Bacteriology — MCQs

A 20-year-old male presented with a painless genital ulcer of 10 days duration, which was circumscribed, indurated, and partially healed. He reported unprotected sexual contact with a commercial sex worker 2 weeks prior to presentation. Examination revealed enlarged inguinal lymph nodes. What is the probable cause of infection based on the history and genital lesion?

Q1548Medium

Which of the following is true about Rheumatic heart disease and Rheumatic fever?

Q1549Easy

Elek's gel precipitation test is done for which bacterium?

Q1550Easy

What is the mode of transmission of Q fever?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1541: True about primary chancre?

A. Painless ulcer
B. Painless lymphadenopathy
C. Covered with exudate
D. All (Correct Answer)

Explanation: The primary chancre is the hallmark of **Primary Syphilis**, caused by the spirochete *Treponema pallidum*. Understanding its clinical presentation is crucial for differentiating it from other genital ulcerative diseases. ### **Explanation of Options:** * **Painless Ulcer (Option A):** The chancre typically begins as a papule that erodes into a clean-based, indurated (hard) ulcer. Unlike chancroid (caused by *H. ducreyi*), the syphilitic chancre is characteristically **painless** because the organism does not provoke an acute inflammatory response in local nerve endings. * **Painless Lymphadenopathy (Option B):** Within 1–2 weeks of the ulcer's appearance, regional lymph nodes (usually inguinal) become enlarged. These nodes are typically **bilateral, firm, discrete, and painless** (often referred to as "shotty" nodes). * **Covered with Exudate (Option C):** While the base of the ulcer is often described as "clean," it frequently exudes a **clear serous fluid**. This fluid is highly infectious and contains a high concentration of spirochetes, which can be visualized via Dark Ground Microscopy (DGM). Since all three clinical features are characteristic of the primary stage of syphilis, **Option D (All)** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Usually 3 weeks (range 10–90 days). * **Induration:** The "hard" feel of the chancre is a classic diagnostic sign (Hunterian chancre). * **Diagnosis:** **Dark Ground Microscopy (DGM)** is the gold standard for primary syphilis. Serological tests (VDRL/RPR) may be negative in the first 1–2 weeks of the chancre's appearance (the "seronegative" phase). * **Differential:** Always differentiate from **Chancroid** (Painful ulcer + Painful lymphadenopathy/Bubo).

Question 1542: Stalactite growth is a feature of which of the following organisms?

A. Haemophilus
B. Pasteurella (Correct Answer)
C. Corynebacterium
D. Mycoplasma

Explanation: **Explanation:** **Correct Option: B (Pasteurella)** The term **"Stalactite growth"** refers to a characteristic growth pattern in liquid media (like broth) where the bacteria grow as a surface pellicle with delicate, thread-like extensions hanging down into the medium, resembling stalactites in a cave. This is a classic laboratory feature of **Yersinia pestis** (formerly classified under the genus *Pasteurella* as *Pasteurella pestis*). In the context of NEET-PG, *Pasteurella* is the designated answer when referring to this specific morphology. This growth occurs when the culture is left undisturbed, often with a drop of oil or butter on the surface to provide a scaffold. **Why other options are incorrect:** * **A. Haemophilus:** Requires X (Hemin) and V (NAD) factors for growth. It typically shows "Satellitism" around *S. aureus* on blood agar, not stalactite growth. * **C. Corynebacterium:** *C. diphtheriae* shows "Granular" growth or pellicle formation in broth, but the specific "stalactite" description is unique to *Yersinia/Pasteurella*. It is better known for its "Chinese letter" arrangement on microscopy. * **D. Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. They produce characteristic **"Fried egg" colonies** on solid media (PPLO agar), not stalactite growth in broth. **High-Yield Clinical Pearls for NEET-PG:** * **Yersinia pestis (Pasteurella pestis):** Shows **Safety-pin appearance** (bipolar staining) with Wayson or Giemsa stain. * **Transmission:** Primarily via the bite of the **Rat flea (*Xenopsylla cheopis*)**. * **Culture:** Optimal growth at **27°C** (lower than the usual 37°C). * **Virulence Factors:** V and W antigens, and the F1 (Fraction 1) capsular antigen.

Question 1543: Classification of pathogenic streptococci into group A, B, C, D, and G is based on which of the following characteristics?

A. Color of colonies on blood agar
B. Antigenicity of cell wall carbohydrate (Correct Answer)
C. Presence or absence of a capsule
D. Presence or absence of M protein

Explanation: **Explanation:** The classification of Streptococci into groups A, B, C, D, and G is known as the **Lancefield Classification**, developed by Rebecca Lancefield in 1933. **1. Why the correct answer is right:** This classification is based on the **antigenicity of the group-specific C-carbohydrate (polysaccharide)** found in the bacterial cell wall. By using specific antisera to identify these carbohydrates, Streptococci are categorized into groups A through V (excluding I and J). This is a serological classification and is the gold standard for identifying pathogenic species like *S. pyogenes* (Group A) and *S. agalactiae* (Group B). **2. Why the incorrect options are wrong:** * **Option A (Color of colonies):** This refers to **Brown’s Classification**, which categorizes Streptococci based on hemolytic patterns on sheep blood agar (Alpha: green/partial; Beta: clear/complete; Gamma: no hemolysis). * **Option C (Capsule):** While some Streptococci (like *S. pneumoniae*) are encapsulated, the presence of a capsule is used for the Quellung reaction, not for Lancefield grouping. * **Option D (M protein):** M protein is a major virulence factor used for **Griffith typing** (sub-typing) of Group A Streptococci, not for the primary group classification. **High-Yield Clinical Pearls for NEET-PG:** * **Group A (GAS):** *S. pyogenes* (Bacitracin sensitive, PYR positive). * **Group B (GBS):** *S. agalactiae* (CAMP test positive, Hippurate hydrolysis positive); leading cause of neonatal meningitis. * **Group D:** Includes *Enterococcus* (can grow in 6.5% NaCl) and *S. bovis* (associated with colon cancer). * **Viridans group & *S. pneumoniae*:** These lack the Lancefield antigen and therefore cannot be classified under this system.

Question 1544: What is true about Campylobacter?

A. Possesses polar flagella (Correct Answer)
B. Grows at 25°C
C. Is a strict aerobe
D. Is psychrophilic

Explanation: **Explanation:** *Campylobacter* species (most commonly *C. jejuni*) are small, curved, Gram-negative bacilli known for their characteristic "comma" or "seagull-wing" shape. **Why Option A is Correct:** *Campylobacter* is highly motile due to the presence of a **single polar flagellum** at one or both ends (monotrichous or amphitrichous). This flagellum provides the organism with its characteristic **"darting motility,"** which is a classic diagnostic feature observed under dark-ground or phase-contrast microscopy. **Why the Other Options are Incorrect:** * **Option B & D:** *Campylobacter jejuni* is **thermophilic**, not psychrophilic. It grows optimally at **42°C** (the body temperature of poultry, its primary reservoir) and fails to grow at 25°C. This temperature requirement is used selectively in laboratories to isolate it from other fecal flora. * **Option C:** *Campylobacter* is a **microaerophilic** organism, not a strict aerobe. It requires a reduced oxygen environment (typically 5% $O_2$, 10% $CO_2$, and 85% $N_2$) for optimal growth. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Uses selective media like **Skirrow’s medium** or **Butzler’s medium**. * **Clinical Presentation:** It is a leading cause of bacterial gastroenteritis (bloody diarrhea). * **Important Association:** Infection with *C. jejuni* (specifically serotype O:19) is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry between bacterial lipooligosaccharides and human gangliosides. * **Oxidase and Catalase:** Most species are positive for both.

Question 1545: Botulism is most commonly seen with which of the following?

A. Food with preservatives
B. Home-canned food (Correct Answer)
C. Commercially canned food
D. Unpasteurized milk

Explanation: **Explanation:** **Botulism** is a severe paralytic illness caused by the neurotoxin produced by *Clostridium botulinum*, an anaerobic, Gram-positive, spore-forming bacillus. **Why Home-canned food is the correct answer:** The most common form of the disease is **Foodborne Botulism**. *C. botulinum* spores are ubiquitous in soil. When food is canned at home, the heating process often fails to reach the high temperatures and pressures required to kill these heat-resistant spores. In the anaerobic environment of a sealed jar, the spores germinate into vegetative cells and release the potent **botulinum toxin**. Low-acid foods (e.g., green beans, corn, beets) are the most frequent culprits. **Analysis of Incorrect Options:** * **Food with preservatives:** Preservatives (like nitrites in cured meats) are specifically added to inhibit the growth of *Clostridium* species. * **Commercially canned food:** Industrial canning uses "retort" processing (high-pressure steam), which ensures the "Botulinum cook" standard is met, making commercial outbreaks extremely rare. * **Unpasteurized milk:** While a risk for *Listeria*, *Brucella*, and *Salmonella*, it is not a typical vehicle for botulism as it is not an anaerobic environment. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** The toxin cleaves **SNARE proteins**, preventing the release of **Acetylcholine** at the neuromuscular junction. * **Clinical Triad:** Symmetric **descending paralysis**, clear sensorium, and absence of fever. * **Infant Botulism:** Most common type overall; associated with **honey** ingestion (ingestion of spores, not pre-formed toxin). * **Diagnosis:** Demonstration of toxin in serum, stool, or suspected food via **Mouse Bioassay** (Gold Standard). * **Treatment:** Equine antitoxin (Heptavalent). Note: Antibiotics are contraindicated in foodborne botulism as they don't neutralize the toxin.

Question 1546: A patient presented with fever, headache, rash, and eschar. You suspect a rickettsial disease. Which of the following is the LEAST likely disease in this case?

A. Indian tick typhus
B. Rickettsial pox
C. Scrub typhus
D. Endemic typhus (Correct Answer)

Explanation: ### Explanation The presence of an **eschar** (a necrotic, black crust at the site of the vector bite) is a crucial clinical diagnostic clue in rickettsial diseases. This question tests your ability to differentiate between rickettsial infections based on this specific cutaneous finding. **1. Why Endemic Typhus is the correct answer:** Endemic typhus (also known as Murine typhus) is caused by *Rickettsia typhi* and transmitted by the rat flea (*Xenopsylla cheopis*). Unlike the spotted fever group or scrub typhus, **Endemic typhus and Epidemic typhus (Louse-borne) typically do not present with an eschar.** Therefore, it is the least likely diagnosis in a patient presenting with an eschar. **2. Analysis of Incorrect Options:** * **Indian Tick Typhus:** Caused by *R. conorii*. As a member of the Spotted Fever Group, it is frequently associated with a characteristic eschar (tache noire) at the tick bite site. * **Rickettsial Pox:** Caused by *R. akari*. It is unique because it presents with a papulovesicular rash and is classically preceded by a **prominent eschar** at the site of the mite bite. * **Scrub Typhus:** Caused by *Orientia tsutsugamushi*. The **eschar** is a hallmark clinical sign of scrub typhus (found in up to 50% of cases), often located in skin folds or axilla. ### Clinical Pearls for NEET-PG: * **Eschar-Positive Diseases:** Scrub typhus, Rickettsial pox, and most Spotted Fever Group diseases (except Rocky Mountain Spotted Fever, where an eschar is rare). * **Eschar-Negative Diseases:** Epidemic typhus, Endemic typhus, and Q fever. * **Weil-Felix Reaction:** * Scrub Typhus: OX-K positive. * Endemic/Epidemic Typhus: OX-19 positive. * Spotted Fever Group: OX-19 and OX-2 positive. * **Q Fever:** Negative for all Weil-Felix antigens.

Question 1547: A 20-year-old male presented with a painless genital ulcer of 10 days duration, which was circumscribed, indurated, and partially healed. He reported unprotected sexual contact with a commercial sex worker 2 weeks prior to presentation. Examination revealed enlarged inguinal lymph nodes. What is the probable cause of infection based on the history and genital lesion?

A. HIV
B. HPV
C. Treponema pallidum (Correct Answer)
D. HSV-2

Explanation: **Explanation:** The clinical presentation is a classic case of **Primary Syphilis**, caused by the spirochete ***Treponema pallidum***. **Why Treponema pallidum is correct:** The hallmark of primary syphilis is the **Hard Chancre**. The key diagnostic features provided in the history are: * **Painless and Indurated:** Unlike other genital ulcers, syphilitic chancres are typically non-tender and have a firm, "button-like" consistency (induration). * **Incubation Period:** The 2-week timeline fits perfectly (average incubation is 3 weeks, range 10–90 days). * **Lymphadenopathy:** It is associated with painless, non-suppurative, rubbery inguinal lymphadenopathy. * **Spontaneous Healing:** The ulcer is "partially healed," reflecting the natural course where the chancre resolves without treatment in 3–6 weeks. **Why other options are incorrect:** * **HIV:** Primarily presents as an acute retroviral syndrome (fever, rash, lymphadenopathy) but does not cause a localized indurated genital ulcer. * **HPV:** Causes **Genital Warts (Condyloma acuminata)**, which are fleshy, cauliflower-like growths, not ulcerative lesions. * **HSV-2:** Causes **Genital Herpes**, characterized by multiple, **painful**, superficial vesicles or ulcers on an erythematous base. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Dark-field microscopy is the gold standard for primary syphilis (shows corkscrew motility). Non-specific serological tests (VDRL/RPR) may be negative in the first 1–2 weeks of the chancre. * **Treatment:** Drug of choice is **Benzathine Penicillin G** (2.4 million units IM, single dose). * **Differential:** Always differentiate from *Haemophilus ducreyi* (Chancroid), which causes a **painful**, soft ulcer with suppurative lymphadenopathy (Bubo).

Question 1548: Which of the following is true about Rheumatic heart disease and Rheumatic fever?

A. Caused by group A Streptococcus (Correct Answer)
B. Caused by group B Streptococcus
C. Carriers have a high risk of developing Rheumatic Fever
D. M-5 protein cross-reacts

Explanation: **Explanation:** **1. Why Option A is Correct:** Acute Rheumatic Fever (ARF) and its sequel, Rheumatic Heart Disease (RHD), are non-suppurative inflammatory complications following a pharyngeal infection with **Group A Beta-Hemolytic Streptococcus (GABHS)**, also known as *Streptococcus pyogenes*. The pathogenesis is based on **Type II Hypersensitivity** (Molecular Mimicry), where antibodies produced against the streptococcal cell wall (specifically the M-protein) cross-react with human tissues like the heart (myocardium), joints, and brain. **2. Why Other Options are Incorrect:** * **Option B:** Group B Streptococcus (*S. agalactiae*) is primarily associated with neonatal sepsis and meningitis, not rheumatic fever. * **Option C:** Carriers (individuals with positive throat cultures but no active immune response/symptoms) have a **very low risk** of developing ARF. The risk is significantly higher in those with symptomatic, untreated exudative pharyngitis. * **Option D:** While M-protein is the chief virulence factor, the classic cross-reactive serotypes associated with ARF are **M-1, 3, 5, 6, 14, 18, 19, and 24**. However, the statement "M-5 protein cross-reacts" is a specific detail; the most fundamental and universally "true" statement among the choices is the causative agent (Group A Strep). **High-Yield NEET-PG Pearls:** * **Jones Criteria:** Used for diagnosis (Major: Joint, Carditis, Nodules, Erythema marginatum, Sydenham chorea). * **ASO Titer:** Evidence of preceding infection (Significant if >200 units). * **Aschoff Bodies:** Pathognomonic histological feature in the myocardium (contains **Anitschkow cells** or "caterpillar cells"). * **Prophylaxis:** Benzathine Penicillin G is the drug of choice for secondary prevention.

Question 1549: Elek's gel precipitation test is done for which bacterium?

A. Corynebacterium diphtheria (Correct Answer)
B. Salmonella choleraesuis
C. Clostridium botulinum
D. Diphtheroids

Explanation: **Explanation:** **Elek’s gel precipitation test** is an *in vitro* toxigenicity test used specifically to identify toxigenic strains of **Corynebacterium diphtheriae**. 1. **Why Option A is correct:** The pathogenicity of *C. diphtheriae* depends on the production of the diphtheria toxin (encoded by the *tox* gene via a lysogenic bacteriophage). Elek’s test works on the principle of **immunodiffusion**. A filter paper strip impregnated with antitoxin is placed on an agar plate, and the test strain is streaked perpendicular to it. If the strain is toxigenic, the secreted toxin reacts with the antitoxin, forming visible white **precipitin lines** (arrowhead formation) at a 45-degree angle. 2. **Why other options are incorrect:** * **Salmonella choleraesuis:** Identified via biochemical tests and serotyping (Widal test is for enteric fever, not toxigenicity). * **Clostridium botulinum:** Toxigenicity is typically demonstrated via the mouse bioassay (lethality test) or ELISA, not Elek’s test. * **Diphtheroids:** These are commensal *Corynebacterium* species (e.g., *C. xerosis*). They are generally non-toxigenic and do not produce the diphtheria toxin; therefore, they yield a negative Elek’s test. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The Elek test is the standard *in vitro* method, while the **Schick test** (historical) was used to check immunity status *in vivo*. * **Culture Media:** *C. diphtheriae* grows on **LSS (Loeffler's Serum Slope)** and **Hoyle’s Tellurite Agar** (producing black colonies). * **Morphology:** Characterized by **Albert’s stain** showing metachromatic granules (Volutin/Babes-Ernst granules) in a cuneiform/Chinese-letter arrangement. * **Rapid Diagnosis:** PCR for the *tox* gene is now the fastest method to detect toxigenic potential.

Question 1550: What is the mode of transmission of Q fever?

A. Bite of infected louse
B. Bite of infected tick
C. Inhalation of aerosol (Correct Answer)
D. Bite of infected mice

Explanation: **Explanation:** **Q Fever** is caused by the obligate intracellular bacterium ***Coxiella burnetii***. Unlike other Rickettsial diseases, the primary mode of transmission to humans is the **inhalation of contaminated aerosols** or dust. 1. **Why Option C is Correct:** *C. burnetii* is highly resistant to environmental stressors (heat, drying) due to its spore-like stage. It is found in high concentrations in the birth products (placenta), urine, and feces of infected livestock (sheep, goats, cattle). When these materials dry, the bacteria become airborne, and humans are infected by inhaling as few as 1 to 10 organisms. 2. **Why Other Options are Incorrect:** * **Option A (Louse):** This is the vector for Epidemic Typhus (*Rickettsia prowazekii*). * **Option B (Tick):** While ticks maintain the cycle among wild animals and can transmit the bacteria to livestock, they are **not** the primary source of infection for humans. * **Option D (Mice):** Rodents are not the typical reservoir for Q fever; it is primarily a zoonosis of farm animals. **High-Yield Clinical Pearls for NEET-PG:** * **Occupational Hazard:** Most common in farmers, veterinarians, and abattoir workers. * **Clinical Presentation:** Presents as an atypical pneumonia or hepatitis (Acute Q fever). Chronic Q fever most commonly manifests as **culture-negative endocarditis**. * **Diagnosis:** Serology is the gold standard (Indirect Immunofluorescence Assay). * **Weil-Felix Test:** Q fever is **Weil-Felix negative** (unlike most other Rickettsial diseases). * **Treatment:** Doxycycline is the drug of choice.

Practice by Chapter

Staphylococci

Practice Questions

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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