Bacteriology — MCQs

A 25-year-old farmer presented with a history of high-grade fever for 7 days and altered sensorium for 2 days. On examination, he was comatose and had conjunctival hemorrhage. Urgent investigations showed a hemoglobin of 11 gm/dl, serum bilirubin 8 mg/dl, and urea 78 mg/dl. The peripheral blood smear was negative for malarial parasites. What is the most likely diagnosis?

Q966Medium

An infant presented with high-grade fever and respiratory distress. Blood culture showed growth of alpha-hemolytic colonies on Gram staining, which were identified as Gram-positive cocci. Which of the following agents is the suspected pathogen likely to be susceptible to?

Q967Easy

Which bacterium causes cholera?

Q968Easy

Which of the following structures is usually spared in a gonococcal infection?

Q969Easy

Tuberculin test positivity is dependent on which of the following?

Q970Easy

Scrub typhus presents with which of the following clinical features?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 961: Which of the following is an exception among lactose non-fermenting bacteria?

A. Shigella sonnei (Correct Answer)
B. Shigella dysenteriae
C. Shigella flexneri
D. Shigella boydii

Explanation: **Explanation:** The classification of Enterobacteriaceae is often based on their ability to ferment lactose on MacConkey agar. While the genus *Shigella* is classically categorized as **Lactose Non-Fermenters (LNF)**, producing pale/colorless colonies, **Shigella sonnei** is a notable exception. **1. Why Option A is Correct:** *Shigella sonnei* is a **Late Lactose Fermenter (LLF)**. Unlike most Shigella species, it possesses the genes to ferment lactose, but it does so slowly. On MacConkey agar, it initially appears as a non-fermenter (pale colonies) at 24 hours, but upon extended incubation (48–72 hours), it produces pink colonies. This is due to the presence of the enzyme $\beta$-galactosidase but a relative deficiency in lactose permease. **2. Why Options B, C, and D are Incorrect:** * **Shigella dysenteriae (Group A):** A true lactose non-fermenter. It is the most virulent species, producing the Shiga toxin. * **Shigella flexneri (Group B):** A true lactose non-fermenter and the most common cause of bacillary dysentery in developing countries. * **Shigella boydii (Group C):** A true lactose non-fermenter, primarily restricted to the Indian subcontinent. **High-Yield Clinical Pearls for NEET-PG:** * **Classification:** Shigella is grouped based on somatic 'O' antigens: A (*S. dysenteriae*), B (*S. flexneri*), C (*S. boydii*), and D (*S. sonnei*). * **Epidemiology:** *S. sonnei* is the most common cause of shigellosis in **developed countries**. * **Biochemical Key:** *S. sonnei* is also **Indole negative** and **Ornithine decarboxylase positive**, distinguishing it from other Shigella species. * **Other Late Lactose Fermenters:** Remember the mnemonic **"Vibrio SSS"** (*Vibrio cholerae*, *Shigella sonnei*, *Serratia*, and *Salmonella Paratyphi B*).

Question 962: What is true about Legionella pneumonia?

A. Occurs in epidemics
B. Treated with penicillin (Correct Answer)
C. Diagnosed from sputum
D. More common in children

Explanation: **Explanation:** *Note: There appears to be a discrepancy in the provided key. **Legionella is naturally resistant to Penicillin** because it produces beta-lactamases and is an intracellular pathogen. The standard treatment is Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin). However, if we must analyze the options based on the provided key:* **1. Why "Treated with Penicillin" is marked correct (Academic Context):** In some older examinations, this option was used to test the knowledge of Legionella’s susceptibility. However, clinically, Legionella is **not** treated with penicillin. If this is the intended answer in a specific question bank, it may be a "distractor" or an error in the source material. In modern NEET-PG standards, the correct treatment is **Macrolides**. **2. Analysis of Incorrect Options:** * **A. Occurs in epidemics:** While outbreaks occur (linked to cooling towers or AC systems), it is more commonly seen as **sporadic** cases of community-acquired or nosocomial pneumonia. * **C. Diagnosed from sputum:** Legionella is difficult to see on a Gram stain (it stains poorly) and is fastidious. Sputum culture has low sensitivity. The gold standard for rapid diagnosis is the **Urinary Antigen Test** (detects Serogroup 1). * **D. More common in children:** Legionella primarily affects **older adults**, smokers, and the immunocompromised. It is rare in children. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Requires **BCYE (Buffered Charcoal Yeast Extract)** agar supplemented with L-cysteine and iron. * **Clinical Feature:** Often presents with **hyponatremia**, high fever, and gastrointestinal symptoms (diarrhea). * **Transmission:** Inhalation of aerosols from contaminated water sources (ACs, showers); **no person-to-person spread.** * **Drug of Choice:** Azithromycin or Levofloxacin.

Question 963: McFadyen reaction is seen with which organism?

A. Clostridium perfringens
B. Clostridium botulinum
C. Bacillus cereus
D. Bacillus anthracis (Correct Answer)

Explanation: **Explanation:** **Bacillus anthracis (Correct Answer):** The McFadyen reaction is a specific staining technique used for the presumptive identification of *Bacillus anthracis* in clinical samples (like blood or tissue). It demonstrates the presence of the **polypeptide capsule** (composed of poly-D-glutamic acid). When a smear is stained with polychrome methylene blue for 30 seconds, the capsule appears as a **pale pink or purple (amorphous) halo** surrounding the blue-stained, truncated bacilli. This reaction is highly characteristic of *B. anthracis* and is used to distinguish it from other non-capsulated *Bacillus* species. **Incorrect Options:** * **Clostridium perfringens:** While it is a capsulated organism, it is identified by the **Nagler reaction** (demonstrating alpha-toxin/lecithinase activity) and "target hemolysis" on blood agar. * **Clostridium botulinum:** Identified primarily by toxin detection (mouse bioassay) or clinical presentation of flaccid paralysis; it does not show a McFadyen reaction. * **Bacillus cereus:** Although related to *B. anthracis*, it is typically non-capsulated and does not exhibit the McFadyen reaction. It is best known for causing food poisoning via emetic or diarrheal toxins. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule:** *B. anthracis* is unique because its capsule is made of **amino acids (Poly-D-glutamic acid)**, whereas most bacterial capsules are polysaccharide. * **Bamboo Stick Appearance:** Chains of *B. anthracis* with squared ends resemble bamboo sticks. * **Medusa Head Colonies:** Characteristic morphology on nutrient agar due to interlacing chains of bacilli. * **String of Pearls Reaction:** Seen when grown on agar containing low concentrations of penicillin.

Question 964: Which of the following is characteristic of Enteropathogenic E. coli?

A. Causes acute gastroenteritis in infants (Correct Answer)
B. Invades intestinal epithelium
C. Produces cholera-like toxin
D. Is non-motile

Explanation: **Explanation:** **Enteropathogenic *E. coli* (EPEC)** is a major cause of endemic and epidemic diarrhea, particularly in infants under the age of two in developing countries. 1. **Why Option A is correct:** EPEC is classically associated with **infantile diarrhea**. It does not produce toxins or invade cells; instead, it utilizes a **Type III secretion system** to inject proteins that cause "attaching and effacing" (A/E) lesions. This leads to the destruction of microvilli, resulting in malabsorption and acute watery diarrhea. 2. **Why other options are incorrect:** * **Option B:** Invasion of the intestinal epithelium is the hallmark of **Enteroinvasive *E. coli* (EIEC)**, which mimics *Shigella* by causing dysentery. * **Option C:** Production of a cholera-like toxin (Heat-labile toxin/LT) is characteristic of **Enterotoxigenic *E. coli* (ETEC)**, the leading cause of Traveler’s diarrhea. * **Option D:** Most *E. coli* strains, including EPEC, are **motile** via peritrichous flagella. Non-motile strains are rare (e.g., some EIEC or O157:H7 variants). **High-Yield Clinical Pearls for NEET-PG:** * **Pathogenesis:** Mediated by **EPEC Adherence Factor (EAF)** plasmid and the **intimin** protein (encoded by the *eae* gene). * **Morphology:** Look for the phrase **"pedestal formation"** in clinical vignettes; this refers to the actin rearrangement in host cells where the bacteria sit. * **Mnemonic:** **EPEC** = **P**ediatric/**P**edestal formation. * **Diagnosis:** Serotyping and PCR for the *eae* gene.

Question 965: A 25-year-old farmer presented with a history of high-grade fever for 7 days and altered sensorium for 2 days. On examination, he was comatose and had conjunctival hemorrhage. Urgent investigations showed a hemoglobin of 11 gm/dl, serum bilirubin 8 mg/dl, and urea 78 mg/dl. The peripheral blood smear was negative for malarial parasites. What is the most likely diagnosis?

A. Brucellosis
B. Weil's disease (Correct Answer)
C. Acute viral hepatitis
D. Q fever

Explanation: ### Explanation The clinical presentation of high-grade fever, jaundice (bilirubin 8 mg/dl), and renal impairment (urea 78 mg/dl) in a farmer constitutes the classic triad of **Weil’s disease**, the severe icteric form of **Leptospirosis**. **1. Why Weil’s Disease is Correct:** Leptospirosis is a zoonosis caused by *Leptospira interrogans*, often transmitted via water contaminated with the urine of infected rodents (common in farmers). * **Clinical Triad:** Jaundice, renal failure, and hemorrhage (e.g., conjunctival suffusion/hemorrhage). * **Neurological Involvement:** Altered sensorium in this patient suggests aseptic meningitis or encephalopathy, common in severe cases. * **Laboratory Findings:** The negative malarial smear helps rule out cerebral malaria, which is the primary differential for fever with jaundice and altered sensorium. **2. Why Other Options are Incorrect:** * **Brucellosis:** Typically presents as undulant fever with musculoskeletal symptoms (back pain, arthritis) and hepatosplenomegaly, but rarely causes acute jaundice or renal failure. * **Acute Viral Hepatitis:** While it causes high bilirubin and altered sensorium (hepatic encephalopathy), it does not typically cause renal failure or conjunctival hemorrhage. * **Q Fever:** Caused by *Coxiella burnetii*, it usually presents as atypical pneumonia or culture-negative endocarditis; it does not typically present with this severe ictero-hemorrhagic profile. **3. High-Yield Pearls for NEET-PG:** * **Gold Standard Test:** Microscopic Agglutination Test (MAT). * **Culture Media:** EMJH medium or Fletcher’s medium. * **Morphology:** Spiral-shaped bacteria with "hooked ends" (Question-mark shape). * **Drug of Choice:** Doxycycline (prophylaxis/mild cases) or IV Penicillin G (severe cases). * **Key Sign:** Conjunctival suffusion (redness without inflammatory exudate) is a pathognomonic sign.

Question 966: An infant presented with high-grade fever and respiratory distress. Blood culture showed growth of alpha-hemolytic colonies on Gram staining, which were identified as Gram-positive cocci. Which of the following agents is the suspected pathogen likely to be susceptible to?

A. Bacitracin
B. Novobiocin
C. Optochin (Correct Answer)
D. Cloxacillin

Explanation: ### Explanation The clinical presentation of high-grade fever and respiratory distress in an infant, combined with the laboratory finding of **alpha-hemolytic, Gram-positive cocci**, strongly suggests **_Streptococcus pneumoniae_** (Pneumococcus). **1. Why Optochin is the Correct Answer:** In microbiology, the **Optochin (Ethylhydrocupreine hydrochloride) sensitivity test** is the gold standard for differentiating _S. pneumoniae_ from other alpha-hemolytic streptococci (Viridans group). _S. pneumoniae_ is exquisitely **sensitive** to Optochin, showing a zone of inhibition ≥14 mm. Another confirmatory test for this pathogen is **Bile Solubility**, as _S. pneumoniae_ possesses autolytic enzymes (amidases) that are activated by bile salts. **2. Analysis of Incorrect Options:** * **Bacitracin (A):** This test is used to differentiate **Group A Streptococci** (_S. pyogenes_, which is beta-hemolytic) from other beta-hemolytic streptococci. _S. pyogenes_ is Bacitracin sensitive. * **Novobiocin (B):** This is used to differentiate coagulase-negative staphylococci (CoNS). **_S. saprophyticus_** is resistant, while _S. epidermidis_ is sensitive. * **Cloxacillin (D):** This is a penicillinase-resistant penicillin used primarily for _Staphylococcus aureus_. While some _S. pneumoniae_ may be susceptible, it is not a diagnostic identification marker like Optochin. **3. Clinical Pearls for NEET-PG:** * **Morphology:** _S. pneumoniae_ are typically described as **"Lancet-shaped"** diplococci. * **Capsule:** It is a capsulated organism; the **Quellung Reaction** (capsular swelling) is a specific identification method. * **Virulence Factor:** The polysaccharide capsule is the most important virulence factor. * **Commonest Cause:** It is the #1 cause of community-acquired pneumonia (CAP) and bacterial meningitis in adults.

Question 967: Which bacterium causes cholera?

A. Vibrio cholerae O1 (Correct Answer)
B. Vibrio cholerae O139
C. Vibrio parahaemolyticus
D. Vibrio cholerae O141

Explanation: **Explanation:** **Vibrio cholerae O1** is the primary causative agent of epidemic and pandemic cholera. The classification of *V. cholerae* is based on the **O-antigen** (somatic antigen). While there are over 200 serogroups, only O1 and O139 are associated with epidemic cholera. Among these, **O1** is the most significant, responsible for seven out of the eight historical pandemics. It is further divided into two biotypes: **Classical** and **El Tor**, and three serotypes: **Ogawa, Inaba, and Hikojima**. **Analysis of Options:** * **Vibrio cholerae O139 (Option B):** Also causes epidemic cholera (first identified in Madras, 1992), but it is restricted to specific geographical areas (mostly South-East Asia) and has not caused a global pandemic like O1. * **Vibrio parahaemolyticus (Option C):** This is a halophilic (salt-loving) bacterium typically associated with **food poisoning** from contaminated seafood. It causes self-limiting watery diarrhea, not the severe "rice-water" stools characteristic of cholera. * **Vibrio cholerae O141 (Option D):** This belongs to the "Non-O1/Non-O139" group (formerly called NCV or NAG vibrios). These may cause sporadic cases of diarrhea or extra-intestinal infections but do not possess the cholera toxin required to cause epidemic cholera. **High-Yield NEET-PG Pearls:** * **Morphology:** Gram-negative, comma-shaped (vibrion) rods showing **darting motility** (liquid-media) or "swarming" (solid-media). * **Culture:** Grows on **TCBS agar** (Thiosulfate Citrate Bile Salts Sucrose), producing characteristic **yellow colonies** due to sucrose fermentation. * **Pathogenesis:** Mediated by **Cholera Toxin (Choleragen)**, which increases intracellular **cAMP**, leading to the hypersecretion of water and electrolytes. * **Clinical Sign:** Classic **"Rice-water stools"** with a fishy odor.

Question 968: Which of the following structures is usually spared in a gonococcal infection?

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

Explanation: **Explanation:** In the male reproductive system, *Neisseria gonorrhoeae* typically spreads via **canalicular spread** (mucosal ascent) rather than hematogenous spread. The infection begins in the urethra and ascends through the reproductive ducts. **1. Why the Testis is spared:** The **testis** is generally resistant to direct gonococcal invasion. While the infection frequently involves the epididymis (causing acute epididymitis), it rarely crosses the "blood-testis barrier" or involves the testicular parenchyma itself. Therefore, while **Epididymo-orchitis** can occur, the primary site of infection is the epididymis, and the testis is considered the "spared" structure in the context of typical gonococcal progression. **2. Analysis of Incorrect Options:** * **Urethra (A):** This is the primary site of infection in males, presenting as acute purulent urethritis (the most common clinical presentation). * **Prostate (B):** Ascending infection can lead to acute or chronic prostatitis as the bacteria move up the urogenital tract. * **Epididymis (D):** This is a common site for complications. Gonococcal epididymitis usually presents with unilateral scrotal pain and swelling. **NEET-PG High-Yield Pearls:** * **Most common site in females:** Endocervix (though the urethra is also frequently involved). * **Fitz-Hugh-Curtis Syndrome:** A complication in females involving perihepatitis ("violin-string" adhesions). * **Culture Media:** Thayer-Martin medium (selective) or Chocolate agar (non-selective). * **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT). * **Treatment:** Ceftriaxone (IM) is the drug of choice, often combined with Azithromycin to cover co-existing *Chlamydia* infection.

Question 969: Tuberculin test positivity is dependent on which of the following?

A. Erythema
B. Nodule formation
C. Induration (Correct Answer)
D. Ulcerative change

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**. It is used to screen for latent tuberculosis infection by injecting 5 TU (Tuberculin Units) of Purified Protein Derivative (PPD) intradermally. **Why Induration is the Correct Answer:** The hallmark of a positive TST is **induration** (a palpable, raised, hardened area). This occurs because sensitized T-lymphocytes migrate to the injection site, releasing cytokines that recruit macrophages and cause local edema and cellular infiltration. According to standard guidelines (CDC/WHO), the test result is interpreted **solely by measuring the transverse diameter of the induration** in millimeters, not the redness. **Analysis of Incorrect Options:** * **A. Erythema:** Redness is a common localized inflammatory response but is highly subjective and does not correlate with T-cell mediated immunity. It must be ignored during measurement. * **B. Nodule formation:** While the area may feel like a nodule, "induration" is the specific clinical term used for the hardening of the skin in this diagnostic context. * **D. Ulcerative change:** Though severe reactions can lead to vesiculation or necrosis, these are complications or "strongly positive" indicators, not the standard criteria for positivity. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** The test must be read **48 to 72 hours** after administration. * **Cut-off values:** * **≥5 mm:** Positive in HIV+, recent contacts of TB cases, or immunosuppressed. * **≥10 mm:** Positive in healthcare workers, immigrants from high-prevalence areas, or IV drug users. * **≥15 mm:** Positive in persons with no known risk factors for TB. * **False Negative:** Seen in miliary TB, sarcoidosis, malnutrition, or AIDS (due to **anergy**). * **False Positive:** Seen in individuals vaccinated with **BCG** or those with atypical mycobacterial infections.

Question 970: Scrub typhus presents with which of the following clinical features?

A. Lymphadenopathy, rash, and eschar (Correct Answer)
B. Lymphadenopathy and rash but no eschar
C. Rash and eschar but no lymphadenopathy
D. Lymphadenopathy but no rash and eschar

Explanation: **Explanation:** Scrub typhus is caused by **_Orientia tsutsugamushi_**, an obligate intracellular bacterium transmitted by the bite of the larval stage (chigger) of **trombiculid mites**. **Why Option A is correct:** The clinical presentation of Scrub typhus is characterized by a classic triad: 1. **Eschar:** This is the hallmark of the disease. It is a painless, necrotic, black crusty lesion that develops at the site of the mite bite. It is most commonly found in areas with skin folds (axilla, groin, neck). 2. **Lymphadenopathy:** Localized lymphadenopathy near the eschar site is common, which often progresses to generalized lymphadenopathy. 3. **Rash:** A maculopapular rash typically appears on the trunk around the 5th to 7th day of illness and spreads to the extremities (centrifugal spread). **Why other options are incorrect:** * **Options B, C, and D** are incorrect because they exclude one or more components of the classic clinical triad. While not every patient presents with all three features (the eschar is found in roughly 50-80% of cases), for the purpose of NEET-PG, the presence of **lymphadenopathy, rash, and eschar** defines the characteristic clinical profile of Scrub typhus. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Trombiculid mite (Leptotrombidium). * **Reservoir:** Mites (via transovarial transmission). * **Diagnosis:** **Weil-Felix test** shows agglutination with **OX-K** strain (OX-19 and OX-2 are negative). The gold standard is the Indirect Immunofluorescence Assay (IFA). * **Drug of Choice:** **Doxycycline** is the first-line treatment; Azithromycin is used in pregnancy. * **Differential:** Unlike Epidemic typhus (centripetal rash), Scrub typhus rash is centrifugal and involves an eschar.

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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