Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 461: At what stage of pregnancy can Treponema pallidum cross the placenta?

A. After 36 weeks
B. After 28 weeks
C. After the second trimester
D. At any stage of pregnancy (Correct Answer)

Explanation: **Explanation:** **Treponema pallidum**, the causative agent of syphilis, is a highly invasive spirochete. The correct answer is **At any stage of pregnancy** because current evidence and clinical guidelines (including the CDC and WHO) confirm that transplacental transmission can occur as early as the first trimester. 1. **Why the correct answer is right:** Historically, it was believed that the "Langhans layer" of the cytotrophoblast acted as a barrier, preventing transmission until after the 16th–18th week of gestation. However, modern histopathological studies have demonstrated that *T. pallidum* can cross the placenta and infect the fetus at any gestational age, including the first trimester. While the inflammatory response (and thus the characteristic stigmata of congenital syphilis) is more pronounced after the development of fetal immunocompetence (around the 4th month), the infection itself can occur much earlier, often leading to early spontaneous abortion. 2. **Why incorrect options are wrong:** * **Options A, B, and C:** These options suggest a late-pregnancy restriction. While the *risk* of transmission increases as pregnancy progresses (highest in the third trimester) and the *severity* of fetal damage is linked to the timing of infection, there is no physiological "lock" that prevents the spirochete from crossing the placenta in early pregnancy. **NEET-PG High-Yield Pearls:** * **Transmission Risk:** The risk of vertical transmission is highest (70–100%) in mothers with **primary or secondary syphilis** and lower (approx. 10%) in late latent syphilis. * **Screening:** All pregnant women should be screened at the first prenatal visit using non-treponemal tests (VDRL/RPR). * **Treatment of Choice:** **Parenteral Penicillin G** is the only effective treatment to prevent maternal-fetal transmission. If the mother is allergic, she must be **desensitized** and treated with Penicillin; macrolides are not an alternative as they do not cross the placenta reliably. * **Hutchinson’s Triad:** Interstitial keratitis, sensorineural deafness, and Hutchinson’s teeth (late congenital syphilis).

Question 462: Blood Tellurite media is used for which organism?

A. Mycobacterium tuberculosis
B. Corynebacterium diphtheriae (Correct Answer)
C. Mycoplasma pneumoniae
D. Yersinia pestis

Explanation: **Explanation:** **Corynebacterium diphtheriae** is the correct answer because it requires specific selective media for isolation. Potassium tellurite acts as a selective agent that inhibits the growth of most normal flora of the upper respiratory tract while allowing *C. diphtheriae* to grow. On **Potassium Tellurite Blood Agar (PTBA)** or **McLeod’s medium**, the organism reduces tellurite to metallic tellurium, resulting in characteristic **black or greyish-black colonies**. This is a crucial diagnostic step in identifying the three biotypes: gravis, mitis, and intermedius. **Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** Primarily cultured on **Lowenstein-Jensen (LJ) medium**, which is egg-based and contains malachite green to inhibit contaminants. * **Mycoplasma pneumoniae:** Lacks a cell wall and requires complex media enriched with sterols (cholesterol) and horse serum, such as **PPLO broth** or **Eaton’s agar**, often producing "fried-egg" colonies. * **Yersinia pestis:** Typically grown on Blood Agar or MacConkey agar (showing non-lactose fermenting colonies). A characteristic selective medium is **CIN (Cefsulodin-Irgasan-Novobiocin) agar**. **NEET-PG High-Yield Pearls:** * **Enrichment Media:** Löffler's Serum Slope (LSS) is used for rapid growth (6–8 hours) and enhances the development of **metachromatic granules** (Albert’s stain). * **Selective Media:** Potassium Tellurite Agar (McLeod’s/Hoyle’s medium). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity. * **Morphology:** Described as "Chinese letter" or cuneiform arrangement due to incomplete separation during binary fission (snapping division).

Question 463: What is the most common cause of meningitis in alcoholics?

A. Klebsiella
B. Staphylococcus
C. Pneumococcus (Correct Answer)
D. Haemophilus

Explanation: **Explanation:** **Pneumococcus (*Streptococcus pneumoniae*)** is the most common cause of bacterial meningitis in adults across most risk groups, including alcoholics. In patients with chronic alcoholism, the risk is significantly heightened due to a suppressed immune system, specifically impaired ciliary clearance in the respiratory tract and diminished splenic function (functional hyposplenism). This makes them highly susceptible to encapsulated organisms, with *S. pneumoniae* being the most frequent isolate. **Analysis of Incorrect Options:** * **A. Klebsiella:** While *Klebsiella pneumoniae* is a notorious cause of severe, "currant-jelly" sputum pneumonia in alcoholics, it is a rare cause of meningitis compared to Pneumococcus. * **B. Staphylococcus:** *Staphylococcus aureus* is a common cause of meningitis following neurosurgery, head trauma, or infective endocarditis, but it is not the primary pathogen associated specifically with alcoholism. * **D. Haemophilus:** *Haemophilus influenzae* (Type B) was a leading cause of meningitis in children, but its incidence has drastically decreased due to vaccination. It is less common than Pneumococcus in the adult alcoholic population. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of meningitis overall (Adults):** *Streptococcus pneumoniae*. * **Most common cause of meningitis in neonates:** *Group B Streptococcus* (followed by *E. coli* and *Listeria*). * **Most common cause of meningitis in adolescents/outbreaks:** *Neisseria meningitidis*. * **Alcoholic Predisposition:** Alcoholics are also at increased risk for *Listeria monocytogenes* meningitis due to impaired cell-mediated immunity; however, *S. pneumoniae* remains statistically more common. * **CSF Finding:** Look for low glucose, high protein, and polymorphonuclear (PMN) leukocytosis in bacterial meningitis.

Question 464: During a community outbreak of Salmonella gastroenteritis, which enrichment medium is the preferred choice for stool samples in the laboratory?

A. Cary Blair medium
B. VR medium
C. Selenite F medium (Correct Answer)
D. Thioglycollate medium

Explanation: **Explanation:** The correct answer is **Selenite F medium**. In cases of gastroenteritis, stool samples contain a high concentration of normal intestinal flora (like *E. coli*) which can overgrow and mask the presence of pathogens like *Salmonella*. **Enrichment media** are liquid media that contain inhibitory substances to suppress commensals while favoring the growth of the pathogen. **Selenite F broth** and **Tetrathionate broth** are specifically designed for the recovery of *Salmonella* species from fecal specimens. Selenite inhibits the growth of coliforms and enterococci, allowing *Salmonella* to multiply during the first 6–12 hours of incubation. **Analysis of Incorrect Options:** * **Cary Blair medium:** This is a **transport medium**, not an enrichment medium. It is used to preserve the viability of pathogens (like *Vibrio cholerae* or *Salmonella*) during transit to the lab but does not actively promote selective multiplication. * **VR (Venkatraman-Ramakrishnan) medium:** This is a specialized **transport medium** specifically used for *Vibrio cholerae*. * **Thioglycollate medium:** This is a multi-purpose, **anaerobic enrichment broth** used to support the growth of a wide variety of fastidious organisms, particularly anaerobes, aerobes, and microaerophiles. It is not selective for enteric pathogens. **High-Yield Clinical Pearls for NEET-PG:** * **Selective Media for Salmonella:** After enrichment in Selenite F, the sample is subcultured onto solid selective media like **Wilson and Blair’s Bismuth Sulfite Agar** (jet black colonies with metallic sheen) or **XLD Agar** (red colonies with black centers). * **Enrichment vs. Transport:** Always distinguish between the two. Enrichment = Liquid + Selective growth; Transport = Semi-solid + Preservation. * **Alkaline Peptone Water (APW):** The preferred enrichment medium for *Vibrio cholerae*.

Question 465: Malta fever is caused by which organism?

A. Legionella
B. Borrelia burgdorferi
C. Brucella melitensis (Correct Answer)
D. Pseudomonas

Explanation: **Explanation:** **Brucella melitensis** is the correct answer as it is the primary causative agent of **Brucellosis**, also known as **Malta fever**, Mediterranean fever, or Undulant fever. Brucella species are small, Gram-negative coccobacilli that are intracellular pathogens. *B. melitensis* (primarily found in goats and sheep) is the most virulent species infecting humans. The disease is characterized by a triad of fever (undulant pattern), profuse sweating (often smelling like wet hay), and joint/muscle pain. **Analysis of Incorrect Options:** * **Legionella:** Causes Legionnaires' disease (atypical pneumonia) and Pontiac fever. It is typically transmitted via aerosolized water from cooling towers or air conditioning systems. * **Borrelia burgdorferi:** A spirochete transmitted by *Ixodes* ticks that causes **Lyme disease**, characterized by the classic *Erythema chronicum migrans* (bull’s eye rash). * **Pseudomonas:** An opportunistic Gram-negative rod known for causing hospital-acquired infections (VAP, HAP), swimmer’s ear (Otitis externa), and infections in cystic fibrosis or burn patients. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Consumption of unpasteurized dairy products (most common) or direct contact with infected animal tissues (occupational hazard for vets/slaughterhouse workers). * **Clinical Feature:** **Undulant fever** (fever that rises and falls like waves) and hepatosplenomegaly. * **Diagnosis:** **Standard Agglutination Test (SAT)** is the screening test of choice (significant titer >1:160). **Rose Bengal Test** is used for rapid screening. * **Culture:** **Castaneda’s medium** (biphasic medium) is the traditional gold standard, though automated systems (Bact/ALERT) are now preferred. * **Treatment:** WHO recommends **Rifampicin + Doxycycline** for 6 weeks.

Question 466: What is a common causative agent of atypical pneumonia?

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Pneumocystis carinii (Correct Answer)
D. Haemophilus influenzae

Explanation: **Explanation:** **Atypical pneumonia** refers to pneumonia that presents with a subacute onset, non-productive cough, and "dissociation" (where clinical signs are mild but chest X-rays show significant interstitial infiltrates). Unlike "typical" pneumonia caused by pyogenic bacteria, these cases do not respond to β-lactam antibiotics. **Why the correct answer is right:** **Pneumocystis carinii** (now reclassified as **Pneumocystis jirovecii**) is a classic cause of atypical pneumonia, particularly in immunocompromised patients (e.g., HIV/AIDS with CD4 <200). It causes an interstitial plasma cell pneumonia characterized by a dry cough, dyspnea, and ground-glass opacities on imaging. While *Mycoplasma pneumoniae* is the most common cause in the general population, *Pneumocystis* is a high-yield "atypical" pathogen in the context of opportunistic infections. **Why the other options are wrong:** * **Streptococcus pneumoniae (B):** The most common cause of **typical/lobar pneumonia**. It presents acutely with high fever, productive cough (rusty sputum), and consolidation. * **Staphylococcus aureus (A) & Haemophilus influenzae (D):** Both are causes of **typical pneumonia**. *S. aureus* often follows viral influenza and can lead to cavitary lesions or pneumatoceles, while *H. influenzae* is common in COPD patients. **High-Yield NEET-PG Pearls:** * **Commonest Atypical Agent:** *Mycoplasma pneumoniae* (associated with Cold Agglutinins). * **Diagnosis of Pneumocystis:** Identified using **Gomori Methenamine Silver (GMS)** stain (shows crushed-ping-pong ball cysts) or Giemsa stain (shows trophozoites). * **Drug of Choice:** Trimethoprim-Sulfamethoxazole (TMP-SMX). * **Radiology:** Characterized by bilateral perihilar "bat-wing" infiltrates.

Question 467: Which of the following structures is typically NOT involved in gonococcal infection?

A. Epididymis
B. Testis (Correct Answer)
C. Prostate
D. Anterior urethra

Explanation: **Explanation:** The correct answer is **Testis**. *Neisseria gonorrhoeae* primarily infects surfaces lined with columnar or cuboidal epithelium. In the male reproductive tract, it causes ascending infections that follow a specific anatomical path, but it rarely involves the testicular parenchyma itself. **Why the Testis is typically NOT involved:** Gonococcal infection in males usually presents as an ascending spread from the urethra to the epididymis. While it frequently causes **acute epididymitis**, the infection typically stops there. If the testis is involved (orchitis), it is almost always secondary to epididymitis (epididymo-orchitis). Isolated orchitis is more characteristic of viral infections (like Mumps) or syphilis, rather than Gonorrhea. **Analysis of Incorrect Options:** * **Anterior Urethra:** This is the primary site of infection. Gonorrhea typically presents as acute anterior urethritis characterized by purulent discharge and dysuria. * **Prostate:** The infection can ascend from the urethra to the prostate gland, leading to acute or chronic gonococcal prostatitis. * **Epididymis:** This is a common site for local complications. Gonococcal epididymitis is a leading cause of scrotal pain and swelling in sexually active young men. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Infection:** *N. gonorrhoeae* prefers non-ciliated columnar epithelium (urethra, cervix, rectum, pharynx, and conjunctiva). It does **not** infect the squamous epithelium of the adult vagina. * **In Males:** The most common complication is **Epididymitis**. * **In Females:** The most common complication is **Pelvic Inflammatory Disease (PID)**, which can lead to Fitz-Hugh-Curtis Syndrome (perihepatitis). * **Disseminated Gonococcal Infection (DGI):** Look for the triad of polyarthralgia, tenosynovitis, and dermatitis. * **Treatment:** Due to increasing resistance, the current recommendation is usually a single IM dose of **Ceftriaxone**.

Question 468: Which of the following biochemical tests is ideal for the identification of Neisseria gonorrhoeae?

A. Sucrose fermentation test
B. Oxidase test (Correct Answer)
C. Penicillin resistance test
D. Coagulase test

Explanation: **Explanation:** The **Oxidase test** is a cornerstone biochemical reaction for the identification of the genus *Neisseria*. All members of this genus produce the enzyme **cytochrome c oxidase**. When a colony is rubbed onto a filter paper impregnated with the oxidase reagent (1% tetramethyl-p-phenylenediamine dihydrochloride), it turns deep purple or black within seconds, indicating a positive result. This test is crucial for differentiating *Neisseria* from other Gram-negative cocci. **Analysis of Incorrect Options:** * **A. Sucrose fermentation test:** *Neisseria gonorrhoeae* only ferments **Glucose** (G for Gonorrhoeae). It does not ferment sucrose, maltose, or lactose. *N. meningitidis* ferments both Glucose and Maltose (M for Meningitidis). * **C. Penicillin resistance test:** While many strains of *N. gonorrhoeae* (PPNG) are now resistant to penicillin, this is an antimicrobial susceptibility test, not a primary biochemical identification test. * **D. Coagulase test:** This test is used specifically to differentiate *Staphylococcus aureus* (positive) from coagulase-negative staphylococci (CoNS). It has no diagnostic value for *Neisseria*. **High-Yield NEET-PG Pearls:** * **Gram Stain:** *N. gonorrhoeae* appears as Gram-negative, kidney-shaped (bean-shaped) diplococci, often found within polymorphonuclear leucocytes (intracellular). * **Culture Media:** It is a fastidious organism requiring enriched media like **Thayer-Martin Medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Catalase Test:** All *Neisseria* species are also **Catalase positive**. * **Drug of Choice:** Due to widespread resistance, the current CDC-recommended treatment is a single IM dose of **Ceftriaxone**.

Question 469: What is a late complication of diphtheria?

A. Airway obstruction
B. Encephalitis
C. Pneumonia
D. Neuropathy (Correct Answer)

Explanation: **Explanation:** The correct answer is **Neuropathy**. This is because the clinical manifestations of Diphtheria are mediated by the **Diphtheria toxin**, which inhibits protein synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2). While the toxin causes local necrosis (pseudomembrane), its systemic absorption leads to distant organ damage, specifically targeting the heart and the nervous system. * **Neuropathy (Late Complication):** Neurological involvement typically appears **3 to 6 weeks** after the onset of the primary infection. It usually begins with **palatal paralysis** (nasal regurgitation of fluids) and ciliary paralysis (loss of accommodation), followed by posterior column damage and peripheral polyneuritis. * **Airway Obstruction (Early/Acute):** This is a mechanical complication caused by the thick, greyish-white **pseudomembrane** extending into the larynx or trachea. It is an acute emergency, not a late sequela. * **Pneumonia:** While it can occur as a secondary bacterial complication, it is not a specific toxemic manifestation of *C. diphtheriae*. * **Encephalitis:** This is not a characteristic feature of Diphtheria; the toxin primarily affects the peripheral nerves (demyelination) rather than the central nervous system. **High-Yield Clinical Pearls for NEET-PG:** * **Myocarditis:** The most common cause of death in Diphtheria; typically occurs in the 2nd week. * **Schick Test:** Used to demonstrate immunity/susceptibility to Diphtheria. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Stain:** Albert’s stain shows metachromatic granules (Volutin/Babes-Ernst granules).

Question 470: Which of the following is the confirmatory test for tuberculosis?

A. Gram's staining
B. AFB (Acid-Fast Bacilli) staining (Correct Answer)
C. Guinea-pig inoculation
D. Tuberculin testing

Explanation: **Explanation:** The diagnosis of Tuberculosis (TB) relies on the identification of *Mycobacterium tuberculosis*. The correct answer is **AFB (Acid-Fast Bacilli) staining**, specifically the **Ziehl-Neelsen (ZN) technique**. 1. **Why AFB Staining is Correct:** *M. tuberculosis* has a high lipid content (mycolic acid) in its cell wall, making it resistant to ordinary stains. Once stained with strong carbol fuchsin, these bacilli resist decolorization by dilute mineral acids (20% sulfuric acid). This "acid-fastness" is the diagnostic hallmark. In clinical practice, finding AFB in sputum is the primary confirmatory method for pulmonary TB. 2. **Analysis of Incorrect Options:** * **Gram’s Staining:** This is ineffective for Mycobacteria because the waxy cell wall prevents the penetration of crystal violet. They are often referred to as "Ghost cells" on Gram stain. * **Guinea-pig Inoculation:** While highly sensitive (historical gold standard), it is no longer used for routine confirmation due to ethical concerns, long turnaround times (up to 8 weeks), and the availability of faster molecular methods like CBNAAT/GeneXpert. * **Tuberculin Testing (Mantoux):** This is a screening tool that indicates delayed hypersensitivity (Type IV) to mycobacterial antigens. A positive result indicates *exposure* or *infection*, but not necessarily *active disease*. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** Culture on **Lowenstein-Jensen (LJ) medium** remains the definitive gold standard (takes 6–8 weeks). * **Rapid Diagnosis:** **CBNAAT (GeneXpert)** is now the preferred initial diagnostic test under NTEP guidelines as it detects both *M. tuberculosis* and Rifampicin resistance. * **Fluorescence:** Auramine-Rhodamine stain is more sensitive than ZN stain for screening large numbers of smears.

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

Practice Questions

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