Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 331: What is the incubation period for Bacillus cereus food poisoning?

A. Staphylococcus aureus
B. Bacillus cereus (Correct Answer)
C. Salmonellosis
D. Vibrio cholera

Explanation: **Explanation:** *Bacillus cereus* is a Gram-positive, spore-forming aerobic rod known for causing two distinct types of food poisoning based on the production of different toxins. The question focuses on the **Emetic type**, which is characterized by a very short incubation period (1–6 hours), similar to *Staphylococcus aureus*. **1. Why Bacillus cereus is correct:** The emetic syndrome is caused by a preformed, heat-stable toxin called **Cereulide**. Because the toxin is already present in the food (classically **reheated rice**), symptoms like nausea and vomiting manifest rapidly after ingestion. There is also a **Diarrheal type** caused by a heat-labile enterotoxin produced in the intestine, which has a longer incubation period (8–16 hours). **2. Why the other options are incorrect:** * **Staphylococcus aureus:** While it also has a short incubation period (1–6 hours) due to preformed enterotoxins, it is typically associated with protein-rich foods (custard, mayonnaise, processed meats) rather than rice. * **Salmonellosis:** This is an invasive infection or toxico-infection with a much longer incubation period, typically **12–72 hours**, presenting with fever, abdominal cramps, and inflammatory diarrhea. * **Vibrio cholerae:** The incubation period is usually **1–3 days**. It produces a potent enterotoxin (Choleragen) in the gut, leading to profuse "rice-water" stools and severe dehydration. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic form:** Heat-stable toxin, 1–6 hours incubation, associated with **Fried Rice**. * **Diarrheal form:** Heat-labile toxin, 8–16 hours incubation, associated with **Meat/Vegetables**. * *Bacillus cereus* is also a significant cause of post-traumatic **endophthalmitis**. * **Nagler Reaction:** Used to identify *C. perfringens*, but *B. cereus* also produces lecithinase, showing a positive reaction on Egg Yolk Agar.

Question 332: The 7th pandemic of cholera is primarily attributed to which of the following?

A. El Tor biotype (Correct Answer)
B. O139 Vibrio cholerae
C. Classical Vibrio cholerae
D. Vibrio mimicus

Explanation: **Explanation:** The 7th pandemic of cholera, which began in 1961 in Indonesia and eventually spread globally, is caused by **Vibrio cholerae O1, El Tor biotype**. This biotype replaced the Classical biotype that was responsible for the previous six pandemics. **Why El Tor is the correct answer:** The El Tor biotype possesses specific characteristics that allowed it to trigger and sustain the 7th pandemic: * **Higher survival rate:** It survives longer in the environment compared to the Classical biotype. * **Asymptomatic carriage:** It causes a higher ratio of asymptomatic-to-symptomatic infections (approx. 50:1), facilitating silent transmission. * **Resistance:** It is generally more resistant to polymyxin B and exhibits different biochemical properties (VP test positive, Hemolysis positive). **Analysis of Incorrect Options:** * **B. O139 Vibrio cholerae:** This strain emerged in 1992 in Madras (Chennai). While it caused large-scale epidemics and was once considered the potential start of an 8th pandemic, it remains localized to parts of Asia and has not officially replaced the 7th pandemic strain. * **C. Classical Vibrio cholerae:** This biotype was responsible for the **1st through 6th pandemics**. It is now largely extinct globally, except for sporadic cases in Bangladesh. * **D. Vibrio mimicus:** This is a distinct species of Vibrio that causes diarrhea through contaminated seafood but is not associated with cholera pandemics. **High-Yield Clinical Pearls for NEET-PG:** * **Biotype Differentiation:** El Tor is **CAMP test positive**, **Polymyxin B resistant**, and agglutinates chicken RBCs; Classical is the opposite for all three. * **Haldane and Pole’s Test:** Used to differentiate biotypes based on susceptibility to Group IV bacteriophage (Classical is sensitive; El Tor is resistant). * **Media of choice:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar; *V. cholerae* produces distinctive **yellow colonies** due to sucrose fermentation.

Question 333: Which of the following infections is caused by anaerobic gram-positive cocci?

A. Puerperal infection (Correct Answer)
B. Food poisoning
C. Endocarditis
D. Septicemia

Explanation: ### Explanation **1. Why Puerperal Infection is Correct:** Anaerobic Gram-positive cocci (AGPC), primarily **Peptostreptococcus** species, are significant members of the normal flora of the female genitourinary tract. They are frequently implicated in polymicrobial infections following childbirth or abortion, such as **puerperal sepsis**, pelvic inflammatory disease (PID), and tubo-ovarian abscesses. Their ability to thrive in the necrotic, anaerobic environment of the postpartum uterus makes them a leading cause of these infections. **2. Why Other Options are Incorrect:** * **Food Poisoning:** This is typically caused by Gram-positive bacilli (e.g., *Clostridium perfringens*, *Bacillus cereus*) or Gram-positive cocci that are **aerobic/facultative anaerobes** (e.g., *Staphylococcus aureus*). * **Endocarditis:** The most common causative agents are aerobic or facultative anaerobic cocci, such as *Viridans group Streptococci*, *Staphylococcus aureus*, or *Enterococci*. Anaerobic cocci are extremely rare causes of endocarditis. * **Septicemia:** While AGPC can cause bacteremia, "Septicemia" as a general clinical entity is most commonly associated with aerobic Gram-negative bacilli (like *E. coli*) or aerobic Gram-positive cocci (like *S. aureus*). **3. Clinical Pearls for NEET-PG:** * **Peptostreptococcus** is the most clinically relevant genus of AGPC. * **Specimen Collection:** Since these are anaerobes, samples must be collected via aspiration (not swabs) and transported in specialized anaerobic transport media. * **Synergy:** AGPC often act synergistically with other anaerobes (like *Bacteroides fragilis*) or facultative anaerobes, leading to foul-smelling discharges—a hallmark of anaerobic infection. * **Treatment:** They are generally sensitive to Penicillin, Metronidazole, and Clindamycin.

Question 334: The Sereny test is positive for which of the following bacteria?

A. Enterotoxigenic Escherichia coli (ETEC)
B. Enteropathogenic Escherichia coli (EPEC)
C. Enteroinvasive Escherichia coli (EIEC) (Correct Answer)
D. Enteroaggregative Escherichia coli (EAEC)

Explanation: **Explanation:** The **Sereny test** is a classic laboratory assay used to detect the **invasiveness** of certain enteric pathogens. It involves the inoculation of a bacterial suspension into the conjunctival sac of a guinea pig or rabbit. A positive result is indicated by the development of severe **keratoconjunctivitis** and purulent exudate within 24–72 hours. **Why EIEC is correct:** Enteroinvasive *Escherichia coli* (EIEC) possesses a large **inv plasmid** (pInv) that encodes for the invasion plasmid antigens (Ipa). This allows the bacteria to invade, multiply within, and spread between colonic epithelial cells, leading to tissue destruction. Because EIEC shares this invasive mechanism with *Shigella* spp., both organisms are **Sereny test positive**. **Why other options are incorrect:** * **ETEC (Option A):** Produces Heat-labile (LT) and Heat-stable (ST) toxins that cause secretory diarrhea without mucosal invasion. * **EPEC (Option B):** Causes "Attaching and Effacing" (A/E) lesions via the *eae* gene (intimin), leading to microvilli destruction but not deep cellular invasion. * **EAEC (Option C):** Characterized by a "stacked-brick" adhesion pattern to the mucosa; it produces toxins but is not inherently invasive. **High-Yield Clinical Pearls for NEET-PG:** * **Organisms positive for Sereny test:** *Shigella* species and EIEC. * **Mechanism:** Both organisms use **actin polymerization** (actin tails) for intracellular movement, similar to *Listeria monocytogenes*. * **Clinical Presentation:** EIEC causes a dysentery-like illness (fever, cramps, blood/mucus in stool) that is clinically indistinguishable from Shigelosis. * **Modern Alternative:** The Sereny test is rarely used today due to animal welfare concerns; it has been replaced by PCR detecting the **ipaH gene**.

Question 335: Which of the following statements about Bacillus anthracis is FALSE?

A. Causes anthrax
B. Toxin is tripartite
C. Capsule made up of polypeptide
D. Most common transmission is by inhalation of spores (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because it is a false statement regarding the epidemiology of *Bacillus anthracis*. While inhalation anthrax (Woolsorter’s disease) is the most severe and fatal form, it is **not** the most common. Approximately **95% of all anthrax cases** globally are **Cutaneous Anthrax**, which occurs through the inoculation of spores into skin abrasions while handling infected animal products (hides, wool, or bone meal). **Analysis of other options:** * **Option A (True):** *B. anthracis* is the causative agent of anthrax, a zoonotic disease primarily affecting herbivores. * **Option B (True):** The anthrax toxin is **tripartite**, consisting of three plasmid-encoded proteins: **Protective Antigen (PA)** (the binding unit), **Edema Factor (EF)** (an adenylate cyclase), and **Lethal Factor (LF)** (a metalloprotease). * **Option C (True):** Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a unique **polypeptide capsule** composed of **poly-D-glutamic acid**, which is essential for evading phagocytosis. **High-Yield NEET-PG Pearls:** * **Morphology:** Large, Gram-positive, non-motile bacilli with "square ends" (Box-car appearance). * **Culture:** Shows **"Medusa head"** colonies on agar and a **"Bamboo stick"** appearance on microscopy. * **String of Pearls Reaction:** Occurs when grown on agar containing penicillin. * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the purple capsule. * **Biological Warfare:** Spores are highly resistant and considered a Category A bioterrorism agent.

Question 336: What is the best rapid diagnostic test for the etiology of acute pyogenic meningitis?

A. Procalcitonin
B. Limulus test
C. Latex agglutination (Correct Answer)
D. Counterimmunoelectrophoresis

Explanation: **Explanation:** Acute pyogenic meningitis is a medical emergency requiring rapid identification of the causative pathogen (e.g., *S. pneumoniae*, *N. meningitidis*, *H. influenzae*) to guide targeted therapy. **Why Latex Agglutination is the Correct Choice:** Latex agglutination is currently the preferred rapid diagnostic test for detecting bacterial antigens in the Cerebrospinal Fluid (CSF). It involves latex beads coated with specific antibodies that clump (agglutinate) in the presence of the corresponding bacterial capsular antigen. * **Advantages:** It is highly sensitive, specific, and provides results within 15–20 minutes. Crucially, it can detect antigens even if the patient has already received antibiotics (which often renders cultures negative). **Analysis of Incorrect Options:** * **Procalcitonin (A):** This is a serum biomarker used to differentiate bacterial from viral infections. While elevated in bacterial meningitis, it does not identify the specific **etiology** (the name of the organism). * **Limulus Test (B):** This test specifically detects **endotoxins** from Gram-negative bacteria using the amoebocyte lysate of the Horseshoe crab (*Limulus polyphemus*). It cannot identify Gram-positive organisms like *S. pneumoniae* and does not specify the species. * **Counterimmunoelectrophoresis (D):** While this also detects bacterial antigens, it is slower, technically more complex, and less sensitive than modern latex agglutination. It has largely been replaced in clinical practice. **NEET-PG High-Yield Pearls:** * **Gold Standard:** CSF Culture remains the gold standard for diagnosis, but it takes 24–48 hours. * **Most Sensitive Rapid Method:** While Latex Agglutination is the standard "best rapid test" in many exams, **PCR** is technically the most sensitive molecular method if available. * **Quellung Reaction:** This is the "gold standard" for serotyping *S. pneumoniae* (capsular swelling).

Question 337: Which of the following diseases is transmitted by a mite vector?

A. Typhus fever (caused by Rickettsia typhi)
B. Epidemic typhus (caused by Rickettsia prowazekii)
C. Rocky Mountain spotted fever (caused by Rickettsia rickettsii)
D. Scrub typhus (caused by Orientia tsutsugamushi) (Correct Answer)

Explanation: **Explanation:** The correct answer is **Scrub typhus (Option D)**. This disease is caused by *Orientia tsutsugamushi* and is transmitted to humans through the bite of the larval stage (chigger) of **trombiculid mites**. The medical concept revolves around identifying specific arthropod vectors for Rickettsial diseases, a high-yield area for NEET-PG. **Analysis of Options:** * **Scrub Typhus (Correct):** Transmitted by **larval mites (chiggers)**. A characteristic clinical feature is the **eschar**—a painless, black, necrotic lesion at the site of the mite bite. * **Typhus fever/Endemic typhus (Option A):** Caused by *Rickettsia typhi*, this is transmitted by the **rat flea** (*Xenopsylla cheopis*). * **Epidemic typhus (Option B):** Caused by *Rickettsia prowazekii*, it is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is known for occurring in crowded conditions like refugee camps. * **Rocky Mountain spotted fever (Option C):** Caused by *Rickettsia rickettsii*, it is transmitted by **hard ticks** (e.g., *Dermacentor* species). **High-Yield Clinical Pearls for NEET-PG:** 1. **Weil-Felix Reaction:** A heterophile agglutination test used for diagnosis. Scrub typhus shows a positive reaction with **OX-K** (negative for OX-19 and OX-2). 2. **Drug of Choice:** Doxycycline is the gold standard treatment for almost all Rickettsial infections, including Scrub typhus. 3. **Vector Mnemonic:** Remember **"Mite-y Scrub"** to link Mites with Scrub typhus. 4. **Reservoir:** For Scrub typhus, the mite acts as both the vector and the reservoir (via transovarial transmission).

Question 338: Pigment production by Staphylococcus aureus occurs in which condition?

A. Erythrasma
B. Bullous impetigo (Correct Answer)
C. Buruli ulcer
D. Food poisoning

Explanation: **Explanation:** The correct answer is **Bullous impetigo**. *Staphylococcus aureus* produces a characteristic golden-yellow carotenoid pigment called **staphyloxanthin**. This pigment is not just a marker for identification; it acts as a virulence factor by serving as an antioxidant, protecting the bacteria from reactive oxygen species (ROS) used by host neutrophils. In **Bullous impetigo**, *S. aureus* (specifically phage group II) produces **Exfoliative Toxins (ETA, ETB)**. These toxins cleave desmoglein-1, leading to subcorneal blisters. The fluid within these bullae often contains the bacteria, and the concentrated production of staphyloxanthin results in the characteristic "golden-yellow" appearance of the pus or crusts (honey-colored crusts) associated with staphylococcal skin infections. **Analysis of Incorrect Options:** * **A. Erythrasma:** Caused by *Corynebacterium minutissimum*. It is characterized by a coral-red fluorescence under Wood’s lamp due to porphyrin production, not staphylococcal pigment. * **C. Buruli ulcer:** Caused by *Mycobacterium ulcerans*. It produces a destructive toxin called mycolactone, leading to chronic necrotizing skin ulcers. * **D. Food poisoning:** Caused by the ingestion of preformed **Staphylococcal enterotoxins** (mostly Type A). While *S. aureus* is the source, the clinical manifestation is due to the toxin's effect on the emetic center, not the pigment-producing growth of the bacteria within the host. **High-Yield Clinical Pearls for NEET-PG:** * **Staphyloxanthin:** Responsible for the "golden" color on nutrient agar; it helps the organism evade host immune responses. * **Culture Media:** Pigment production is enhanced on **Milk Agar** or Fatty acid-rich media. * **Differentiation:** *S. aureus* (Golden yellow) vs. *S. epidermidis* (White/Albus) vs. *S. saprophyticus* (White/Yellow). * **Catalase Test:** All Staphylococci are Catalase positive, differentiating them from Streptococci.

Question 339: Epidemic typhus is transmitted by:

A. Mite
B. Louse (Correct Answer)
C. Flea
D. Tick

Explanation: **Explanation:** **Epidemic typhus** is caused by the bacterium ***Rickettsia prowazekii***. The primary vector for this disease is the **human body louse** (*Pediculus humanus corporis*). Transmission occurs when the louse feeds on an infected human, the bacteria multiply in the louse's gut, and are then excreted in its feces. When the louse bites another human, the person scratches the site, inadvertently rubbing the infected feces into the bite wound or other abrasions. **Analysis of Options:** * **B. Louse (Correct):** Specifically the body louse. It is associated with crowded, unsanitary conditions (wars, famine, refugee camps), leading to "epidemic" outbreaks. * **A. Mite:** Mites (specifically chiggers) transmit **Scrub typhus**, caused by *Orientia tsutsugamushi*. * **C. Flea:** The rat flea (*Xenopsylla cheopis*) transmits **Endemic (Murine) typhus**, caused by *Rickettsia typhi*. * **D. Tick:** Ticks are vectors for the **Spotted Fever Group**, such as Rocky Mountain Spotted Fever (*R. rickettsii*) and Indian Tick Typhus (*R. conorii*). **High-Yield Clinical Pearls for NEET-PG:** * **Brill-Zinsser Disease:** This is a recrudescent (latent) form of epidemic typhus that occurs years after the primary infection, acting as a reservoir for *R. prowazekii*. * **Weil-Felix Test:** A classic heterophile agglutination test used for diagnosis. Epidemic typhus shows a positive reaction with **OX-19**. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial infections. * **Mnemonic:** "Epidemic is Bad (Body louse)" vs. "Endemic is Mouse (Flea/Rat)."

Question 340: Which of the following is NOT caused by Bartonella henselae?

A. Oroya fever (Correct Answer)
B. Cat scratch disease
C. Bacillary angiomatosis
D. Subacute bacterial endocarditis (SABE)

Explanation: **Explanation:** The correct answer is **Oroya fever** because it is caused by ***Bartonella bacilliformis***, not *Bartonella henselae*. *Bartonella bacilliformis* is transmitted by the sandfly (*Lutzomyia*) and is endemic to the Andes mountains. It causes two distinct phases of **Carrion’s disease**: 1. **Oroya fever:** An acute febrile illness characterized by severe hemolytic anemia. 2. **Verruga peruana:** A chronic phase presenting with eruptive vascular skin nodules. **Analysis of Incorrect Options:** * **Cat scratch disease:** This is the most common clinical manifestation of ***B. henselae***. It typically presents with regional lymphadenopathy following a cat scratch or bite. * **Bacillary angiomatosis:** Caused by both ***B. henselae*** and ***B. quintana***. It involves proliferation of small blood vessels in the skin and visceral organs, primarily seen in immunocompromised patients (e.g., HIV/AIDS). * **Subacute bacterial endocarditis (SABE):** *Bartonella* species (especially *B. henselae* and *B. quintana*) are well-known causes of **culture-negative endocarditis**, often associated with exposure to cats or body lice. **High-Yield Clinical Pearls for NEET-PG:** * **B. quintana:** Causes **Trench fever** (transmitted by body lice) and is common in homeless populations. * **Warthin-Starry stain:** The silver stain of choice used to visualize *Bartonella* species in tissue sections. * **Stellate Necrotizing Granulomas:** The characteristic histopathological finding in lymph nodes of Cat Scratch Disease. * **Treatment:** Azithromycin is the first-line treatment for Cat Scratch Disease; Doxycycline or Erythromycin is used for Bacillary Angiomatosis.

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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