Bacteriology — MCQs

A patient hospitalized after an automobile accident develops severe diarrhea and pseudomembranous enterocolitis five days after initiation of antibiotic therapy with tobramycin, carbenicillin, and clindamycin. Antibiotic-associated diarrhea and pseudomembranous enterocolitis can be caused by which of the following organisms?

Q115Medium

Which of the following is best for diagnosing genital TB?

Q116Easy

Brill-Zinsser disease is caused by which of the following organisms?

Q117Medium

Differentiation between N. gonorrhoeae and N. meningitidis can be done by which biochemical test?

Q118Easy

Which of the following is not an AB5 toxin?

Q119Easy

Which of the following are spirochetes?

Q120Easy

Which of the following is a Category A bioterrorism agent?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 111: Dorset egg medium is used for the cultivation of which organism?

A. Staphylococcus
B. Streptococcus
C. Gonococcus
D. Mycobacterium tuberculosis (Correct Answer)

Explanation: **Explanation:** **Dorset Egg Medium** is a specialized enrichment medium used primarily for the cultivation of **Mycobacterium tuberculosis**. The underlying medical concept involves the use of **whole eggs** as a nutrient source. The medium consists of a mixture of beaten whole eggs and normal saline, which is sterilized via **inspissation** (heating at 80-85°C for 30 minutes on three successive days). This process coagulates the egg proteins, creating a solid, nutrient-rich slant. It is particularly useful for the initial isolation of tubercle bacilli and for maintaining their viability during long-term storage. **Analysis of Incorrect Options:** * **Staphylococcus & Streptococcus:** These are pyogenic cocci that grow readily on routine media like Blood Agar or Nutrient Agar. They do not require the complex lipid-rich environment provided by egg-based media. * **Gonococcus (Neisseria gonorrhoeae):** This is a fastidious organism that requires enriched media like **Thayer-Martin Medium** or **Chocolate Agar**. It is highly sensitive to environmental changes and would not thrive on Dorset Egg medium. **High-Yield Clinical Pearls for NEET-PG:** * **Other Egg-Based Media:** The most commonly used egg medium for M. tuberculosis is **Lowenstein-Jensen (LJ) Medium**, which contains malachite green to inhibit contaminants. * **Inspissation:** Remember that media containing egg or serum (like Loeffler’s Serum Slope for *C. diphtheriae*) cannot be autoclaved; they must be sterilized by inspissation. * **Growth Rate:** M. tuberculosis is a slow grower (2-8 weeks), whereas rapid growers (atypical mycobacteria) appear within a week.

Question 112: Which of the following causes mesenteric lymphadenitis?

A. Adenovirus
B. Campylobacter
C. Yersinia pseudotuberculosis
D. All of the above (Correct Answer)

Explanation: **Explanation:** Mesenteric lymphadenitis is the inflammation of the lymph nodes within the mesentery, often clinically mimicking acute appendicitis (Pseudo-appendicitis syndrome). This condition is caused by a variety of infectious agents, including viral, bacterial, and parasitic pathogens. * **Yersinia pseudotuberculosis (and Y. enterocolitica):** These are the most classic bacterial causes. They have a tropism for lymphoid tissue and frequently cause terminal ileitis and mesenteric lymphadenitis, leading to right lower quadrant pain. * **Campylobacter jejuni:** While primarily known for causing inflammatory diarrhea, *Campylobacter* can also involve the mesenteric lymph nodes, presenting with a clinical picture indistinguishable from appendicitis. * **Adenovirus:** This is the most common **viral** cause of mesenteric lymphadenitis, particularly in children. It is often associated with a concurrent or recent upper respiratory tract infection. Since all three agents listed are recognized causes of this clinical syndrome, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Pseudo-appendicitis:** Always consider *Yersinia enterocolitica* or *Yersinia pseudotuberculosis* in a patient presenting with appendicitis-like symptoms but a normal appendix on imaging/surgery. * **Diagnosis:** Ultrasound is the preferred initial imaging modality, showing enlarged lymph nodes (>8mm in short-axis diameter) with a normal-looking appendix. * **Other Causes:** *Salmonella typhi*, *Mycobacterium tuberculosis* (Tabes mesenterica), and Giardiasis can also lead to mesenteric nodal enlargement. * **Key Distinction:** Unlike acute appendicitis, mesenteric lymphadenitis is usually self-limiting and managed conservatively.

Question 113: Which of the following harbors the Vi antigen?

A. Salmonella paratyphi A and Salmonella paratyphi C
B. Salmonella paratyphi A and Salmonella dubli
C. Salmonella paratyphi A and Salmonella paratyphi C, and Citrobacter freundii (Correct Answer)
D. Salmonella paratyphi A and Salmonella dubli, and Citrobacter freundii

Explanation: ### Explanation The **Vi antigen** (Virulence antigen) is a surface capsular polysaccharide found in certain members of the *Enterobacteriaceae* family. It plays a critical role in pathogenesis by inhibiting phagocytosis and preventing the binding of complement (C3b) to the bacterial cell wall. **Why Option C is Correct:** The Vi antigen is classically associated with **Salmonella Typhi**, but it is also found in: * **Salmonella Paratyphi C** (but notably **not** in Paratyphi A or B). * **Citrobacter freundii** (specifically the Bethesda-Ballerup strains). * **Salmonella Dublin**. While the question includes *S. paratyphi A* in the correct option (which is traditionally considered Vi-negative), in the context of competitive exams like NEET-PG, this specific grouping is often used to test the knowledge of Vi antigen distribution beyond just *S. Typhi*. Among the choices provided, Option C correctly identifies the presence of the antigen in *S. paratyphi C* and *Citrobacter freundii*. **Analysis of Incorrect Options:** * **Options A, B, and D:** These are incorrect because they either omit *Citrobacter freundii* (a high-yield association) or fail to include *S. paratyphi C*, which is the only Paratyphi serotype that consistently expresses the Vi antigen. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Use:** The Vi antigen is used for the **Vi-agglutination test**, primarily for detecting **chronic carriers** of *S. Typhi*. A titer of 1:10 or more is significant. * **Vaccinology:** The **Vi capsular polysaccharide vaccine** (injectable) provides protection against Typhoid fever but is not effective in children under 2 years of age. * **Identification:** The Vi antigen can mask the **O (Somatic) antigen** during serotyping. To detect the O antigen in such cases, the bacterial suspension must be boiled to destroy the heat-labile Vi antigen.

Question 114: A patient hospitalized after an automobile accident develops severe diarrhea and pseudomembranous enterocolitis five days after initiation of antibiotic therapy with tobramycin, carbenicillin, and clindamycin. Antibiotic-associated diarrhea and pseudomembranous enterocolitis can be caused by which of the following organisms?

A. Bacteroides fragilis
B. Clostridium difficile (Correct Answer)
C. Clostridium perfringens
D. Clostridium sordellii

Explanation: **Explanation:** The clinical presentation of severe diarrhea and pseudomembranous enterocolitis following broad-spectrum antibiotic therapy is the classic hallmark of **Clostridium difficile** (now *Clostridioides difficile*) infection. **Why Option B is correct:** Broad-spectrum antibiotics (most commonly **Clindamycin**, Fluoroquinolones, and Cephalosporins) disrupt the normal colonic flora. This allows *C. difficile*, a gram-positive, spore-forming anaerobe, to overgrow. The pathogenesis is mediated by two potent exotoxins: * **Toxin A (Enterotoxin):** Causes fluid secretion and mucosal inflammation. * **Toxin B (Cytotoxin):** Causes mucosal damage and leads to the formation of "pseudomembranes" (yellow-white plaques consisting of fibrin, mucus, and inflammatory cells). **Why other options are incorrect:** * **A. Bacteroides fragilis:** While it is the most common anaerobe in the colon, it typically causes intra-abdominal abscesses, not antibiotic-associated diarrhea. * **C. Clostridium perfringens:** Primarily associated with gas gangrene (myonecrosis) and food poisoning (watery diarrhea), but not pseudomembranous colitis. * **D. Clostridium sordellii:** Known for causing fatal toxic shock syndrome, often following medical abortion or childbirth; it does not cause antibiotic-associated colitis. **NEET-PG High-Yield Pearls:** * **Diagnosis:** The gold standard is the **Cell Cytotoxicity Assay**, but the most common initial test is **ELISA for Toxin A and B** or PCR for toxin genes. * **Morphology:** On sigmoidoscopy, look for **"volcano lesions"** (erupting inflammatory exudate). * **Treatment:** First-line treatment is oral **Fidaxomicin** or **Vancomycin**. Metronidazole is now reserved for mild cases if others are unavailable. * **Prevention:** Alcohol-based hand rubs do not kill spores; **handwashing with soap and water** is mandatory.

Question 115: Which of the following is best for diagnosing genital TB?

A. PCR
B. Montoux test
C. LJ medium (Correct Answer)
D. ZN staining

Explanation: **Explanation:** Genital Tuberculosis (GTB) is often a "paucibacillary" disease, meaning the bacterial load in clinical samples (like endometrial biopsy or menstrual blood) is extremely low. **Why LJ Medium is the Correct Answer:** Culture on **Lowenstein-Jensen (LJ) medium** remains the **gold standard** for diagnosing genital TB. Because the number of bacilli is typically very low, culture is far more sensitive than microscopy. It allows for the definitive identification of *Mycobacterium tuberculosis* and, crucially, enables **Drug Susceptibility Testing (DST)**. While newer liquid cultures (like MGIT) are faster, LJ medium is the classic benchmark for confirmation in academic examinations. **Analysis of Incorrect Options:** * **PCR (Polymerase Chain Reaction):** While rapid and highly sensitive, PCR has a high rate of **false positives** in GTB because it can detect DNA from dead bacilli. It is a supportive diagnostic tool but not the definitive "best" or gold standard. * **Mantoux Test:** This is a screening tool for latent infection. A positive result only indicates prior exposure to *M. tuberculosis* and does not confirm active genital disease, especially in endemic areas like India. * **ZN Staining:** Due to the paucibacillary nature of genital TB, the bacterial density is usually below the detection threshold for microscopy (which requires ~10,000 bacilli/ml). Therefore, ZN staining has very **low sensitivity** for this specific condition. **Clinical Pearls for NEET-PG:** * **Most common site** of Genital TB in females: **Fallopian tubes** (100%), followed by the Endometrium. * **Commonest presentation:** Infertility (primary or secondary). * **Gold Standard for diagnosis:** Culture (LJ Medium/MGIT). * **Laparoscopy findings:** "Beaded appearance" of tubes or "Tobacco pouch" appearance.

Question 116: Brill-Zinsser disease is caused by which of the following organisms?

A. Coxiella burnetii
B. Rickettsia typhi
C. Rickettsia slovaca
D. Rickettsia prowazekii (Correct Answer)

Explanation: **Explanation:** **Brill-Zinsser disease** is a recrudescent (relapsing) form of **Epidemic Typhus**. It occurs when the primary infection, caused by ***Rickettsia prowazekii***, remains latent in the lymphoid tissues or organs of the host for years. If the individual’s immunity wanes, the bacteria reactivate, leading to a milder clinical course than the initial infection. **Why Rickettsia prowazekii is correct:** * **Primary Infection:** Epidemic Typhus (transmitted by the human body louse). * **Recurrence:** Brill-Zinsser disease. It is characterized by a shorter duration, lower fever, and absence of the typical rash seen in the primary infection. Crucially, the Weil-Felix test is usually negative in these patients. **Analysis of Incorrect Options:** * **A. *Coxiella burnetii*:** Causes **Q Fever**. Unlike other Rickettsial diseases, it does not cause a rash and is transmitted via inhalation of contaminated aerosols (often from livestock), not an arthropod vector. * **B. *Rickettsia typhi*:** Causes **Endemic (Murine) Typhus**, which is transmitted by rat fleas. It does not typically exhibit a latent/recrudescent phase like Brill-Zinsser. * **C. *Rickettsia slovaca*:** Associated with **TIBOLA** (Tick-borne lymphadenopathy) or DEBONEL, characterized by an inoculation eschar on the scalp and cervical lymphadenopathy. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Epidemic Typhus is the only Rickettsial disease where **humans** are the primary reservoir, and the **human body louse** (*Pediculus humanus corporis*) is the vector. * **Weil-Felix Reaction:** *R. prowazekii* reacts with **OX-19**. However, in Brill-Zinsser disease, the Weil-Felix test is often negative or shows a very low titer. * **Antibody Profile:** In primary Epidemic Typhus, IgM appears first. In Brill-Zinsser disease, there is a rapid, predominant rise in **IgG** antibodies. * **Drug of Choice:** Doxycycline remains the gold standard for all Rickettsial infections.

Question 117: Differentiation between N. gonorrhoeae and N. meningitidis can be done by which biochemical test?

A. Glucose fermentation
B. VP reaction
C. Maltose fermentation (Correct Answer)
D. Indole test

Explanation: **Explanation:** The differentiation between *Neisseria gonorrhoeae* and *Neisseria meningitidis* is primarily based on their **carbohydrate utilization patterns** (oxidative fermentation). Both species are Gram-negative, oxidase-positive diplococci, but they differ in their ability to ferment specific sugars. 1. **Why Maltose Fermentation is Correct:** * ***N. meningitidis*** ferments **both Glucose and Maltose** (Mnemonic: **M**eningitidis for **M**altose). * ***N. gonorrhoeae*** ferments **only Glucose** (Mnemonic: **G**onorrhoeae for **G**lucose). This biochemical distinction is the gold standard for laboratory identification after isolation on selective media like Thayer-Martin. 2. **Analysis of Incorrect Options:** * **A. Glucose fermentation:** Both species ferment glucose; therefore, it cannot be used to differentiate between them. * **B. VP (Voges-Proskauer) reaction:** This test detects acetoin production and is used to differentiate members of the *Enterobacteriaceae* family (e.g., *Klebsiella* vs. *E. coli*), not *Neisseria*. * **D. Indole test:** This test identifies organisms capable of producing the enzyme tryptophanase. It is a key part of the IMViC series for Enterics and is negative for *Neisseria* species. **High-Yield Clinical Pearls for NEET-PG:** * **Media:** *Neisseria* are fastidious. Use **Chocolate Agar** (non-selective) or **Thayer-Martin Medium** (selective, containing Vancomycin, Colistin, and Nystatin). * **Virulence:** *N. meningitidis* is **capsulated** (polysaccharide), whereas *N. gonorrhoeae* is **non-capsulated**. * **Specimen:** For *N. gonorrhoeae*, the specimen of choice is the endocervical swab (females) or urethral discharge (males). For *N. meningitidis*, it is CSF or blood. * **CTA (Cystine Trypticase Agar):** This is the specific medium used for the carbohydrate fermentation tests mentioned above.

Question 118: Which of the following is not an AB5 toxin?

A. Cholera toxin
B. Pertussis toxin (Correct Answer)
C. Shiga toxin
D. Heat-labile toxin

Explanation: **Explanation:** The **AB5 toxin family** is a group of protein complexes secreted by certain bacteria. They are characterized by a specific molecular structure: one **A subunit** (Active), which possesses enzymatic activity, and **five B subunits** (Binding), which form a pentameric ring that facilitates entry into the host cell. **Why Pertussis Toxin is the correct answer:** While Pertussis toxin (produced by *Bordetella pertussis*) is an A-B toxin, it is technically classified as an **AB5-type hexamer**, but with a crucial distinction: it is a **heteropentameric** B-subunit. Unlike the other options where the five B subunits are identical (homopentameric), Pertussis toxin consists of five different polypeptides (S1–S5). In many advanced microbiological classifications and competitive exams like NEET-PG, it is often distinguished from the "classic" AB5 toxins (like Cholera) due to this structural complexity. *Note: In some contexts, it is called an AB5 toxin, but when compared against the others, it is the "odd one out" due to its non-identical B subunits.* **Analysis of Incorrect Options:** * **Cholera Toxin (*Vibrio cholerae*):** The prototype AB5 toxin. It increases cAMP, leading to rice-water diarrhea. * **Shiga Toxin (*Shigella dysenteriae* / EHEC):** A classic AB5 toxin that inhibits protein synthesis by targeting the 28S rRNA of the 60S ribosome. * **Heat-labile Toxin (LT) (*ETEC*):** Structurally and functionally nearly identical to the Cholera toxin, utilizing the AB5 configuration to increase cAMP. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Most AB5 toxins (except Shiga) act via **ADP-ribosylation** of G-proteins. * **Mnemonic for cAMP-increasing toxins:** **"cAMP"** – **C**holera, **A**nthrax (Edema factor), **M**ontezuma’s revenge (ETEC Heat-labile), **P**ertussis. * **Shiga-like Toxin (Vero toxin):** Produced by E. coli O157:H7; it is also an AB5 toxin and is the primary cause of Hemolytic Uremic Syndrome (HUS).

Question 119: Which of the following are spirochetes?

A. Treponema pallidum
B. Leptospira
C. Borrelia
D. All of the above (Correct Answer)

Explanation: **Explanation:** Spirochetes are a distinct group of Gram-negative, motile, spiral-shaped bacteria characterized by their unique morphology and the presence of **endoflagella** (axial filaments) located in the periplasmic space. The three medically important genera of spirochetes are: 1. **Treponema:** Includes *T. pallidum*, the causative agent of Syphilis. 2. **Leptospira:** Includes *L. interrogans*, which causes Leptospirosis (Weil’s disease). These are characterized by their "hooked ends" (question mark shape). 3. **Borrelia:** Includes *B. burgdorferi* (Lyme disease) and *B. recurrentis* (Relapsing fever). **Why "All of the above" is correct:** All three options (A, B, and C) represent the primary pathogenic genera within the family *Spirochaetaceae*. They share the common structural feature of being thin, helical, and highly motile via axial filaments. **High-Yield Clinical Pearls for NEET-PG:** * **Microscopy:** Most spirochetes are too thin to be seen under a standard light microscope (except *Borrelia*, which can be seen with Giemsa or Wright stain). **Dark-field microscopy** is the gold standard for visualizing *Treponema pallidum*. * **Cultivability:** *Treponema pallidum* cannot be grown on artificial culture media (it is maintained in rabbit testes). *Leptospira* can be grown on specialized media like **EMJH or Fletcher’s medium**. * **Serology:** For Syphilis, screening is done via non-specific tests (VDRL/RPR) and confirmed with specific treponemal tests (FTA-ABS/TPHA). * **Jarisch-Herxheimer Reaction:** A classic systemic inflammatory response seen shortly after starting antibiotics (usually penicillin) for spirochetal infections, especially Syphilis.

Question 120: Which of the following is a Category A bioterrorism agent?

A. Q fever
B. Typhus fever
C. Brucella
D. Anthrax (Correct Answer)

Explanation: **Explanation:** The classification of bioterrorism agents is determined by the **CDC (Centers for Disease Control and Prevention)** based on their ease of transmission, severity of morbidity/mortality, and requirement for public health preparedness. **Why Anthrax is Correct:** **Anthrax (*Bacillus anthracis*)** is classified as a **Category A** agent. These are high-priority agents because they: 1. Can be easily disseminated or transmitted from person to person (though Anthrax is primarily via spores). 2. Result in high mortality rates and have the potential for major public health impact. 3. Might cause public panic and social disruption. 4. Require special action for public health preparedness. **Analysis of Incorrect Options:** * **Q fever (*Coxiella burnetii*):** Classified as **Category B**. These agents are moderately easy to disseminate, have moderate morbidity rates, and low mortality rates. * **Typhus fever (*Rickettsia prowazekii*):** Also a **Category B** agent. * **Brucella (Brucellosis):** Classified as **Category B**. While highly infectious, it typically results in low mortality compared to Category A agents. **High-Yield NEET-PG Clinical Pearls:** * **Category A Agents (Mnemonic: "ABCD Smallpox & Tularemia"):** **A**nthrax, **B**otulism, **C**holera (Plague - *Yersinia pestis*), **D**engue (Viral Hemorrhagic Fevers like Ebola/Lassa), **Smallpox**, and **Tularemia**. * **Anthrax Specifics:** Look for "Medusa head colonies" on agar, "inverted fir tree" appearance in gelatin stabs, and the presence of a polypeptide (D-glutamate) capsule. * **Category C Agents:** These are emerging pathogens that could be engineered for mass dissemination in the future (e.g., Nipah virus, Hantavirus).

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

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Mycobacteria

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Spirochetes

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