Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 21: Vi antigen is present in all of the following bacteria except?

A. S.typhi
B. S.paratyphi A (Correct Answer)
C. S.paratyphi C
D. S.dublin

Explanation: **Explanation:** The **Vi (Virulence) antigen** is a surface capsular polysaccharide (K antigen) found in certain strains of *Salmonella*. It acts as a protective layer that masks the underlying O-antigen, preventing phagocytosis and complement-mediated killing, thereby increasing the virulence of the organism. **Why S. paratyphi A is the correct answer:** Among the common causes of enteric fever, **Salmonella paratyphi A** is uniquely characterized by the **absence of the Vi antigen**. This is a high-yield diagnostic point: because it lacks the Vi antigen, patients with Paratyphi A infections will not show a rise in anti-Vi antibodies, and vaccines targeting the Vi antigen (like the Typhim Vi vaccine) do not provide protection against this specific serotype. **Analysis of Incorrect Options:** * **S. typhi:** This is the most well-known carrier of the Vi antigen. It is the primary target for Vi-based typhoid vaccines. * **S. paratyphi C:** Unlike Paratyphi A and B, Paratyphi C expresses the Vi antigen. * **S. dublin:** This is a primarily zoonotic (bovine) serotype that is known to express the Vi antigen, occasionally causing systemic infections in humans. **NEET-PG High-Yield Pearls:** 1. **Detection:** If the Vi antigen is present, it may mask the O-antigen during agglutination tests (Widal). To unmask the O-antigen, the bacterial suspension must be **boiled** to destroy the heat-labile Vi antigen. 2. **Carrier State:** Detection of Vi antibodies is a useful screening tool for identifying **chronic carriers** of *S. typhi*. 3. **Vaccinology:** The Vi polysaccharide vaccine is a subunit vaccine used for typhoid prophylaxis, but it is ineffective against *S. paratyphi A*.

Question 22: Atypical pneumonia is caused by all of the following pathogens except:

A. Mycoplasma
B. Adenovirus
C. Chlamydia
D. Haemophilus (Correct Answer)

Explanation: **Explanation:** **Atypical pneumonia** refers to pneumonia that presents with a subacute onset, non-productive cough, and systemic symptoms (headache, malaise) rather than the classic signs of consolidation. Radiologically, it often shows diffuse interstitial infiltrates ("patchy" appearance) that look worse than the clinical presentation. **Why Haemophilus is the correct answer:** *Haemophilus influenzae* is a classic cause of **Typical Pneumonia**. It is a Gram-negative coccobacillus that typically causes lobar consolidation, productive cough with purulent sputum, and high fever. It possesses a polysaccharide capsule (Type b being the most virulent) and is a common cause of community-acquired pneumonia (CAP), especially in patients with underlying COPD. **Why the other options are incorrect:** * **Mycoplasma pneumoniae:** The most common cause of atypical pneumonia ("Walking Pneumonia"). It lacks a cell wall (resistant to beta-lactams) and is associated with cold agglutinins. * **Adenovirus:** A common viral cause of atypical pneumonia, particularly in military recruits and children. * **Chlamydia:** Both *C. pneumoniae* and *C. psittaci* are well-known causes of atypical pneumonia, often presenting with a persistent dry cough and pharyngitis. **High-Yield Clinical Pearls for NEET-PG:** * **Mycoplasma:** Look for "Bullous Myringitis" or "Erythema Multiforme" in the clinical vignette. * **Legionella:** Another atypical pathogen; look for hyponatremia, diarrhea, and history of exposure to air conditioners/water towers. * **Treatment:** Atypical pathogens lack a cell wall or are intracellular; therefore, they are **intrinsically resistant to Beta-lactams**. The drugs of choice are **Macrolides** (Azithromycin), Tetracyclines (Doxycycline), or Fluoroquinolones.

Question 23: What is the recommended approach for BCG vaccination in an HIV-positive newborn?

A. Contraindicated (Correct Answer)
B. Double dilution
C. Half dilution
D. Dose doubled

Explanation: **Explanation:** The correct answer is **A. Contraindicated**. **1. Why it is correct:** BCG (Bacillus Calmette-Guérin) is a **live-attenuated vaccine** derived from *Mycobacterium bovis*. In individuals with compromised immune systems, such as HIV-positive newborns, the body cannot effectively contain the attenuated bacteria. This poses a significant risk of **Disseminated BCG infection**, which is often fatal. According to WHO and National AIDS Control Organization (NACO) guidelines, BCG should be withheld in infants known to be HIV-infected. If the infant's status is unknown but they show clinical signs/symptoms suggestive of HIV, the vaccine is also contraindicated. **2. Why incorrect options are wrong:** * **B, C, & D (Dilution/Dose adjustments):** Modifying the concentration or volume of the vaccine does not mitigate the risk of dissemination in an immunocompromised host. The issue is not the quantity of the bacteria, but the host's inability to mount a cell-mediated immune response against a live pathogen. There is no clinical protocol involving "double" or "half" dilutions for BCG in the context of HIV. **3. High-Yield Clinical Pearls for NEET-PG:** * **General Rule:** All live vaccines (BCG, OPV, MMR, Yellow Fever, Varicella) are generally contraindicated in symptomatic HIV/immunocompromised states. * **Exception:** In asymptomatic HIV-infected children, WHO suggests MMR and Measles vaccines can be given, but BCG remains strictly contraindicated if the HIV status is confirmed positive. * **Normal BCG Dose:** 0.1 ml (0.05 ml for neonates <1 month) given intradermally over the left deltoid. * **Diagnostic Tip:** If a child presents with hepatosplenomegaly and lymphadenopathy after BCG vaccination, suspect **SCID** (Severe Combined Immunodeficiency) or **HIV**.

Question 24: Trench mouth is caused by which of the following microorganisms?

A. Beta-hemolytic streptococci
B. Borrelia vincenti (Correct Answer)
C. Epstein Barr virus
D. Diphtheria

Explanation: **Explanation:** **Trench Mouth**, clinically known as **Acute Necrotizing Ulcerative Gingivitis (ANUG)** or Vincent’s Angina, is a severe form of gingivitis characterized by painful, bleeding gums and ulceration of the interdental papillae. **Why Borrelia vincenti is correct:** The disease is caused by a **synergistic polymicrobial infection** involving a fusiform bacillus (*Fusobacterium fusiforme*) and a spirochete (**Borrelia vincenti**). This combination is often referred to as the "fusospirochetal complex." These organisms are part of the normal oral flora but become pathogenic under conditions of poor oral hygiene, severe stress, or malnutrition (common in soldiers in "trenches," hence the name). **Why other options are incorrect:** * **Beta-hemolytic streptococci:** These are primarily associated with pharyngitis (Strep throat), scarlet fever, and skin infections like impetigo, but not the necrotic gingivitis seen in Trench mouth. * **Epstein Barr virus (EBV):** EBV causes Infectious Mononucleosis and is associated with Oral Hairy Leukoplakia and Burkitt Lymphoma, but it does not cause acute necrotizing gingival lesions. * **Diphtheria:** Caused by *Corynebacterium diphtheriae*, it presents with a thick, grey "pseudomembrane" over the tonsils and pharynx, not localized necrotic ulcers of the gums. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Pain, "punched-out" interdental papillae, and halitosis (fetid breath). * **Diagnosis:** Primarily clinical; Gram stain shows a characteristic mixture of Gram-negative fusiform rods and spirochetes. * **Predisposing factors:** Smoking, psychological stress, and HIV infection (often an early sign of immunosuppression). * **Treatment:** Debridement and antibiotics (Metronidazole is highly effective against the anaerobic components).

Question 25: Which of the following organisms is identified by interferon?

A. Staphylococcus
B. Leptospira
C. Campylobacter
D. Tuberculosis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Tuberculosis**. The identification of *Mycobacterium tuberculosis* (MTB) infection can be achieved through **Interferon-Gamma Release Assays (IGRAs)**, such as the QuantiFERON-TB Gold test. **Why Tuberculosis is Correct:** IGRAs are *in vitro* blood tests used to diagnose latent TB infection. The test works on the principle of cell-mediated immunity: T-lymphocytes of an individual previously sensitized with MTB antigens (ESAT-6 and CFP-10) will release **Interferon-gamma (IFN-γ)** when re-exposed to these specific antigens in a lab setting. Unlike the Tuberculin Skin Test (Mantoux), IGRAs do not cross-react with the BCG vaccine. **Why Other Options are Incorrect:** * **Staphylococcus:** Identified primarily through Gram stain (Gram-positive cocci in clusters), culture on blood agar, and biochemical tests like Catalase and Coagulase. * **Leptospira:** Diagnosed via Dark-field microscopy, culture on specialized media (EMJH medium), or the Microscopic Agglutination Test (MAT), which detects antibodies, not interferon. * **Campylobacter:** Identified by its characteristic "seagull-wing" morphology on Gram stain, stool culture on Skirrow’s medium, and its thermophilic nature (growth at 42°C). **NEET-PG High-Yield Pearls:** * **IGRA Advantage:** It has higher specificity than the Mantoux test because the antigens used (ESAT-6/CFP-10) are absent from the *M. bovis* BCG strain and most non-tuberculous mycobacteria. * **Limitation:** IGRAs cannot differentiate between **Latent TB Infection (LTBI)** and **Active TB disease**. * **Cytokine Profile:** IFN-γ is a critical Th1 cytokine required for macrophage activation to contain MTB within granulomas. Deficiency in the IFN-γ pathway increases susceptibility to severe mycobacterial infections.

Question 26: What is the primary toxin involved in streptococcal toxic shock syndrome?

A. Pyrogenic exotoxin (Correct Answer)
B. Erythrogenic exotoxin
C. Hemolysin
D. Neurotoxin

Explanation: **Explanation:** Streptococcal Toxic Shock Syndrome (STSS) is a severe, life-threatening condition primarily caused by **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*. **1. Why Option A is Correct:** The pathogenesis of STSS is driven by **Streptococcal Pyrogenic Exotoxins (SPEs)**, most notably **SPE-A and SPE-C**. These act as **Superantigens**. Unlike regular antigens, superantigens bypass normal antigen processing and bind directly to the MHC Class II molecules on antigen-presenting cells and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells (up to 20%), resulting in a "cytokine storm" (IL-1, IL-6, TNF-α, and IFN-γ). This overwhelming inflammatory response causes capillary leak, hypotension, and multi-organ failure. **2. Why Other Options are Incorrect:** * **Option B (Erythrogenic exotoxin):** While these are also pyrogenic exotoxins, the term is historically associated with the rash of **Scarlet Fever**. Though related, "Pyrogenic exotoxin" is the more accurate term for the systemic shock syndrome. * **Option C (Hemolysin):** *S. pyogenes* produces hemolysins like Streptolysin O and S. These are responsible for cell lysis (beta-hemolysis on blood agar) but do not trigger the systemic superantigen response required for STSS. * **Option D (Neurotoxin):** *Streptococcus* does not produce neurotoxins; these are characteristic of organisms like *Clostridium botulinum* or *Clostridium tetani*. **High-Yield Clinical Pearls for NEET-PG:** * **STSS vs. Staph TSS:** STSS is often associated with **necrotizing fasciitis** and bacteremia, whereas Staphylococcal TSS (caused by TSST-1) is often associated with tampon use or focal infections without bacteremia. * **M Protein:** The M protein (specifically types 1 and 3) is the chief virulence factor that helps the bacteria resist phagocytosis and is strongly linked to STSS strains. * **Treatment:** Clindamycin is often added to penicillin because it inhibits protein synthesis, thereby shutting down toxin production (the "Eagle Effect").

Question 27: Triple sugar iron agar shows gas production with a red slant and yellow butt. What organism is cultured?

A. Escherichia coli
B. Shigella flexneri (Correct Answer)
C. Pseudomonas
D. None of the above

Explanation: ### Explanation The **Triple Sugar Iron (TSI) Agar** test is a differential medium used to identify Gram-negative enteric bacilli based on their ability to ferment glucose, lactose, and sucrose, and produce hydrogen sulfide ($H_2S$) or gas. **1. Why Shigella flexneri is correct:** * **Red Slant (Alkaline/K):** Indicates the organism is a **non-lactose fermenter (NLF)**. Once the small amount of glucose (0.1%) is exhausted, the bacteria utilize peptones in the slant aerobically, producing ammonia and raising the pH. * **Yellow Butt (Acid/A):** Indicates **glucose fermentation**. Since glucose is fermented anaerobically in the butt, the acid produced maintains a yellow color. * **Gas Production:** While most *Shigella* species are anaerogenic (do not produce gas), **Shigella flexneri (specifically serotype 6)** and *Shigella boydii* can occasionally produce gas during fermentation, matching the question's description (K/A with gas). **2. Why other options are incorrect:** * **Escherichia coli:** It is a **lactose fermenter**. It produces a **Yellow Slant/Yellow Butt (A/A)** with significant gas production. * **Pseudomonas:** It is a **non-fermenter**. It cannot ferment glucose or lactose, resulting in a **Red Slant/Red Butt (K/K)**. * **None of the above:** Incorrect, as *S. flexneri* fits the biochemical profile described. **High-Yield Clinical Pearls for NEET-PG:** * **K/A (Red/Yellow):** Non-lactose fermenters (*Shigella, Salmonella, Proteus, Yersinia*). * **A/A (Yellow/Yellow):** Lactose fermenters (*E. coli, Klebsiella, Enterobacter*). * **$H_2S$ Production (Blackening):** *Salmonella typhi* (slight black ring), *Proteus*, and *Citrobacter*. * **Key Exception:** *Shigella sonnei* is a **late lactose fermenter**, which may show a change in the slant upon prolonged incubation.

Question 28: Which gene is primarily responsible for the pathogenesis of Helicobacter pylori?

A. CagA (Correct Answer)
B. CagB
C. TaxA
D. Hbx

Explanation: **Explanation:** **Helicobacter pylori** is a Gram-negative, microaerophilic bacterium strongly associated with chronic gastritis, peptic ulcer disease (PUD), and gastric adenocarcinoma. **Why CagA is Correct:** The **CagA (Cytotoxin-associated gene A)** is the most important virulence factor in the pathogenesis of *H. pylori*. It is located on a 40-kb genomic fragment known as the **cag Pathogenicity Island (cag PAI)**. The bacterium uses a **Type IV secretion system** (a molecular syringe) to inject the CagA protein into gastric epithelial cells. Once inside, CagA undergoes phosphorylation and disrupts host cell signaling, leading to: * Alteration of the cell cytoskeleton (the "hummingbird phenotype"). * Disruption of tight junctions. * Induction of a pro-inflammatory response (IL-8 production). * Increased risk of gastric malignancy (CagA is considered a bacterial oncoprotein). **Analysis of Incorrect Options:** * **CagB:** While part of the *cag* pathogenicity island, it is a structural component of the secretion system rather than the primary effector protein responsible for pathogenesis. * **TaxA:** This is not associated with *H. pylori*. It is often confused with *Tax*, an oncogene associated with HTLV-1 (Human T-cell Lymphotropic Virus). * **Hbx:** This refers to the HBx protein associated with Hepatitis B Virus (HBV) oncogenesis, not bacterial pathogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **VacA (Vacuolating cytotoxin):** Another key virulence factor that induces large vacuoles in gastric cells and promotes apoptosis. * **Urease:** Essential for survival in the acidic stomach; it neutralizes gastric acid by producing ammonia. This is the basis for the **Urea Breath Test** and **Rapid Urease Test (RUT)**. * **MALToma:** *H. pylori* is the only bacterium classified as a **Class I Carcinogen** by the WHO. Eradication of the infection can lead to regression of low-grade MALT lymphoma.

Question 29: Which among the following is bile esculin positive and shows growth in 6.5% sodium chloride?

A. Streptococcus agalactiae
B. Streptococcus pneumoniae
C. Enterococcus faecalis (Correct Answer)
D. Streptococcus viridans

Explanation: **Explanation:** The correct answer is **Enterococcus faecalis**. This question tests the biochemical differentiation of Catalase-negative, Gram-positive cocci (Streptococci and Enterococci). **Why Enterococcus faecalis is correct:** Enterococci (formerly classified as Group D Streptococci) are unique due to their ability to survive in harsh environments. 1. **Bile Esculin Test:** Enterococci can grow in the presence of 40% bile and hydrolyze esculin to esculetin, which reacts with ferric ions to turn the medium black. 2. **6.5% NaCl Tolerance:** This is the definitive test to differentiate Enterococci from non-enterococcal Group D Streptococci (like *S. bovis*). Enterococci can withstand high salt concentrations, whereas most other Streptococci cannot. **Why the other options are incorrect:** * **A. Streptococcus agalactiae (Group B):** It is CAMP test positive and hippurate hydrolysis positive. It does not grow in 6.5% NaCl or hydrolyze bile esculin. * **B. Streptococcus pneumoniae:** It is characterized by alpha-hemolysis, bile solubility, and optochin sensitivity. It is inhibited by high salt concentrations. * **D. Streptococcus viridans:** This is a heterogeneous group of alpha-hemolytic streptococci that are optochin resistant and bile insoluble. They do not grow in 6.5% NaCl. **High-Yield Clinical Pearls for NEET-PG:** * **Enterococci** are notorious for **intrinsic resistance** to many antibiotics (including cephalosporins) and are a leading cause of nosocomial UTIs and subacute bacterial endocarditis (SBE) following GI/GU procedures. * **VRE (Vancomycin-Resistant Enterococci):** A major infection control challenge; resistance is mediated by the *vanA* gene (changing D-Ala-D-Ala to D-Ala-D-Lac). * **PYR Test:** Enterococci are **PYR positive**, a rapid test used for presumptive identification.

Question 30: Which serovar of Chlamydia trachomatis causes Lymphogranuloma venereum (LGV)?

A. Trachoma
B. Inclusion conjunctivitis
C. Non-gonococcal urethritis (NGU)
D. Lymphogranuloma venereum (LGV) (Correct Answer)

Explanation: **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium classified into different serovars based on major outer membrane protein (MOMP) antigens. These serovars exhibit distinct tissue tropisms and clinical manifestations. **1. Why the Correct Answer is Right:** **Lymphogranuloma venereum (LGV)** is caused by **serovars L1, L2, and L3**. Unlike the oculogenital strains, these serovars are more invasive and possess a predilection for lymphatic tissue. The disease typically presents in three stages: a painless primary lesion, followed by painful inguinal lymphadenopathy (the "bubo" stage), and potentially chronic proctocolitis or lymphatic obstruction (elephantiasis) if left untreated. **2. Analysis of Incorrect Options:** * **A. Trachoma:** Caused by **serovars A, B, Ba, and C**. It is a leading cause of preventable blindness worldwide, characterized by chronic follicular conjunctivitis and subsequent scarring (entropion/trichiasis). * **B. Inclusion Conjunctivitis:** Caused by **serovars D–K**. In adults, it is often associated with genital infections (paratrachoma), while in neonates, it occurs via birth canal transmission. * **C. Non-gonococcal Urethritis (NGU):** Also caused by **serovars D–K**. These are the most common cause of sexually transmitted bacterial urethritis and cervicitis, often leading to complications like Pelvic Inflammatory Disease (PID) or epididymitis. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign of Greenblatt:** A characteristic clinical finding in LGV where the inguinal ligament creates a depression between inflamed superficial and deep inguinal lymph nodes. * **Frei Test:** A delayed hypersensitivity skin test previously used for LGV diagnosis (now largely replaced by NAAT). * **Drug of Choice:** Doxycycline (100 mg twice daily) is the preferred treatment for LGV, typically for a longer duration (21 days) compared to standard chlamydial urethritis.

Practice by Chapter

Staphylococci

Practice Questions

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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