Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 391: Which of the following statements about pneumococcus is false?

A. Capsule aids in virulence.
B. It is the commonest cause of otitis media.
C. It causes a mild form of meningitis. (Correct Answer)
D. The respiratory tract of carriers is the most important source of infection.

Explanation: **Explanation:** **Pneumococcus (*Streptococcus pneumoniae*)** is a leading cause of invasive bacterial diseases. The statement that it causes a "mild form of meningitis" is **false** because pneumococcal meningitis is, in fact, one of the most severe forms of pyogenic meningitis. It is associated with high mortality rates (up to 20-30%) and a high frequency of permanent neurological sequelae, such as deafness and cognitive deficits, compared to other pathogens like *Neisseria meningitidis*. **Analysis of other options:** * **Option A (Capsule):** The polysaccharide capsule is the primary virulence factor. It prevents phagocytosis by host immune cells. Non-capsulated strains are generally avirulent. * **Option B (Otitis Media):** *S. pneumoniae* remains the #1 most common bacterial cause of acute otitis media and community-acquired pneumonia (CAP) across all age groups. * **Option D (Carriers):** Humans are the only natural reservoir. Asymptomatic nasopharyngeal carriage (especially in children) is the primary source of spread via respiratory droplets. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Culture:** Shows **Alpha-hemolysis** on blood agar; colonies exhibit a "draughtsman" or "checkerboard" appearance due to autolysis. * **Identification:** Bile soluble and **Optochin sensitive** (distinguishes it from *S. viridans*). * **Quellung Reaction:** Swelling of the capsule when treated with specific antiserum (Positive). * **Vaccines:** PPSV23 (T-independent, for adults) and PCV13 (Conjugate, T-dependent, for children).

Question 392: Citron bodies are seen in which of the following bacteria?

A. Clostridium perfringens
B. Clostridium difficile
C. Clostridium septicum (Correct Answer)
D. Clostridium tertium

Explanation: **Explanation:** **Citron bodies** (also known as "lemon-shaped" or "leaf-shaped" cells) are a characteristic morphological feature of **Clostridium septicum**. When viewed in smears from tissue exudates or cultures, these organisms appear as pleomorphic, swollen, gram-positive rods that often show intense staining. **Why Clostridium septicum is correct:** * *C. septicum* is a highly invasive, motile, anaerobic bacterium. * In clinical specimens (like gas gangrene muscle or peritoneal fluid), it exhibits marked pleomorphism. The development of these "citron bodies" is a diagnostic hallmark used to differentiate it from other Clostridia. **Why other options are incorrect:** * **Clostridium perfringens:** Known for its "box-car" shaped, thick, non-motile rods. It is the most common cause of gas gangrene but does not form citron bodies. * **Clostridium difficile:** Characterized by "drumstick" appearance due to subterminal spores; it is primarily associated with pseudomembranous colitis. * **Clostridium tertium:** A non-toxigenic, aerotolerant species often associated with neutropenic patients; it does not exhibit the classic citron morphology. **High-Yield Clinical Pearls for NEET-PG:** 1. **Malignancy Association:** Isolation of *C. septicum* in a blood culture is a major "red flag" for underlying **colorectal carcinoma** or hematologic malignancies (due to the breakdown of the mucosal barrier). 2. **Gas Gangrene:** While *C. perfringens* is the #1 cause, *C. septicum* is the most common cause of **nontraumatic (spontaneous) gas gangrene**. 3. **Culture:** It is highly motile and produces a characteristic "spreading" or "swarming" growth on agar plates, similar to *Proteus*.

Question 393: Lancefield grouping of streptococci is based on which characteristic?

A. M proteins
B. Carbohydrate (C) antigen on the cell wall (Correct Answer)
C. Bile solubility
D. Sugar fermentation

Explanation: ### Explanation **Lancefield grouping** is a serological classification system developed by Rebecca Lancefield to categorize catalase-negative, coagulase-negative bacteria, primarily the genus *Streptococcus*. #### Why Option B is Correct: The classification is based on the **group-specific polysaccharide (C) antigen** located in the bacterial cell wall. Using specific antisera, streptococci are divided into groups labeled **A to V** (with some omissions). For example, Group A Streptococcus (GAS) refers to *S. pyogenes*, and Group B Streptococcus (GBS) refers to *S. agalactiae*. #### Why Other Options are Incorrect: * **Option A (M proteins):** While M protein is the major virulence factor of *S. pyogenes*, it is used for **Griffith typing** (subtyping within Group A), not the primary Lancefield grouping. * **Option C (Bile solubility):** This is a biochemical test used specifically to differentiate *Streptococcus pneumoniae* (bile soluble) from other alpha-hemolytic viridans streptococci (bile insoluble). *S. pneumoniae* does not possess a Lancefield antigen. * **Option D (Sugar fermentation):** This refers to biochemical properties used for species identification (e.g., the Christie-Atkins-Munch-Peterson or CAMP test, or trehalose fermentation), but it is not the basis of the Lancefield serological system. #### High-Yield Clinical Pearls for NEET-PG: * **Group A (S. pyogenes):** Bacitracin sensitive; causes Pharyngitis, Rheumatic fever, and PSGN. * **Group B (S. agalactiae):** CAMP test positive; leading cause of neonatal meningitis and sepsis. * **Group D:** Includes *Enterococcus* and *S. bovis* (associated with colonic malignancy). * **Note:** *Streptococcus pneumoniae* and the Viridans group **do not** have Lancefield antigens and are thus "non-groupable."

Question 394: The tuberculin test is associated with all of the following findings except:

A. Indicates cell-mediated immunity against the bacteria.
B. Likely to be positive in prior exposure to mycobacteria.
C. Can distinguish between infection and disease. (Correct Answer)
D. May be false negative in immunosuppression.

Explanation: **Explanation:** The Tuberculin Skin Test (TST), also known as the Mantoux test, is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**. **1. Why Option C is the Correct Answer:** The Tuberculin test **cannot distinguish between latent infection and active disease**. A positive result simply indicates that the individual’s immune system has been sensitized to mycobacterial antigens. It does not provide information on whether the bacteria are dormant (latent TB) or actively replicating and causing symptoms (active TB disease). Diagnosis of active disease requires clinical correlation, radiology, and microbiology (sputum microscopy/culture). **2. Analysis of Other Options:** * **Option A:** TST is a direct measure of **cell-mediated immunity (CMI)**. It relies on sensitized T-lymphocytes migrating to the injection site and releasing cytokines. * **Option B:** Prior exposure to *M. tuberculosis*, non-tuberculous mycobacteria (NTM), or the **BCG vaccine** can lead to a positive TST, as the immune system "remembers" the antigens. * **Option D:** In immunosuppressed states (e.g., HIV with low CD4 count, sarcoidosis, malnutrition, or steroid use), the body may fail to mount a T-cell response, leading to a false negative result, known as **anergy**. **High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** Results are read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **Induration thresholds:** In India, an induration of **≥10 mm** is generally considered positive. In HIV-positive patients, **≥5 mm** is significant. * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)**, like QuantiFERON-TB Gold, is more specific than TST as it is not affected by prior BCG vaccination.

Question 395: Which is the common feature of spirillary rat bite fever?

A. Lymphadenopathy (Correct Answer)
B. Endocarditis
C. Hepatosplenomegaly
D. Amegakaryocytic thrombocytopenia

Explanation: **Explanation:** Rat-bite fever (RBF) is caused by two distinct organisms: *Streptobacillus moniliformis* (streptobacillary RBF) and *Spirillum minus* (spirillary RBF, also known as **Sodoku**). **Why Lymphadenopathy is correct:** In **Spirillary RBF (*S. minus*)**, the initial bite wound typically heals but then reactivates, becoming indurated and occasionally ulcerating (chancre-like). A hallmark feature that distinguishes it from the streptobacillary form is **marked regional lymphadenopathy** (lymphangitis and lymphadenitis) accompanying the site of the bite. This reflects the lymphatic spread of the spirochete-like organism. **Analysis of Incorrect Options:** * **B. Endocarditis:** While a known complication of *Streptobacillus moniliformis*, it is extremely rare in *Spirillum minus* infections. * **C. Hepatosplenomegaly:** Though systemic involvement occurs, hepatosplenomegaly is not a "common" or defining diagnostic feature compared to the localized lymphatic involvement. * **D. Amegakaryocytic thrombocytopenia:** This is not a feature of RBF. While some systemic infections cause reactive thrombocytopenia, amegakaryocytic forms are typically related to bone marrow failure or specific viral triggers (e.g., CMV). **High-Yield Clinical Pearls for NEET-PG:** * **Sodoku (*S. minus*):** Characterized by a prolonged incubation (1–4 weeks), reactivation of the bite site, and **lymphadenopathy**. Arthritis is rare. * **Streptobacillary RBF:** Characterized by a short incubation (<7 days), rapid onset of fever, and **migratory polyarthritis** (50% of cases). The bite wound usually heals without reactivation. * **Diagnosis:** *S. minus* cannot be cultured on artificial media; it is diagnosed via **dark-field microscopy** of blood/exudate or animal inoculation. * **Treatment:** Penicillin G is the drug of choice for both forms.

Question 396: Which of the following is true about Mycobacterium tuberculosis?

A. Strict aerobes (Correct Answer)
B. Gram negative
C. Thin cell wall
D. All of the above

Explanation: **Explanation:** *Mycobacterium tuberculosis* (MTB) is the causative agent of tuberculosis, characterized by unique structural and metabolic properties that are frequently tested in NEET-PG. **Why Option A is Correct:** *Mycobacterium tuberculosis* is a **strict (obligate) aerobe**. It requires high oxygen concentrations for growth and energy production. This metabolic requirement explains its clinical predilection for the **apices of the lungs**, where the ventilation-perfusion ratio is highest, providing an oxygen-rich environment conducive to its multiplication. **Why Other Options are Incorrect:** * **Option B (Gram-negative):** MTB is not classified as Gram-negative. While it possesses an outer membrane, it is technically **Gram-positive** due to its genetic lineage. However, it does not take up Gram stain well because of its high lipid content; instead, it is identified using the **Ziehl-Neelsen (Acid-Fast) stain**. * **Option C (Thin cell wall):** MTB is famous for having an exceptionally **thick, lipid-rich cell wall**. Approximately 60% of the cell wall weight is comprised of lipids, specifically **mycolic acids**. This thick barrier makes the organism resistant to desiccation, common antibiotics, and acidic/alkaline compounds. **High-Yield Clinical Pearls for NEET-PG:** * **Generation Time:** Very slow (15–20 hours), leading to long incubation periods and the need for prolonged culture (up to 6–8 weeks on LJ medium). * **Virulence Factor:** **Cord Factor** (trehalose dimycolate) is responsible for the parallel "serpentine cord" growth pattern and inhibits neutrophil migration. * **Culture Media:** Lowenstein-Jensen (LJ) medium is the gold standard (solid); MGIT (Mycobacteria Growth Indicator Tube) is used for rapid liquid culture. * **Acid-Fastness:** Attributed to the presence of long-chain mycolic acids in the cell wall.

Question 397: Which specific test is used for the diagnosis of Treponema pallidum infection?

A. VDRL
B. Kahn's test
C. TPHA (Correct Answer)
D. Widal test

Explanation: **Explanation:** The diagnosis of Syphilis (*Treponema pallidum*) is divided into two categories: Non-specific (Nontreponemal) tests and **Specific (Treponemal) tests**. **Why TPHA is the correct answer:** **TPHA (Treponema pallidum Hemagglutination Assay)** is a specific treponemal test. It detects IgM and IgG antibodies directed specifically against the surface antigens of *T. pallidum*. Unlike non-specific tests, TPHA remains positive for life even after successful treatment, making it an excellent confirmatory test but unsuitable for monitoring treatment response. **Analysis of Incorrect Options:** * **VDRL (Venereal Disease Research Laboratory):** This is a non-specific (nontreponemal) screening test. It detects **reagin antibodies** (anti-cardiolipin), not the bacteria itself. It is used for screening and monitoring treatment efficacy but can yield biological false positives (e.g., in SLE, leprosy, or pregnancy). * **Kahn’s test:** An older, obsolete tube flocculation test for syphilis. Like VDRL, it is a non-specific test and has been largely replaced by modern assays. * **Widal test:** This is a serological test used for the diagnosis of **Enteric (Typhoid) fever**, detecting antibodies against *Salmonella typhi* O and H antigens. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test of Choice:** VDRL / RPR. * **Confirmatory Test of Choice:** FTA-ABS (Fluorescent Treponemal Antibody Absorption) or TPHA. * **Neurosyphilis:** VDRL on CSF is the gold standard for diagnosis. * **Treatment Monitoring:** Only non-specific tests (VDRL/RPR) are used to monitor response to therapy, as their titers fall after treatment. Specific tests (TPHA/FTA-ABS) stay "positive for life."

Question 398: In a neonate with meningitis caused by gram-positive cocci with a CAMP test positive, what is the most common source of infection?

A. Genital tract (Correct Answer)
B. Respiratory tract
C. Hematogenous
D. May be any

Explanation: **Explanation:** The clinical presentation describes **Group B Streptococcus (GBS)**, also known as ***Streptococcus agalactiae***. This organism is a Gram-positive coccus characterized by a positive **CAMP test** (production of a diffusible protein that enhances the beta-hemolysis of *Staphylococcus aureus*). **1. Why the Genital Tract is Correct:** *Streptococcus agalactiae* is a normal commensal of the female **genital tract** and lower gastrointestinal tract, colonizing approximately 10–30% of pregnant women. Neonatal infection occurs via **vertical transmission**—either through ascending infection (premature rupture of membranes) or, more commonly, during passage through the birth canal. It is the leading cause of neonatal sepsis and meningitis. **2. Why Other Options are Incorrect:** * **B. Respiratory tract:** While GBS can cause pneumonia in neonates, the primary *source* is the maternal birth canal, not a respiratory droplet transmission. * **C. Hematogenous:** While the bacteria spread hematogenously *within* the neonate to reach the meninges, the question asks for the **source** of the infection, which is the maternal flora. * **D. May be any:** This is incorrect because there is a specific, well-documented epidemiological link between maternal vaginal colonization and neonatal GBS disease. **High-Yield Clinical Pearls for NEET-PG:** * **Identification:** GBS is Catalase negative, Bacitracin resistant, and Hippurate hydrolysis positive. * **CAMP Factor:** It is a polypeptide that acts synergistically with Staphylococcal β-hemolysin (sphingomyelinase C). * **Prevention:** Universal screening of pregnant women at 35–37 weeks gestation and intrapartum antibiotic prophylaxis (usually Penicillin G) significantly reduces the risk of early-onset disease. * **Other Neonatal Meningitis Causes:** *E. coli* (Gram-negative bacilli) and *Listeria monocytogenes* (Gram-positive bacilli).

Question 399: Which genes encode virulence factors of M. tuberculosis?

A. Kat-G gene
B. rpoV
C. Erp gene
D. All of the above (Correct Answer)

Explanation: **Explanation:** The virulence of *Mycobacterium tuberculosis* (M.tb) is polygenic, involving various genes that facilitate survival within host macrophages, evasion of the immune system, and resistance to oxidative stress. 1. **Kat-G gene:** This gene encodes the enzyme **Catalase-peroxidase**. While primarily known for activating the prodrug Isoniazid (INH), its biological role as a virulence factor is crucial. It protects the bacterium against reactive oxygen species (ROS) and oxidative bursts triggered by host phagocytes. Loss of *katG* often leads to a significant reduction in virulence in animal models. 2. **rpoV gene:** This gene encodes a principal sigma factor (Sigma A) that regulates the transcription of several essential genes. It is vital for the intracellular survival of M.tb during the early stages of infection and helps the pathogen adapt to the stressful environment inside the macrophage. 3. **Erp (Exported Repetitive Protein) gene:** This gene encodes a surface-exposed protein essential for optimal multiplication during the initial phase of infection. Studies show that *erp* mutants are significantly impaired in their ability to replicate within macrophages and cause disease. **Conclusion:** Since all three genes contribute to the pathogen's ability to survive, replicate, and cause disease within the host, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Kat-G Mutation:** The most common cause of high-level resistance to **Isoniazid (INH)**. * **rpoB Mutation:** The most common cause of resistance to **Rifampicin** (targets RNA polymerase). * **Cord Factor (Trehalose dimycolate):** The most important morphological virulence factor, responsible for the "serpentine cord" appearance and inhibition of neutrophil migration. * **LAM (Lipoarabinomannan):** Inhibits phagosome-lysosome fusion.

Question 400: Which bacteria are capable of growing in acidic media?

A. Vibrio cholerae
B. Lactobacillus (Correct Answer)
C. Natronobacterium
D. Bacillus firmus

Explanation: ### Explanation The ability of bacteria to grow at specific pH levels is a key diagnostic and physiological characteristic. Bacteria are classified based on their optimal pH range: **Acidophiles** (pH < 5.5), **Neutrophiles** (pH 5.5–8.0), and **Alkaliphiles** (pH > 8.0). **Why Lactobacillus is Correct:** *Lactobacillus* species are classic **acidophiles**. They ferment carbohydrates to produce lactic acid, which lowers the environmental pH. They are not only acid-tolerant but thrive in acidic conditions (often pH 4.5 or lower). This property is clinically significant in the vaginal flora (Döderlein’s bacilli), where they maintain an acidic environment to inhibit the growth of pathogens. **Analysis of Incorrect Options:** * **Vibrio cholerae:** This is a classic **alkaliphile**. It is highly sensitive to stomach acid and thrives in alkaline media (pH 8.5–9.0). This is why transport media like Venkatraman-Ramakrishnan (VR) medium have a high pH. * **Natronobacterium:** These are **extreme alkaliphiles** (and often halophiles) found in soda lakes, typically requiring a pH of 9.0 to 10.0 to grow. * **Bacillus firmus:** This is another example of an **alkaliphilic** bacterium, often used in research to study alkaline adaptation mechanisms. **High-Yield NEET-PG Pearls:** 1. **Döderlein’s Bacilli:** These are *Lactobacillus* species in the vagina that maintain a pH of ~4.0, protecting against Bacterial Vaginosis. 2. **Culture Media for Vibrio:** Because *V. cholerae* loves alkali, selective media like **TCBS** (Thiosulfate-Citrate-Bile Salts-Sucrose) and enrichment media like **Alkaline Peptone Water (APW)** utilize high pH to inhibit other enteric flora. 3. **Helicobacter pylori:** While it survives in the acidic stomach, it is actually a **neutrophile**. It survives by producing **Urease**, which creates a neutralizing "ammonia cloud" around the bacterium.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

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