Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 101: Both OX 2 and OX 19 are seen as positive tests in case of which of the following conditions?

A. Scrub typhus
B. Rocky mountain spotted fever (Correct Answer)
C. Epidemic typhus
D. Trench fever

Explanation: ### Explanation The question refers to the **Weil-Felix Reaction**, a heterophile agglutination test used for the presumptive diagnosis of Rickettsial infections. It utilizes the cross-reactivity between antibodies produced against Rickettsial antigens and the somatic (O) antigens of certain *Proteus* strains (*P. vulgaris* OX-19, OX-2, and *P. mirabilis* OX-K). **Why Rocky Mountain Spotted Fever (RMSF) is correct:** RMSF is caused by *Rickettsia rickettsii* (Spotted Fever Group). In this group, antibodies typically react with **both OX-19 and OX-2** strains of *Proteus vulgaris*. While OX-19 usually shows a stronger reaction, the positivity of both is a characteristic diagnostic pattern for the Spotted Fever group. **Analysis of Incorrect Options:** * **Scrub Typhus (A):** Caused by *Orientia tsutsugamushi*. It characteristically reacts **only with OX-K** (Kauffman strain). OX-19 and OX-2 are negative. * **Epidemic Typhus (C):** Caused by *Rickettsia prowazekii* (Typhus Group). This group shows a strong positive reaction with **OX-19 only**, with OX-2 being negative or weakly positive. * **Trench Fever (D):** Caused by *Bartonella quintana*. The Weil-Felix test is **negative** in Trench fever, Q fever (*Coxiella burnetii*), and Rickettsialpox. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Pattern Summary:** * **OX-19 (+), OX-2 (+):** Spotted Fever Group (e.g., RMSF). * **OX-19 (+), OX-2 (-):** Typhus Group (Epidemic & Endemic Typhus). * **OX-K (+):** Scrub Typhus. * **Negative Weil-Felix:** Always remember that **Q Fever** and **Rickettsialpox** do not show agglutination (Negative test). * **Gold Standard:** While Weil-Felix is high-yield for exams due to its historical importance, the **Indirect Fluorescent Antibody (IFA)** test is the current gold standard for Rickettsial diagnosis.

Question 102: Which of the following is a characteristic feature on culture of Streptococcus pneumoniae organism?

A. Alpha-hemolytic colonies inhibited by optochin on blood agar and lysed by bile (Correct Answer)
B. Beta-hemolytic colonies that are bacitracin-resistant on blood agar
C. Beta-hemolytic colonies that are inhibited by bacitracin on blood agar
D. Catalase-negative organisms that hydrolyze esculin in 40% bile and 6.5% NaCl

Explanation: **Explanation:** *Streptococcus pneumoniae* (Pneumococcus) is a Gram-positive, lancet-shaped diplococcus. Its identification in the laboratory relies on three hallmark characteristics: 1. **Alpha-hemolysis:** On blood agar, it produces a greenish discoloration due to partial hemolysis. 2. **Optochin Sensitivity:** It is uniquely sensitive to optochin (ethylhydrocupreine hydrochloride). A zone of inhibition ≥14 mm around the disc is diagnostic. 3. **Bile Solubility:** The addition of bile salts (e.g., sodium deoxycholate) activates autolytic enzymes (amidases) within the bacteria, leading to the rapid lysis of the colonies. **Analysis of Incorrect Options:** * **Option B:** Describes *Streptococcus agalactiae* (Group B Strep), which is beta-hemolytic and bacitracin-resistant. * **Option C:** Describes *Streptococcus pyogenes* (Group A Strep), which is beta-hemolytic and highly sensitive to bacitracin. * **Option D:** Describes *Enterococcus* species. While they are catalase-negative, they are characterized by their ability to grow in 6.5% NaCl and hydrolyze esculin in the presence of 40% bile (Bile Esculin Agar). **High-Yield Facts for NEET-PG:** * **Quellung Reaction:** Swelling of the capsule when mixed with specific antiserum; the gold standard for serotyping. * **Draughtsman/Checkerboard Appearance:** Older colonies of Pneumococci show central indentation due to autolysis. * **Virulence Factor:** The polysaccharide capsule is the most important virulence factor (anti-phagocytic). * **Commonest Cause:** It is the #1 cause of Community-Acquired Pneumonia (CAP) and bacterial meningitis in adults.

Question 103: RPR test is done for the diagnosis of:

A. Malaria
B. Syphilis (Correct Answer)
C. Leishmaniasis
D. None of the above

Explanation: **Explanation:** **RPR (Rapid Plasma Reagin)** is a macroscopic, non-treponemal screening test used for the diagnosis of **Syphilis**, caused by *Treponema pallidum*. It detects non-specific **reaginic antibodies** (IgM and IgG) produced by the host in response to lipid moieties released from damaged host cells and the treponeme itself. * **Mechanism:** The test uses a modified **VDRL antigen** (cardiolipin-cholesterol-lecithin) with added **charcoal particles**. These particles allow for macroscopic visualization of flocculation (clumping) without the need for a microscope, making it faster and easier than the VDRL test. **Why other options are incorrect:** * **Malaria:** Diagnosis is primarily made via peripheral blood smears (thick and thin) or Rapid Diagnostic Tests (RDTs) detecting antigens like HRP-2 or LDH. * **Leishmaniasis:** Diagnosis involves demonstrating amastigotes in bone marrow/splenic aspirates or serological tests like the rK39 immunochromatographic test. **High-Yield Clinical Pearls for NEET-PG:** 1. **Screening vs. Confirmatory:** RPR and VDRL are **screening tests** (high sensitivity). Positive results must be confirmed with **treponemal-specific tests** like TPHA or FTA-ABS (high specificity). 2. **Prozone Phenomenon:** False negatives can occur in secondary syphilis due to excessively high antibody titers; this is corrected by diluting the serum. 3. **Biological False Positives (BFP):** Conditions like SLE, leprosy, malaria, and pregnancy can cause false-positive RPR results. 4. **Monitoring:** Unlike treponemal tests (which remain positive for life), RPR titers decrease with successful treatment, making it ideal for **monitoring therapeutic response**.

Question 104: In the gut, anaerobic bacteria outnumber the aerobic bacteria by what approximate ratio?

A. 10:1
B. 100:1
C. 1000:1 (Correct Answer)
D. 10,000:1

Explanation: **Explanation:** The human gastrointestinal tract, particularly the colon, serves as a massive reservoir for microbial flora. The environment within the distal gut is profoundly **hypoxic** (low oxygen), which favors the proliferation of obligate anaerobes. **1. Why 1000:1 is correct:** In the large intestine, the oxygen tension is extremely low. While facultative anaerobes (like *E. coli*) consume available oxygen, they create a niche for obligate anaerobes to thrive. Quantitative studies of fecal matter demonstrate that anaerobic bacteria (such as *Bacteroides fragilis*, *Clostridium* spp., and *Bifidobacterium*) reach concentrations of $10^{11}$ to $10^{12}$ CFU/gram, whereas aerobic/facultative bacteria reach only $10^8$ to $10^9$ CFU/gram. This creates a consistent ratio of approximately **1000:1**. **2. Analysis of Incorrect Options:** * **10:1 and 100:1:** These ratios significantly underestimate the dominance of anaerobes. While these ratios might be seen in more proximal parts of the small intestine where transit is faster and oxygen levels are slightly higher, they do not represent the gut as a whole. * **10,000:1:** While some specific micro-niches might reach this level, the standard consensus for medical examinations and textbook references (like Harrison’s or Jawetz) is the 1000:1 ratio. **Clinical Pearls for NEET-PG:** * **Most common anaerobe in the gut:** *Bacteroides fragilis* (clinically significant for intra-abdominal abscesses). * **Most numerous bacteria in the gut:** *Bacteroides* species (specifically *B. vulgatus*) and *Bifidobacterium*. * **Function:** These anaerobes provide "colonization resistance," preventing the overgrowth of pathogens like *Clostridioides difficile*. * **Vitamin Synthesis:** Gut flora are essential for the synthesis of **Vitamin K** and **Vitamin B12**.

Question 105: Which of the following statements is true about Vibrio vulnificus?

A. Causes diarrhea commonly
B. Is halophilic (Correct Answer)
C. Drug of choice is penicillin
D. Produces Shiga toxin

Explanation: ### Explanation **Vibrio vulnificus** is a highly virulent, Gram-negative, comma-shaped bacterium found in warm coastal waters. **1. Why the correct answer is right:** *Vibrio vulnificus* is **halophilic** (salt-loving). Like most members of the *Vibrionaceae* family (except *Vibrio cholerae*), it requires a high concentration of sodium chloride (NaCl) for growth. It thrives in estuarine and marine environments, particularly in warm seawater. **2. Why the incorrect options are wrong:** * **Option A:** While *V. vulnificus* can cause mild gastroenteritis, it is primarily notorious for causing **primary septicemia** (following ingestion of raw oysters) and **necrotizing fasciitis** (following wound exposure to seawater). It is not a "common" cause of diarrhea compared to *V. cholerae* or *V. parahaemolyticus*. * **Option C:** Penicillin is ineffective. The drug of choice is typically a combination of **Doxycycline and a third-generation Cephalosporin** (e.g., Ceftriaxone). * **Option D:** Shiga toxin is produced by *Shigella dysenteriae* type 1 and Enterohemorrhagic *E. coli* (EHEC). *V. vulnificus* produces virulence factors like cytolysins, proteases, and a thick polysaccharide capsule. **3. Clinical Pearls for NEET-PG:** * **Risk Factor:** Patients with **chronic liver disease (hemochromatosis/cirrhosis)** are at extremely high risk for fatal septicemia due to the organism's requirement for free iron. * **Clinical Presentation:** Look for a history of **raw oyster consumption** or **seawater-contaminated wounds** followed by rapid development of hemorrhagic bullae and sepsis. * **Culture:** It is a **Lactose Fermenter (LF)**, which distinguishes it from most other Vibrios on MacConkey agar. On TCBS agar, it typically appears as green colonies (sucrose non-fermenter).

Question 106: A patient presented with interstitial pneumonia and denied being bitten by insects. Of the following rickettsial diseases, which could have been caused without an insect bite?

A. Scrub typhus
B. Rickettsial pox
C. Brill-Zinsser disease
D. Q fever (Correct Answer)

Explanation: **Explanation:** The correct answer is **Q fever** (caused by *Coxiella burnetii*). Unlike other members of the Rickettsiales order, *Coxiella burnetii* is unique because it does not require an arthropod vector for transmission to humans. **Why Q fever is correct:** * **Transmission:** It is primarily a zoonosis transmitted via **inhalation** of contaminated aerosols or dust from the birth products, feces, or urine of infected livestock (cattle, sheep, goats). It can also be transmitted through the consumption of unpasteurized milk. * **Clinical Presentation:** It characteristically presents as an undifferentiated febrile illness, **interstitial pneumonia**, or hepatitis. * **Biological Feature:** *C. burnetii* forms spore-like structures that allow it to survive harsh environmental conditions, facilitating airborne spread. **Why other options are incorrect:** * **Scrub typhus:** Caused by *Orientia tsutsugamushi*, it is transmitted by the bite of a **larval mite (chigger)**. It typically presents with an eschar at the bite site. * **Rickettsial pox:** Caused by *Rickettsia akari*, it is transmitted by the **mite** *Liponyssoides sanguineus*. * **Brill-Zinsser disease:** This is a recrudescent form of Epidemic typhus (*Rickettsia prowazekii*). While the initial infection is transmitted by the **human body louse**, the reactivation occurs years later without a new bite; however, the primary disease is strictly vector-borne. **High-Yield Clinical Pearls for NEET-PG:** * **Q fever** is the only rickettsial disease that **does not present with a rash** and is **Weil-Felix reaction negative**. * It is a highly infectious agent (Category B bioterrorism agent); a single organism can cause disease. * **Chronic Q fever** most commonly manifests as culture-negative endocarditis. * **Drug of choice:** Doxycycline.

Question 107: Diagnosis of tetanus is made by?

A. Culture of bacteria from wound
B. Clinically (Correct Answer)
C. Fourfold rise in antibody against tetanus toxin
D. Gram staining of biopsy from wound

Explanation: **Explanation:** The diagnosis of **Tetanus** is primarily **clinical**, based on the patient’s history (e.g., injury, lack of immunization) and the presence of characteristic physical findings like **trismus** (lockjaw), **risus sardonicus** (facial spasms), and **opisthotonus** (generalized arching of the back). **Why the correct answer is right:** * **Clinical Diagnosis:** *Clostridium tetani* is a non-invasive organism; the disease is caused by its potent exotoxin, **tetanospasmin**. Since the symptoms are toxin-mediated and occur even with a negligible bacterial load, the diagnosis must be made at the bedside to initiate life-saving treatment (Antitoxin and debridement) immediately. **Why other options are incorrect:** * **Option A & D:** Isolation of *C. tetani* from a wound is unreliable. The organism is recovered in only about **30% of cases**. Conversely, the bacteria may be present in a wound without causing disease in an immune individual. Gram staining (showing "drumstick" appearance) is suggestive but not diagnostic of the disease state. * **Option C:** Tetanus toxin is so lethal that a dose sufficient to cause disease is often **too small to be immunogenic**. Therefore, patients do not develop a measurable antibody response during the infection, making serology useless for acute diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Spatula Test:** A bedside clinical test where touching the oropharynx with a spatula triggers a reflex bite (positive) instead of a gag reflex (negative). * **Morphology:** *C. tetani* is a Gram-positive, motile anaerobic bacillus with terminal, spherical spores (**Drumstick appearance**). * **Culture:** On blood agar, it produces a thin film of growth called **swarming growth**. * **Treatment Priority:** Clinical suspicion warrants immediate administration of **Human Tetanus Immunoglobulin (HTIG)** and wound management.

Question 108: A child developed diarrhea after a tour with her parents. Which of the following is TRUE about Enterotoxigenic E.coli?

A. A common cause of travelers diarrhea
B. Rarely seen in acute watery diarrhea in infants
C. Causes diarrhea due to invasion of mucosa
D. Transmitted by feco-oral route and through fomites (Correct Answer)

Explanation: **Explanation:** Enterotoxigenic *Escherichia coli* (ETEC) is a major cause of bacterial diarrheal illness. The correct answer is **D** because ETEC is primarily transmitted via the **feco-oral route**, typically through contaminated food or water. While ingestion of contaminated consumables is the primary vehicle, transmission can also occur via **fomites** (inanimate objects) or direct person-to-person contact in settings with poor hygiene. **Analysis of Options:** * **Option A:** While ETEC is indeed the **most common cause of traveler’s diarrhea**, this option is often considered "less correct" in specific MCQ formats if the transmission mechanism (Option D) is being tested as a fundamental microbiological fact. However, in many contexts, A is also clinically true. * **Option B:** This is incorrect. ETEC is a **leading cause** of acute watery diarrhea in infants and children in developing countries, often resulting in significant dehydration. * **Option C:** This is incorrect. ETEC is **non-invasive**. It causes diarrhea by adhering to the small intestinal mucosa via colonization factors and releasing toxins (LT and ST). It does not cause mucosal inflammation or invasion (unlike EIEC or *Shigella*). **High-Yield Clinical Pearls for NEET-PG:** * **Pathogenesis:** Produces two types of enterotoxins: 1. **Heat-labile toxin (LT):** Increases **cAMP** (Mechanism similar to Cholera toxin). 2. **Heat-stable toxin (ST):** Increases **cGMP**. * **Clinical Presentation:** "Rice water" stools similar to cholera but usually milder. * **Diagnosis:** Gold standard is detecting toxins via ELISA or PCR for toxin genes. * **Mnemonic:** **"LT** increases **A**denylate cyclase; **ST** increases **G**uanylate cyclase" (**L**abile/**A**ir; **S**table/**G**round).

Question 109: Which stain is used for Corynebacterium diphtheriae?

A. Giemsa
B. Albert's stain (Correct Answer)
C. PAS
D. India ink

Explanation: **Explanation:** **Albert’s stain** is the specific differential stain used for the presumptive identification of *Corynebacterium diphtheriae*. The diagnostic hallmark of this bacterium is the presence of **metachromatic granules** (also known as Volutin or Babes-Ernst granules). These granules are composed of polymetaphosphate and possess a unique property: when stained with toluidine blue or malachite green (components of Albert’s stain), they appear **bluish-black**, while the bacillary body stains **green**. This contrast, often arranged in a "Chinese letter" or cuneiform pattern, is a high-yield diagnostic feature. **Analysis of Incorrect Options:** * **Giemsa:** Primarily used for staining blood parasites (Plasmodium), *Chlamydia*, *Rickettsia*, and certain fungi like *Histoplasma*. * **PAS (Periodic Acid-Schiff):** Used to demonstrate glycogen and fungal cell walls (e.g., *Candida*, *Cryptococcus*). It is not used for routine bacterial identification. * **India Ink:** A negative staining technique used specifically to visualize the polysaccharide capsule of ***Cryptococcus neoformans*** by creating a clear halo against a dark background. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** The gold standard for isolation is **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the definitive method to detect the diphtheria toxin (toxigenicity test). * **Other Special Stains:** Apart from Albert’s, Neisser’s and Ponder’s stains can also be used to visualize metachromatic granules. * **Mechanism:** The diphtheria toxin acts by inhibiting **EF-2** (Elongation Factor 2) via ADP-ribosylation, halting protein synthesis.

Question 110: A group of 7 friends developed non-bloody diarrhea, cramping abdominal pain, and fever 6-8 hours after attending a birthday party. Stool samples showed the presence of Salmonella enteritidis. Which of the following statements is FALSE?

A. There is no vaccine available for this illness.
B. Salmonella enteritidis causes typhoid fever. (Correct Answer)
C. Antibiotic therapy is not indicated for this type of infection.
D. Bacteremia occurs in less than 10% of cases.

Explanation: This question tests the ability to differentiate between **Typhoidal** and **Non-Typhoidal Salmonella (NTS)**, a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer (B)** **Salmonella enteritidis** is a leading cause of **Non-Typhoidal Salmonellosis (Gastroenteritis)**, not Typhoid fever. Typhoid (Enteric) fever is caused exclusively by *Salmonella Typhi* and *Salmonella Paratyphi* (A, B, and C). While NTS presents with rapid-onset diarrhea (6–48 hours) due to contaminated food (poultry/eggs), Typhoid fever has a longer incubation period (7–14 days) and presents as a systemic febrile illness rather than primary gastroenteritis. ### **Analysis of Other Options** * **Option A (True):** Vaccines (like Ty21a or Vi polysaccharide) are only available for *S. Typhi*. There is **no vaccine** for NTS infections. * **Option C (True):** In healthy individuals, NTS gastroenteritis is self-limiting. **Antibiotics are generally not indicated** as they do not shorten the illness and may actually prolong fecal shedding of the bacteria. They are reserved only for infants, the elderly, or the immunocompromised. * **Option D (True):** Unlike Typhoid fever (where bacteremia is a hallmark), NTS is usually localized to the GI tract. Bacteremia occurs in **less than 5–10%** of cases, typically in patients with underlying conditions like HIV or Sickle Cell Disease. ### **High-Yield Clinical Pearls for NEET-PG** * **Incubation Period:** NTS (hours to 2 days) vs. Typhoid (1–2 weeks). * **Reservoir:** NTS is **zoonotic** (animals/poultry); *S. Typhi* is **strictly human**. * **Sickle Cell Connection:** Patients with Sickle Cell Anemia are predisposed to **Osteomyelitis** caused by *Salmonella* (NTS). * **Stool Culture:** NTS shows NLF (Non-Lactose Fermenting) colonies with black centers (H2S production) on Hektoen Enteric or XLD agar.

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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