or Fidaxomicin (Metronidazole is now a second-line agent). * **Diagnosis:** Detection of Toxin A and B in stool via ELISA or PCR (GDH assay is used for screening). * **Morphology:** On colonoscopy, look for "yellowish-white raised plaques." * **Risk Factor:** Most common antibiotic associated is Clindamycin, but most frequent causes currently are Cephalosporins and Fluoroquinolones.

Question 946: Urinary tract infection by a gram-positive organism in a young sexually active female is commonly due to which bacterium?

A. Bacillus subtilis
B. Escherichia coli
C. Staphylococcus saprophyticus (Correct Answer)
D. Streptococcus pyogenes

Explanation: **Explanation:** The correct answer is **Staphylococcus saprophyticus**. **1. Why it is correct:** *Staphylococcus saprophyticus* is a coagulase-negative, gram-positive coccus that is a leading cause of uncomplicated Urinary Tract Infections (UTIs) specifically in **young, sexually active females** (often referred to as "Honeymoon Cystitis"). It possesses specific adhesins (like Uers) that allow it to adhere effectively to the uroepithelium. While *E. coli* is the overall most common cause of UTI, *S. saprophyticus* is the most common **gram-positive** cause in this specific demographic. **2. Why the other options are incorrect:** * **Bacillus subtilis (A):** This is a gram-positive spore-forming rod that is generally considered a non-pathogenic environmental contaminant and does not cause UTIs. * **Escherichia coli (B):** While *E. coli* is the #1 cause of UTIs globally, it is a **Gram-negative** rod. The question specifically asks for a **Gram-positive** organism. * **Streptococcus pyogenes (D):** Also known as Group A Streptococcus (GAS), it primarily causes pharyngitis and skin infections (impetigo, cellulitis). It is not a recognized cause of urinary tract infections. **3. High-Yield NEET-PG Pearls:** * **Novobiocin Resistance:** *S. saprophyticus* is characteristically resistant to Novobiocin, which differentiates it from *S. epidermidis* (which is sensitive). * **Catalase/Coagulase:** It is Catalase-positive but Coagulase-negative (CoNS). * **Urease:** It is Urease-positive, which can lead to an increase in urine pH. * **Demographic:** If a clinical vignette mentions a "young female" with UTI symptoms and a "Gram-positive coccus in clusters," always think of *S. saprophyticus*.

Question 947: Lymphangitis is caused by which bacterium?

A. Staphylococcus
B. Streptococcus (Correct Answer)
C. Pneumococcus
D. Neisseria

Explanation: **Explanation:** **Lymphangitis** is the inflammation of the lymphatic channels, typically occurring as a result of an infection distal to the site. **Why Streptococcus is the correct answer:** The most common causative organism for acute lymphangitis is **Streptococcus pyogenes (Group A Beta-Hemolytic Streptococcus)**. These bacteria produce enzymes like hyaluronidase and streptokinase that break down connective tissue barriers, allowing the organism to spread rapidly into the lymphatic system. Clinically, this presents as painful, erythematous "red streaks" extending from the site of infection toward regional lymph nodes. **Analysis of Incorrect Options:** * **Staphylococcus:** While *Staphylococcus aureus* is a leading cause of localized skin infections (like abscesses or furuncles), it tends to remain localized due to the production of coagulase, which walls off the infection. It is a much less common cause of spreading lymphangitis compared to Streptococcus. * **Pneumococcus (*S. pneumoniae*):** This organism primarily causes respiratory tract infections (pneumonia, otitis media) and meningitis. It is not a typical pathogen associated with skin or lymphatic spread. * **Neisseria:** *N. meningitidis* and *N. gonorrhoeae* are associated with meningitis and sexually transmitted infections, respectively. They do not typically cause primary lymphangitis. **NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Look for the classic "red streaks" on a limb moving proximally. * **Lymphadenitis vs. Lymphangitis:** Lymphadenitis refers to the inflammation of the lymph **nodes** themselves, whereas lymphangitis is the inflammation of the **vessels**. * **Other Causes:** In specific contexts, consider *Sporothrix schenckii* (nodular lymphangitis/Rose gardener’s disease) or *Pasteurella multocida* (following animal bites). * **Treatment:** Penicillin remains the drug of choice for Streptococcal lymphangitis.

Question 948: Which of the following statements about syphilis is FALSE?

A. Treponema pallidum immobilization (TPI) test gives a definite diagnosis.
B. Treponema pallidum hemagglutination assay (TPHA) is the earliest serological test to become positive. (Correct Answer)
C. VDRL becomes positive one week after the appearance of the chancre.
D. Serologically, syphilis is indistinguishable from yaws.

Explanation: **Explanation:** The statement in **Option B** is false because the **Fluorescent Treponemal Antibody-Absorption (FTA-ABS)** test is the earliest serological test to become positive in syphilis (usually during the first week of the primary chancre). TPHA, while highly specific, typically becomes positive later (around the 3rd to 4th week). **Analysis of other options:** * **Option A:** The **TPI test** is considered the "gold standard" for specificity. It uses live Treponemes and is the most definitive test, though it is rarely performed now due to technical complexity. * **Option C:** Non-specific tests like **VDRL and RPR** typically take 1–2 weeks after the appearance of the primary chancre (or 4–6 weeks after infection) to show reactivity. * **Option D:** *Treponema pallidum* (syphilis) and *Treponema pertenue* (yaws) are morphologically and serologically **identical**. They cannot be distinguished by current blood tests; diagnosis relies on clinical presentation and geographical history. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test of Choice:** VDRL/RPR (Non-treponemal). * **Confirmatory Test of Choice:** FTA-ABS or TPHA (Treponemal). * **Test for Neurosyphilis:** CSF-VDRL is highly specific (though poorly sensitive). * **Prozone Phenomenon:** Can cause a false-negative VDRL in secondary syphilis due to excessively high antibody titers; requires serum dilution. * **Biological False Positives (BFP):** Seen in VDRL due to conditions like SLE, leprosy, malaria, and pregnancy.

Question 949: Pea-soup stool is characteristically seen in which of the following conditions?

A. Cholera
B. Botulism
C. Typhoid (Correct Answer)
D. Polio

Explanation: **Explanation:** **Typhoid fever** (Enteric fever), caused by *Salmonella Typhi*, is the correct answer. The characteristic **"pea-soup stool"** typically appears during the second or third week of the infection. This occurs due to the inflammation and ulceration of **Peyer’s patches** in the terminal ileum, leading to a greenish, liquid, and foul-smelling diarrhea that resembles thick pea soup. **Analysis of Incorrect Options:** * **Cholera:** Characterized by **"rice-water stools"** (watery, non-foul smelling, with mucus flecks). It is caused by *Vibrio cholerae* enterotoxin, leading to massive secretory diarrhea. * **Botulism:** Caused by *Clostridium botulinum* toxin. It typically presents with **descending paralysis** and autonomic symptoms; gastrointestinal symptoms are usually limited to constipation rather than diarrhea. * **Polio:** While the Poliovirus is transmitted via the feco-oral route and replicates in the gut, it does not cause a specific characteristic stool. Its hallmark is **asymmetrical flaccid paralysis**. **Clinical Pearls for NEET-PG:** * **Widal Test:** Most reliable in the **2nd week** of Typhoid. * **Blood Culture:** Most sensitive in the **1st week** (90% positive). * **Rose Spots:** Faint, blanching maculopapular rashes seen on the chest/abdomen during the 2nd week. * **Relative Bradycardia (Faget’s sign):** A high fever with a disproportionately low pulse rate, a classic sign of Typhoid. * **Drug of Choice:** Ceftriaxone (for empirical/multidrug-resistant cases) or Azithromycin.

Question 950: A patient presents with signs of pneumonia. The bacteria obtained from sputum was a Gram-positive coccus which showed alpha hemolysis on sheep agar. Which of the following tests will help to confirm the diagnosis?

A. Bile solubility (Correct Answer)
B. Coagulase test
C. Bacitracin test
D. CAMP test

Explanation: **Explanation:** The clinical presentation of pneumonia combined with the laboratory finding of **Gram-positive cocci** showing **alpha-hemolysis** (greenish discoloration) on blood agar strongly suggests **_Streptococcus pneumoniae_**. **1. Why Bile Solubility is the Correct Answer:** _Streptococcus pneumoniae_ possesses an intracellular autolytic enzyme called **amidase**. Bile salts (like sodium deoxycholate) lower the surface tension, which activates these autolytic enzymes, leading to the lysis of the bacterial cell wall. In a bile solubility test, the addition of bile salts to a broth culture of _S. pneumoniae_ results in the clearing of the turbidity. This test is the gold standard for differentiating _S. pneumoniae_ (bile soluble) from other alpha-hemolytic streptococci, such as the Viridans group (bile insoluble). **2. Why Other Options are Incorrect:** * **Coagulase test:** Used to differentiate _Staphylococcus aureus_ (positive) from coagulase-negative staphylococci (CONS). Staphylococci are catalase-positive and do not show alpha-hemolysis. * **Bacitracin test:** Used to identify **Group A Streptococci** (_S. pyogenes_), which are **beta-hemolytic**. _S. pyogenes_ is sensitive to bacitracin. * **CAMP test:** Used to identify **Group B Streptococci** (_S. agalactiae_), which are also **beta-hemolytic**. It detects a synergistic effect between the CAMP factor of GBS and the beta-lysin of _S. aureus_. **High-Yield Clinical Pearls for NEET-PG:** * **Optochin Sensitivity:** _S. pneumoniae_ is sensitive to Optochin (ethylhydrocupreine hydrochloride), while Viridans streptococci are resistant. * **Quellung Reaction:** A capsular swelling test used for rapid identification of _S. pneumoniae_. * **Morphology:** Classically described as **"Flame-shaped"** or **"Lancet-shaped"** diplococci. * **Culture:** Shows "Draughtsman" or "Carrom coin" appearance due to central autolysis of older colonies.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

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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