Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 201: Rickettsia tsutsugamushi is a small gram-negative, obligate intracellular organism that causes which of the following diseases?

A. Epidemic typhus
B. Endemic typhus
C. Scrub typhus (Correct Answer)
D. Q fever

Explanation: **Explanation:** **Orientia tsutsugamushi** (formerly *Rickettsia tsutsugamushi*) is the causative agent of **Scrub Typhus**. It is a small, Gram-negative, obligate intracellular bacterium. Unlike other Rickettsiae, it lacks a typical peptidoglycan layer and lipopolysaccharide, which is a high-yield distinction. 1. **Why Scrub Typhus is correct:** The disease is transmitted by the bite of the larval stage (chigger) of **Leptotrombidium mites**. A characteristic clinical feature is the **eschar**—a painless, black, crusty lesion at the site of the mite bite—followed by fever, lymphadenopathy, and a maculopapular rash. 2. **Why other options are incorrect:** * **Epidemic Typhus:** Caused by *Rickettsia prowazekii* and transmitted by the **human body louse**. It is associated with crowded, unsanitary conditions. * **Endemic (Murine) Typhus:** Caused by *Rickettsia typhi* and transmitted by **rat fleas** (*Xenopsylla cheopis*). * **Q Fever:** Caused by *Coxiella burnetii*. Unlike Rickettsiae, it does not cause a rash, is transmitted via inhalation of contaminated aerosols (not an arthropod vector), and is **Weil-Felix negative**. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Larval mites (Chiggers). * **Reservoir:** Mites (via transovarial transmission). * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** strains (Proteus antigen), while OX-19 and OX-2 are negative. * **Drug of Choice:** Doxycycline is the gold standard treatment for all Rickettsial diseases. * **Geography:** Part of the "Tsutsugamushi Triangle" (includes India).

Question 202: Which of the following is NOT a member of the Enterobacteriaceae family?

A. Pseudomonas (Correct Answer)
B. Klebsiella
C. Salmonella
D. Proteus

Explanation: **Explanation:** The **Enterobacteriaceae** family consists of Gram-negative, non-spore-forming bacilli that are characterized by being **Oxidase negative**, fermenting glucose, and reducing nitrates to nitrites. **Why Pseudomonas is the correct answer:** *Pseudomonas aeruginosa* is a **Non-fermenter** and belongs to the Pseudomonadaceae family. Unlike Enterobacteriaceae, it is **Oxidase positive** and an obligate aerobe. This distinction is a classic "high-yield" point in microbiology: Enterobacteriaceae are Oxidase negative, while *Pseudomonas*, *Vibrio*, and *Campylobacter* are Oxidase positive. **Why the other options are incorrect:** * **B. Klebsiella:** A classic member of Enterobacteriaceae. It is a lactose-fermenting, non-motile, mucoid encapsulated bacillus often associated with pneumonia and UTIs. * **C. Salmonella:** A member of the family known for causing enteric fever and gastroenteritis. It is a non-lactose fermenter (NLF) and is motile. * **D. Proteus:** A member of the family famous for its "swarming motility" on agar and its ability to produce urease, which contributes to the formation of staghorn renal calculi. **High-Yield Clinical Pearls for NEET-PG:** 1. **Oxidase Test:** The most rapid way to differentiate *Pseudomonas* (Positive) from Enterobacteriaceae (Negative). 2. **Culture:** *Pseudomonas* produces a characteristic fruity/grape-like odor and pigments like **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent). 3. **Resistance:** *Pseudomonas* is notorious for multi-drug resistance and is a leading cause of nosocomial infections (ventilator-associated pneumonia and burn wound sepsis).

Question 203: Nocardia can be differentiated from other Actinomyces by which characteristic?

A. ZN staining (Correct Answer)
B. Fontana stain
C. Gram staining
D. Oxygen requirement

Explanation: **Explanation:** The primary laboratory method to differentiate **Nocardia** from **Actinomyces** is **Acid-fast staining (Modified Ziehl-Neelsen stain)**. 1. **Why ZN Staining is Correct:** Nocardia species have **mycolic acids** in their cell walls (though shorter chains than Mycobacteria). This makes them **weakly acid-fast**. When using a modified ZN stain (using 1% sulfuric acid instead of 20% as a decolorizer), Nocardia appears as filamentous, branching, acid-fast bacilli. In contrast, **Actinomyces is non-acid-fast**. 2. **Analysis of Incorrect Options:** * **Fontana Stain:** This is a silver impregnation stain used primarily for visualizing **Spirochetes** (like *Treponema pallidum*) and fungal elements, not for differentiating aerobic/anaerobic actinomycetes. * **Gram Staining:** Both Nocardia and Actinomyces are **Gram-positive** branching filaments. Therefore, Gram staining cannot reliably distinguish between the two genera. * **Oxygen Requirement:** While Nocardia is an obligate **aerobe** and most pathogenic Actinomyces are **anaerobes**, oxygen requirement is a physiological trait rather than a staining characteristic. In the context of rapid laboratory identification and classic "differentiation" questions in microbiology, staining properties (Acid-fastness) are the gold standard. **High-Yield Clinical Pearls for NEET-PG:** * **Nocardia:** Aerobic, weakly acid-fast, found in soil. Causes pulmonary infections in immunocompromised patients and **Mycetoma** (Madura foot). * **Actinomyces:** Anaerobic, non-acid-fast, commensal of the oral cavity. Characterized by **"Sulfur granules"** in pus and "lumpy jaw" (cervicofacial actinomycosis). * **Decolorizer Trick:** Remember that Nocardia requires **1% H₂SO₄**, whereas *M. tuberculosis* requires 25% H₂SO₄ and *M. leprae* requires 5% H₂SO₄.

Question 204: Anchovy sauce pus or chocolate brown pus is the clinical feature of which condition?

A. Amoebic liver abscess (Correct Answer)
B. Pyogenic liver abscess
C. Peritoneal abscess
D. Hydatid liver

Explanation: **Explanation:** The characteristic **"anchovy sauce"** or chocolate brown appearance of pus is a classic diagnostic hallmark of an **Amoebic Liver Abscess (ALA)**, caused by *Entamoeba histolytica*. 1. **Why it is correct:** The pus in ALA is not true "pus" (which typically contains neutrophils and bacteria) but rather **liquefactive necrosis** of liver tissue. The reddish-brown color and thick consistency result from the destruction of hepatocytes and the release of blood pigments. It is typically odorless and sterile unless secondary bacterial infection occurs. 2. **Why other options are incorrect:** * **Pyogenic liver abscess:** Usually presents with yellow or greenish, foul-smelling pus containing polymorphonuclear leukocytes and bacteria (e.g., *E. coli, Klebsiella*). * **Peritoneal abscess:** Generally contains purulent material or fecaloid matter if secondary to bowel perforation, lacking the specific "anchovy" appearance. * **Hydatid liver cyst:** Caused by *Echinococcus granulosus*, it contains "hydatid sand" in a clear, colorless fluid (rock-water consistency), not thick brown pus. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** ALA most commonly affects the **Right Lobe** of the liver (due to the anatomy of portal venous drainage). * **Diagnosis:** The gold standard for identifying the organism is seeing **trophozoites** in the abscess wall (rarely found in the pus itself). * **Serology:** Enzyme-linked immunosorbent assay (ELISA) for anti-amoebic antibodies is highly sensitive. * **Treatment:** **Metronidazole** is the drug of choice, followed by a luminal amoebicide (e.g., Diloxanide furoate) to eradicate the intestinal carrier state.

Question 205: Red leg disease is caused by which of the following?

A. Pseudomonas
B. Moudly sugar cane fibre
C. Coniosporium
D. Aeromonas (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Aeromonas** Red leg disease (also known as bacterial hemorrhagic septicemia) is a classic infection caused by **Aeromonas hydrophila**. It primarily affects amphibians (frogs) and fish. The disease is characterized by cutaneous hemorrhages, particularly on the legs and abdomen, leading to a distinct "red leg" appearance. In humans, *Aeromonas* is clinically significant as it causes gastroenteritis (often following ingestion of contaminated water or seafood) and wound infections, especially after exposure to freshwater or soil. **Analysis of Incorrect Options:** * **A. Pseudomonas:** While *Pseudomonas* species can cause various infections in aquatic animals and humans (like "swimmer's ear" or ecthyma gangrenosum), they are not the specific causative agent of the classic "red leg disease." * **B. Mouldy sugar cane fibre:** Exposure to mouldy sugar cane fibre (specifically the actinomycete *Thermoactinomyces sacchari*) leads to **Bagassosis**, an occupational hypersensitivity pneumonitis. * **C. Coniosporium:** *Coniosporium corticale* is a fungus found under the bark of maple trees. Inhalation of its spores leads to **Maple bark stripper’s disease**, another form of hypersensitivity pneumonitis. **High-Yield Clinical Pearls for NEET-PG:** * **Aeromonas hydrophila** is a Gram-negative, oxidase-positive, facultative anaerobic rod. * It is often confused with *Vibrio* species but can be differentiated because *Aeromonas* is **halotolerant** (can grow without salt), whereas most *Vibrio* are halophilic. * In humans, it is a known cause of **necrotizing fasciitis** following trauma in freshwater environments. * It is resistant to Penicillins and first-generation Cephalosporins but usually sensitive to Fluoroquinolones and Third-generation Cephalosporins.

Question 206: Neisseria vaccine strains (MCV-4 and MPSV-4) are not protective against which of the following serogroups?

A. Serogroup A
B. Serogroup B (Correct Answer)
C. Serogroup C
D. Serogroup W135

Explanation: **Explanation:** The correct answer is **Serogroup B**. The primary reason for this lies in the biochemical nature of the capsular polysaccharide of *Neisseria meningitidis* Serogroup B. 1. **Why Serogroup B is the correct answer:** The capsule of Serogroup B is composed of **α-2,8-linked sialic acid (neuraminic acid)**. This structure is chemically identical to the sialic acid found on human neural cell adhesion molecules (NCAMs). Because the body recognizes this as "self-antigen," the polysaccharide is **poorly immunogenic**. Furthermore, using it in a vaccine carries a theoretical risk of inducing autoimmunity. Therefore, standard quadrivalent vaccines (MCV-4 and MPSV-4) include only A, C, Y, and W-135. Protection against Serogroup B requires specialized vaccines based on **recombinant proteins** (e.g., Bexsero, Trumenba) rather than capsular polysaccharides. 2. **Why other options are incorrect:** * **Serogroup A, C, and W135:** These serogroups have unique capsular polysaccharides that are highly immunogenic and distinct from human tissues. They are standard components of both the **Meningococcal Polysaccharide Vaccine (MPSV-4)** and the **Meningococcal Conjugate Vaccine (MCV-4)**. **High-Yield Clinical Pearls for NEET-PG:** * **MCV-4 (Conjugate):** Preferred over MPSV-4 as it induces T-cell dependent immunity, provides longer protection, and reduces nasopharyngeal carriage (herd immunity). * **Waterhouse-Friderichsen Syndrome:** Characterized by bilateral adrenal hemorrhage and fulminant meningococcemia. * **Thayer-Martin Medium:** The selective medium of choice for *Neisseria* (contains Vancomycin, Colistin, and Nystatin). * **Complement Deficiency:** Individuals with deficiencies in late complement components (**C5-C9**, Membrane Attack Complex) are at a significantly increased risk for recurrent *Neisseria* infections.

Question 207: What is the commonest cause of cellulitis?

A. Staphylococcus
B. Streptococcus (Correct Answer)
C. E. coli
D. Hemophilus

Explanation: **Explanation:** **1. Why Streptococcus is the Correct Answer:** The most common cause of cellulitis is **Group A Streptococcus (Streptococcus pyogenes)**. The underlying medical concept involves the production of spreading factors such as **hyaluronidase** and **streptokinase**. These enzymes break down the extracellular matrix and fibrin clots, allowing the infection to spread rapidly through the subcutaneous tissues in a diffuse, non-circumscribed manner, which is the hallmark of cellulitis. **2. Analysis of Incorrect Options:** * **Staphylococcus aureus (Option A):** While a frequent cause of skin and soft tissue infections, *S. aureus* typically produces **coagulase**, which leads to fibrin deposition and localized, walled-off infections like abscesses, furuncles, or carbuncles. It is the most common cause of *purulent* cellulitis (associated with pus/drainage), but not cellulitis in general. * **E. coli (Option C):** This is a gram-negative enteric organism. It is an uncommon cause of cellulitis, usually seen only in immunocompromised patients or those with specific exposures (e.g., surgical site infections in the perineal area). * **Hemophilus influenzae (Option D):** Historically, *H. influenzae* type b was a common cause of facial/periorbital cellulitis in children, often characterized by a "buccal" or violaceous (purplish) hue. However, its incidence has drastically decreased due to the Hib vaccine. **3. NEET-PG High-Yield Pearls:** * **Erysipelas:** A superficial form of cellulitis with well-demarcated borders and prominent lymphatic involvement, almost exclusively caused by *Streptococcus pyogenes*. * **Dog/Cat Bites:** Think *Pasteurella multocida*. * **Saltwater Exposure:** Think *Vibrio vulnificus*. * **Freshwater Exposure:** Think *Aeromonas hydrophila*. * **Diabetes/Peripheral Vascular Disease:** Cellulitis is often polymicrobial (including anaerobes).

Question 208: What is the specific test for the diagnosis of rickettsial infection?

A. Weil-Felix reaction
B. Agglutination of rickettsial suspension (Correct Answer)
C. Paul-Bunnell test
D. Rose-Waaler test

Explanation: ### Explanation **1. Why "Agglutination of rickettsial suspension" is correct:** The diagnosis of Rickettsial infections can be categorized into non-specific and specific tests. The **Agglutination of rickettsial suspension** (Microagglutination test) is a **specific serological test** because it uses the actual Rickettsial antigens to detect homologous antibodies in the patient's serum. It is highly specific and can differentiate between various Rickettsial species (e.g., *R. prowazekii* vs. *R. typhi*), unlike heterophile tests. **2. Why the other options are incorrect:** * **A. Weil-Felix reaction:** While commonly used, this is a **non-specific heterophile agglutination test**. It relies on the cross-reactivity between Rickettsial antibodies and the alkali-stable polysaccharide antigens of certain *Proteus* strains (OX-19, OX-2, and OX-K). It is often negative in Q fever and Rickettsialpox. * **C. Paul-Bunnell test:** This is a heterophile antibody test used specifically for the diagnosis of **Infectious Mononucleosis** (Epstein-Barr Virus), where patient antibodies agglutinate sheep red blood cells. * **D. Rose-Waaler test:** This is an agglutination test used to detect **Rheumatoid Factor** (IgM against the Fc portion of IgG) in Rheumatoid Arthritis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The Indirect Fluorescent Antibody (IFA) test is currently considered the gold standard for Rickettsial diagnosis. * **Weil-Felix Patterns:** * **Epidemic/Endemic Typhus:** OX-19 (+++) * **Scrub Typhus:** OX-K (+++) * **Rocky Mountain Spotted Fever:** OX-19 (+) and OX-2 (+) * **Culture:** Rickettsiae are obligate intracellular pathogens and are typically grown in the yolk sac of embryonated eggs or cell cultures (not routine). * **Drug of Choice:** Doxycycline is the treatment of choice for almost all Rickettsial infections.

Question 209: What is the key differentiating feature between Staphylococcus aureus and Staphylococcus saprophyticus?

A. Catalase positivity
B. Coagulase positivity (Correct Answer)
C. Novobiocin susceptibility
D. Oxidase positivity

Explanation: ### Explanation The genus *Staphylococcus* is characterized as Gram-positive cocci in clusters that are **Catalase positive**. The primary laboratory algorithm to differentiate species within this genus is the **Coagulase test**. **1. Why Coagulase positivity is the correct answer:** * ***Staphylococcus aureus*** is the most clinically significant **Coagulase-positive** staphylococcus. It produces the enzyme coagulase, which converts fibrinogen to fibrin, causing plasma to clot. * ***Staphylococcus saprophyticus*** belongs to the group known as **Coagulase-Negative Staphylococci (CoNS)**. Therefore, the presence or absence of coagulase is the gold-standard differentiating feature between these two. **2. Analysis of Incorrect Options:** * **Catalase positivity (A):** Both *S. aureus* and *S. saprophyticus* are catalase-positive. This test differentiates *Staphylococci* (positive) from *Streptococci* (negative). * **Novobiocin susceptibility (C):** This test is used to differentiate between CoNS species. *S. saprophyticus* is **Novobiocin resistant**, whereas *S. epidermidis* is Novobiocin sensitive. Since *S. aureus* is already identified by the coagulase test, novobiocin is not the primary differentiator for it. * **Oxidase positivity (D):** Almost all Staphylococci are **oxidase negative**. This test is typically used to identify organisms like *Pseudomonas* or *Neisseria*. **3. NEET-PG High-Yield Pearls:** * **S. aureus:** Look for "Golden yellow colonies" on Nutrient Agar (due to staphyloxanthin pigment) and $\beta$-hemolysis on Blood Agar. * **S. saprophyticus:** A common cause of Urinary Tract Infections (UTIs) in young, sexually active females ("Honeymoon cystitis"). * **Mnemonic for Novobiocin:** *"On the **Staph** retreat, there was **NO** **S**aprophyticus **RE**sistance"* (Novobiocin: **S**aprophyticus = **Re**sistant; **E**pidermidis = **S**ensitive).

Question 210: Which of the following is NOT true regarding pseudomembranous colitis?

A. It is caused by Clostridium difficile.
B. The organism is a normal commensal of the gut.
C. It is due to the production of phospholipase A. (Correct Answer)
D. It is treated by vancomycin.

Explanation: **Explanation:** Pseudomembranous colitis (PMC) is a severe inflammation of the colon typically occurring after broad-spectrum antibiotic use. **Why Option C is the correct answer (The False Statement):** The pathogenesis of *Clostridium difficile* is mediated by two potent exotoxins: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. These toxins act by inactivating Rho GTPases, leading to actin depolymerization and cell death. **Phospholipase A** (specifically Alpha-toxin) is the primary virulence factor for *Clostridium perfringens* (causing gas gangrene), not *C. difficile*. **Analysis of other options:** * **Option A:** *Clostridium difficile* is the definitive causative agent of PMC. * **Option B:** It is a normal commensal in the gut of about 3-5% of healthy adults and up to 70% of neonates. Disease occurs when antibiotic therapy (especially Clindamycin, Fluoroquinolones, or Cephalosporins) disrupts normal flora, allowing *C. difficile* overgrowth. * **Option D:** Oral **Vancomycin** or Fidaxomicin are the first-line treatments. Metronidazole is an alternative for mild cases. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The gold standard is the **Tissue Culture Cytotoxicity Assay**, but the most common rapid test is detecting toxins in stool via **ELISA** or **PCR** (GDH antigen screening). * **Morphology:** Endoscopy reveals characteristic "yellowish-white plaques" (pseudomembranes) on the colonic mucosa. * **Risk Factors:** Recent antibiotic use and Proton Pump Inhibitors (PPIs). * **Complication:** Toxic megacolon and perforation.

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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