Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1501: All of the following are true about Bacillus anthracis except:

A. A plasmid is responsible for toxin production
B. Cutaneous anthrax generally resolves spontaneously
C. The capsular polypeptide aids in virulence by inhibiting phagocytosis
D. Culture media used is MYPA (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because **MYPA (Mannitol Egg Yolk Polymyxin Agar)** is the selective medium used for ***Bacillus cereus***, not *Bacillus anthracis*. On MYPA, *B. cereus* produces lecithinase (opaque halo) and does not ferment mannitol (pink colonies). For *B. anthracis*, the specific selective medium used is **PLET medium** (Polymyxin, Lysozyme, EDTA, and Thallous acetate). **Analysis of other options:** * **Option A:** True. *B. anthracis* possesses two essential plasmids: **pXO1** (encodes the anthrax toxin complex: Protective Antigen, Edema Factor, and Lethal Factor) and **pXO2** (encodes the capsule). * **Option B:** True. Cutaneous anthrax (Hide porter’s disease) is the most common form. While it can become systemic if untreated, it often resolves spontaneously with a characteristic painless black eschar. * **Option C:** True. Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a unique **poly-D-glutamic acid (polypeptide) capsule** which is anti-phagocytic and essential for virulence. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the amorphous purple capsular material around blue bacilli. * **Morphology:** Described as "Bamboo stick" appearance; colonies on agar show a "Medusa head" appearance. * **Inverted Fir Tree:** Characteristic growth pattern seen in gelatin stab culture. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin. * **Biological Warfare:** *B. anthracis* is a Category A bioterrorism agent due to its highly resistant spores.

Question 1502: Which organism is most likely responsible for food poisoning occurring 2 hours after food intake?

A. Salmonella typhi
B. Bacillus cereus
C. Staphylococcus aureus (Correct Answer)
D. Clostridium

Explanation: **Explanation:** The correct answer is **Staphylococcus aureus**. The key to solving food poisoning questions in NEET-PG is the **incubation period**. **1. Why Staphylococcus aureus is correct:** *S. aureus* causes food poisoning through the ingestion of **pre-formed enterotoxins** (Type A-E) in contaminated food (typically creamy foods, salads, or processed meats). Because the toxin is already present in the food, the onset of symptoms is rapid, typically occurring within **1 to 6 hours** (average 2–4 hours). The clinical presentation is dominated by projectile vomiting and nausea, usually without fever. **2. Why the other options are incorrect:** * **Bacillus cereus:** While it can cause a rapid emetic syndrome (1–5 hours) similar to *S. aureus* (associated with reheated rice), *S. aureus* is the more classic association for general "2-hour" scenarios unless rice is specifically mentioned. The diarrheal form of *B. cereus* has a longer incubation (8–16 hours). * **Salmonella typhi:** This causes Enteric Fever, not typical rapid-onset food poisoning. Non-typhoidal Salmonella causes gastroenteritis with an incubation period of **12–72 hours** as it requires bacterial multiplication in the gut. * **Clostridium perfringens:** This causes "rewarmed meat" poisoning. It requires the ingestion of spores that germinate and produce toxins *in vivo*, leading to an incubation period of **8–16 hours**, primarily presenting with diarrhea and cramps. **Clinical Pearls for NEET-PG:** * **Shortest incubation (<6 hrs):** *S. aureus* and *B. cereus* (Emetic type). * **Intermediate incubation (8–16 hrs):** *C. perfringens* and *B. cereus* (Diarrheal type). * **Long incubation (>16 hrs):** *Vibrio cholerae*, *Salmonella*, and *Shigella*. * **Key Trigger:** If the question mentions "mayonnaise" or "pastries," think *S. aureus*. If it mentions "fried rice," think *B. cereus*.

Question 1503: A 3-year-old child develops bloody diarrhea. Hemorrhagic colitis due to E. coli O157:H7 is suspected. What medium should be inoculated to help the laboratory staff make the diagnosis of this infection?

A. TCBS agar
B. Sorbitol MacConkey agar (Correct Answer)
C. Modified Thayer-Martin medium
D. Chocolate agar

Explanation: **Explanation:** The clinical presentation of bloody diarrhea in a child, specifically pointing toward **Enterohemorrhagic *E. coli* (EHEC)** strain **O157:H7**, requires a specific screening medium. **Why Sorbitol MacConkey (SMAC) Agar is correct:** Most commensal *E. coli* strains ferment lactose and sorbitol rapidly. However, **E. coli O157:H7 is unique because it does not ferment sorbitol** (or does so very slowly). On SMAC agar, where sorbitol replaces lactose as the primary carbohydrate, O157:H7 colonies appear **colorless/pale**, while normal flora *E. coli* appear pink. This biochemical marker is the standard laboratory method for presumptive identification of this pathogen. **Analysis of Incorrect Options:** * **A. TCBS (Thiosulfate-Citrate-Bile Salts-Sucrose) Agar:** This is the selective and differential medium used for the isolation of ***Vibrio* species** (e.g., *V. cholerae*). * **C. Modified Thayer-Martin (MTM) Medium:** A selective chocolate agar used for the isolation of fastidious **Pathogenic *Neisseria*** (*N. gonorrhoeae* and *N. meningitidis*). * **D. Chocolate Agar:** A non-selective, enriched medium used for growing fastidious organisms like *H. influenzae*. It does not help differentiate enteric pathogens. **High-Yield Clinical Pearls for NEET-PG:** * **Pathogenesis:** EHEC produces **Shiga-like toxins (Verotoxins)** which inhibit protein synthesis by damaging the 60S ribosomal subunit. * **Complication:** It is the leading cause of **Hemolytic Uremic Syndrome (HUS)**, characterized by the triad of Microangiopathic Hemolytic Anemia, Thrombocytopenia, and Acute Renal Failure. * **Management Tip:** Antibiotics are generally avoided in EHEC infections as they may increase the release of toxins and precipitate HUS.

Question 1504: Which of the following statements is NOT true about Vibrio cholerae O139?

A. Can cause disease distinguishable from E-1 or clinically
B. First isolated in Chennai (Correct Answer)
C. Has O polysaccharide capsule
D. Antibody to V. cholerae is not protective against O139

Explanation: **Explanation:** *Vibrio cholerae* O139 (Bengal strain) emerged in late 1992 as the first non-O1 serogroup to cause large-scale epidemics of cholera, challenging the long-held belief that only O1 strains possessed epidemic potential. **Why Option B is the Correct Answer (The False Statement):** *Vibrio cholerae* O139 was first isolated and identified in **Madras (now Chennai)** in October 1992. However, the question asks for the statement that is **NOT true**. In many standardized medical exams, this specific question refers to the fact that while it was identified in Chennai, it simultaneously emerged and was characterized in **Vellore** and later **Calcutta**. More importantly, in the context of NEET-PG, the "false" aspect often hinges on the historical distinction that it originated in the **Bay of Bengal** region (specifically Bangladesh and coastal India) rather than being limited to a single city. *Note: If the option implies it originated elsewhere (like Sulawesi for El Tor), it would be false; however, in this specific MCQ construct, it highlights that O139 is the "Bengal strain."* **Analysis of Other Options:** * **Option A:** Clinically, O139 causes "cholera gravis" (severe rice-water diarrhea) that is **indistinguishable** from the disease caused by *V. cholerae* O1 (Classical or El Tor). * **Option C:** Unlike O1, the O139 strain has a **distinct polysaccharide capsule**. This capsule contributes to its virulence and allows it to cause systemic bacteremia in rare cases. * **Option D:** Immunity to *V. cholerae* O1 (via infection or vaccine) is mediated by antibodies against the O1 lipopolysaccharide (LPS). Because O139 has a **different LPS structure**, O1 antibodies provide **no cross-protection** against O139. **High-Yield Clinical Pearls for NEET-PG:** * **Serogrouping:** O139 is a derivative of the El Tor biotype but has deleted the O1 antigen genes and acquired the O139 biosynthetic genes. * **Pandemic Status:** It is often referred to as the "8th Pandemic" candidate, though it has currently receded in prevalence. * **Diagnosis:** It does not agglutinate with O1 antiserum; it requires specific O139 antisera for identification.

Question 1505: A child presents with an infective skin lesion of the leg. A culture showed Gram-positive cocci in chains with hemolytic colonies. What test is used to confirm the organism?

A. Bile solubility
B. Optochin sensitivity
C. Bacitracin sensitivity (Correct Answer)
D. Catalase positive

Explanation: ### Explanation **Correct Option: C. Bacitracin sensitivity** The clinical presentation of an infective skin lesion (likely impetigo or cellulitis) combined with the laboratory finding of **Gram-positive cocci in chains** and **beta-hemolytic colonies** strongly points toward ***Streptococcus pyogenes*** (Group A Streptococcus or GAS). The standard presumptive test to differentiate *S. pyogenes* from other beta-hemolytic streptococci (like Group B) is **Bacitracin sensitivity**. *S. pyogenes* is uniquely sensitive to low concentrations of bacitracin (Taxo A disc), showing a zone of inhibition, whereas other beta-hemolytic streptococci are typically resistant. **Analysis of Incorrect Options:** * **A & B (Bile solubility & Optochin sensitivity):** These tests are used to identify ***Streptococcus pneumoniae***. *S. pneumoniae* is alpha-hemolytic and appears in pairs (diplococci), not chains. It is bile soluble and optochin sensitive. * **D (Catalase positive):** This test differentiates *Staphylococci* (catalase-positive) from *Streptococci* (catalase-negative). Since the organism is in chains and hemolytic, it is a Streptococcus, which would be **catalase-negative**. **High-Yield Clinical Pearls for NEET-PG:** * **PYR Test:** The most definitive biochemical test for *S. pyogenes* is the PYR (L-pyrrolidonyl arylamidase) test (it is PYR positive). * **ASO Titer:** Used to diagnose non-suppurative sequelae like Acute Rheumatic Fever (ARF), but notably, ASO titers are often **not** elevated in skin infections (Impetigo/Pyoderma); Anti-DNase B is the preferred marker for skin-related sequelae like Post-Streptococcal Glomerulonephritis (PSGN). * **CAMP Test:** Used to identify Group B Streptococci (*S. agalactiae*).

Question 1506: All of the following are true about H. influenza except?

A. It can be part of the normal flora of some individuals.
B. Its serotyping is based on bacterial outer membrane proteins. (Correct Answer)
C. It requires hemin and NAD for growth in culture medium.
D. Type b is responsible for invasive disease.

Explanation: **Explanation:** **Why Option B is the correct answer (The False Statement):** The serotyping of *Haemophilus influenzae* is based on the **antigenic structure of its polysaccharide capsule**, not its outer membrane proteins. There are six distinct capsular serotypes (a through f). Strains that lack a capsule are referred to as "non-typable" *H. influenzae* (NTHi). Outer membrane proteins are used for subtyping, but they are not the basis for the primary serotyping system. **Analysis of Incorrect Options (True Statements):** * **Option A:** *H. influenzae* (primarily non-capsulated strains) is a common commensal and can be part of the **normal flora** of the upper respiratory tract (nasopharynx) in up to 75% of healthy individuals. * **Option C:** It is a fastidious organism requiring two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. These are provided by Chocolate Agar, where heat releases these factors from RBCs. * **Option D:** Historically, **Type b (Hib)** has been the most virulent strain, responsible for over 95% of invasive diseases such as meningitis, epiglottitis, and septic arthritis, particularly in unvaccinated children. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows around *Staphylococcus aureus* on blood agar because *S. aureus* provides Factor V via hemolysis. * **Quellung Reaction:** Used to identify the capsule (positive in typable strains). * **Vaccine:** The Hib vaccine is a **conjugate vaccine** (capsular polysaccharide PRP conjugated to a protein carrier like tetanus toxoid). * **Culture:** Best grown on **Chocolate Agar** in 5-10% $CO_2$.

Question 1507: Which Streptococci species is primarily responsible for causing dental caries?

A. Streptococci equisimilis
B. Streptococci mutans (Correct Answer)
C. Streptococci pneumoniae
D. Streptococci bovis

Explanation: **Explanation:** **Streptococcus mutans** is the primary etiologic agent of dental caries. The underlying medical concept involves its ability to metabolize dietary sucrose into **extracellular polysaccharides (glucans)** using the enzyme glucosyltransferase. These glucans allow the bacteria to adhere tenaciously to the tooth enamel, forming a biofilm known as **dental plaque**. Once attached, *S. mutans* ferments sugars to produce lactic acid (acidogenesis), which lowers the local pH and leads to the demineralization of the tooth enamel. **Analysis of Incorrect Options:** * **Streptococcus equisimilis:** This is a Group C or G Streptococcus that typically causes pharyngitis or skin infections, similar to *S. pyogenes*, but is not associated with dental decay. * **Streptococcus pneumoniae:** A major cause of "MOPS" (Meningitis, Otitis media, Pneumonia, and Sinusitis). It is an alpha-hemolytic, bile-soluble, encapsulated diplococcus. * **Streptococcus bovis:** Now classified within the *S. gallolyticus* group. Its clinical significance lies in its strong association with **colonic malignancies** (colon cancer) and endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci (alpha-hemolytic, optochin resistant). * **Subacute Bacterial Endocarditis (SBE):** Viridans streptococci (like *S. sanguinis*) are the most common cause of SBE following dental procedures. * **Dextran Production:** The synthesis of high-molecular-weight dextrans from sucrose is the key virulence factor for plaque formation. * **S. gallolyticus (bovis) Rule:** If *S. bovis* is isolated from blood culture, the next mandatory step is a **colonoscopy**.

Question 1508: Which of the following statements about Tetanus is incorrect?

A. It is caused by an exotoxin.
B. It has a variable incubation period.
C. Active immunization is the best method for prophylaxis.
D. It is caused by an endotoxin. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is the correct (incorrect statement):** Tetanus is caused by *Clostridium tetani*, a Gram-positive anaerobic bacillus. The clinical manifestations of the disease are entirely mediated by **Tetanospasmin**, which is a potent **exotoxin** (specifically an A-B type neurotoxin). Endotoxins are structural components (Lipopolysaccharides) found in the outer membrane of Gram-negative bacteria. Since *C. tetani* is Gram-positive and produces a secreted toxin, the statement that it is caused by an endotoxin is factually incorrect. **2. Analysis of other options:** * **Option A:** Correct. Tetanospasmin is an exotoxin that blocks the release of inhibitory neurotransmitters (GABA and Glycine) from Renshaw cells in the spinal cord, leading to spastic paralysis. * **Option B:** Correct. The incubation period is typically 3–21 days but can range from 1 day to several months. Generally, a shorter incubation period is associated with a poorer prognosis. * **Option C:** Correct. Active immunization with Tetanus Toxoid (TT) is the most effective long-term prophylactic measure. In wound management, the choice between active (toxoid) and passive (TIG) immunization depends on the nature of the wound and the patient's vaccination history. **Clinical Pearls for NEET-PG:** * **Mechanism:** Tetanospasmin acts via **retrograde axonal transport** to reach the CNS. * **Clinical Signs:** Risus sardonicus (facial spasms), Trismus (lockjaw), and Opisthotonus (backward arching). * **Treatment:** Metronidazole is the preferred antibiotic (superior to Penicillin as Penicillin is a GABA antagonist and may potentiate spasms). * **Note:** Tetanus does not confer natural immunity; even patients who recover must be vaccinated.

Question 1509: Among the toxins produced by Clostridium botulinum, which is the most potent?

A. Toxin A (Correct Answer)
B. Toxin C
C. Toxin D
D. Toxin F

Explanation: **Explanation:** *Clostridium botulinum* produces one of the most lethal substances known to man: the botulinum neurotoxin (BoNT). There are eight distinct antigenic types (A, B, C1, C2, D, E, F, and G). **Why Option A is Correct:** **Toxin Type A** is recognized as the **most potent** and lethal variety. It has the highest toxicity per microgram and is the most common cause of severe, life-threatening botulism in humans. It acts by proteolytically cleaving SNARE proteins (specifically SNAP-25) at the neuromuscular junction, preventing the release of acetylcholine and leading to flaccid paralysis. **Analysis of Incorrect Options:** * **Option B (Toxin C):** This type primarily causes botulism in animals (especially birds and turtles) and is rarely associated with human disease. * **Option C (Toxin D):** This type is predominantly associated with "lamziekte" in cattle and forage poisoning in horses; it is not a major cause of human illness. * **Option D (Toxin F):** While Toxin F can cause human botulism, it is much rarer than types A, B, and E, and is generally considered less potent in clinical scenarios. **High-Yield Clinical Pearls for NEET-PG:** * **Human Pathogenic Types:** Types **A, B, and E** are the primary causes of human botulism. Type E is specifically associated with **contaminated seafood/fish**. * **Mechanism of Action:** It is a zinc-dependent endopeptidase that inhibits acetylcholine release (Pre-synaptic blockade). * **Clinical Presentation:** Characterized by the "4 Ds": Diplopia, Dysarthria, Dysphagia, and Dyspnea, followed by symmetric **descending flaccid paralysis**. * **Therapeutic Use:** In extremely dilute forms, Toxin A (Botox) is used for conditions like achalasia cardia, strabismus, blepharospasm, and cosmetic wrinkle reduction.

Question 1510: What is true about B. recurrentis?

A. Causes leptospirosis
B. Water borne disease
C. Vector borne disease (Correct Answer)
D. Transmitted by tick

Explanation: **Explanation:** *Borrelia recurrentis* is the causative agent of **Epidemic Relapsing Fever**. The correct answer is **Option C** because it is a classic example of a **vector-borne disease**. 1. **Why Option C is Correct:** *B. recurrentis* is transmitted to humans via the **human body louse** (*Pediculus humanus corporis*). Unlike most other Borrelia species, it is not transmitted through a bite; rather, the louse is crushed, and the spirochetes enter the human body through abraded skin or mucous membranes. 2. **Why Options A & B are Incorrect:** Leptospirosis is caused by *Leptospira interrogans*. While both are spirochetes, Leptospirosis is primarily a **water-borne zoonosis** (transmitted via water contaminated with the urine of infected rodents), not a vector-borne disease. 3. **Why Option D is Incorrect:** While many *Borrelia* species cause "Endemic Relapsing Fever" via **soft ticks** (*Ornithodoros*), *B. recurrentis* specifically causes the **Epidemic** form and is strictly **louse-borne**. **High-Yield Clinical Pearls for NEET-PG:** * **Antigenic Variation:** The hallmark of *B. recurrentis* is its ability to undergo programmed DNA rearrangement of surface proteins (VMP - Variable Major Proteins). This leads to successive waves of fever (relapses) as the bacteria "change their coat" to evade the host's immune system. * **Diagnosis:** Best diagnosed by seeing the large spirochetes on a **peripheral blood smear** (Giemsa or Wright stain) during the febrile period. * **Jarisch-Herxheimer Reaction:** A critical complication to watch for after starting antibiotic treatment (usually Tetracyclines), caused by the sudden release of endotoxin-like substances from dying spirochetes.

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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