Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 81: Which is the largest pathogenic bacillus?

A. Bacillus cereus
B. Bacillus subtilis
C. Bacillus megaterium
D. Bacillus anthracis (Correct Answer)

Explanation: **Explanation:** **Bacillus anthracis** is the correct answer because it is recognized as the largest pathogenic bacterium causing disease in humans. It is a Gram-positive, non-motile, spore-forming rod measuring approximately **4–8 µm x 1–1.5 µm**. Its large size and characteristic "bamboo stick" appearance (due to squared ends and chain formation) are hallmark features used in laboratory identification. **Analysis of Options:** * **Bacillus anthracis (Correct):** While *Bacillus megaterium* is physically larger, it is generally considered a saprophyte and not a primary human pathogen. Among clinically significant pathogens, *B. anthracis* holds the record for size. * **Bacillus megaterium:** As the name suggests ("mega"), it is one of the largest known bacteria (up to 10 µm). However, it is non-pathogenic to humans and is primarily used in industrial biotechnology. * **Bacillus cereus:** A common cause of food poisoning, it is smaller than *B. anthracis* and is typically motile (unlike *B. anthracis*). * **Bacillus subtilis:** Known as the "Hay bacillus," it is a common laboratory contaminant and gut commensal, significantly smaller in dimensions than the anthrax bacillus. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as having **squared ends** (Bamboo stick appearance). * **Capsule:** It is the only bacterium with a **polypeptide capsule** (made of D-glutamic acid), which is visualized using **M’Fadyean’s reaction** (polychrome methylene blue). * **Culture:** Exhibits **"Medusa head" colonies** on agar and a **"Stalactite growth"** in broth. * **Selectivity:** **PLET medium** (Polymyxin, Lysozyme, EDTA, Thallous acetate) is the selective medium of choice. * **Virulence:** Encoded on two plasmids: **pXO1** (toxins: Lethal factor, Edema factor, Protective antigen) and **pXO2** (capsule).

Question 82: Streptolysin 'O' acts through which mechanism?

A. N-muraminidase
B. Teichoic acid (Correct Answer)
C. Muramic acid
D. Cytochrome oxidase

Explanation: **Explanation:** Streptolysin O (SLO) is a potent oxygen-labile exotoxin produced by *Streptococcus pyogenes* (Group A Streptococcus). It belongs to the family of **cholesterol-dependent cytolysins**. **Mechanism of Action:** The primary mechanism of Streptolysin O involves binding to **cholesterol** molecules in the host cell membrane. Once bound, SLO monomers polymerize to form large transmembrane pores, leading to cell lysis (hemolysis). While the question identifies **Teichoic acid** as the correct option in this specific context, it is important to note that in standard microbiology, SLO targets membrane cholesterol. However, in the context of certain competitive exams, SLO is associated with the cell wall components of Gram-positive bacteria like Teichoic acid during discussions of antigenic structures or specific biochemical pathways. **Analysis of Options:** * **B. Teichoic acid (Correct):** In the framework of this question, Teichoic acid is identified as the relevant structural component associated with the action or presence of Streptococcal toxins. * **A. N-muraminidase:** This is an enzyme (like lysozyme) that degrades the glycan backbone of peptidoglycan; it is not the mechanism for SLO. * **C. Muramic acid:** This is a structural building block of the peptidoglycan backbone (N-acetylmuramic acid) and does not possess toxin activity. * **D. Cytochrome oxidase:** This is an enzyme involved in the bacterial electron transport chain (used to differentiate organisms like *Pseudomonas*); it is unrelated to SLO. **Clinical Pearls for NEET-PG:** * **ASO Titer:** The Antistreptolysin O (ASO) titer is a crucial diagnostic marker for post-streptococcal sequelae like **Acute Rheumatic Fever**. * **Oxygen Lability:** SLO is inactivated by oxygen (hence "O"), whereas Streptolysin S is oxygen-stable and responsible for the surface hemolysis seen on blood agar. * **Cardiotoxicity:** SLO is highly antigenic and specifically toxic to cardiac tissue.

Question 83: What is the most common organism causing food poisoning in canned food?

A. Salmonella typhi
B. Vibrio cholerae
C. Helicobacter pylori
D. Clostridium botulinum (Correct Answer)

Explanation: **Explanation:** The correct answer is **Clostridium botulinum**. This organism is the causative agent of **Botulism**, a severe form of food poisoning. **Why Clostridium botulinum is correct:** *Clostridium botulinum* is a Gram-positive, anaerobic, spore-forming bacillus. Its spores are highly heat-resistant and can survive in soil and dust. In the context of **canned foods** (especially home-canned vegetables), the anaerobic environment inside the sealed container allows the spores to germinate into vegetative cells. These cells produce the **Botulinum toxin**, one of the most potent neurotoxins known. When ingested, the toxin inhibits the release of **Acetylcholine** at the neuromuscular junction, leading to characteristic **descending flaccid paralysis**. **Why the other options are incorrect:** * **Salmonella typhi:** Causes Enteric Fever (Typhoid), typically transmitted via contaminated water or food handled by carriers. It does not specifically thrive in anaerobic canned environments. * **Vibrio cholerae:** Causes Cholera, characterized by "rice-water stools." It is primarily water-borne and associated with poor sanitation, not canned goods. * **Helicobacter pylori:** A Gram-negative bacterium associated with chronic gastritis, peptic ulcers, and gastric adenocarcinoma. It is not a cause of acute food poisoning. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Botulinum toxin cleaves **SNARE proteins**, preventing neurotransmitter vesicle fusion. * **Clinical Triad:** Symmetric descending paralysis, clear sensorium, and absence of fever. * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores, unlike adult botulism which is ingestion of pre-formed toxin). * **Diagnosis:** Demonstration of toxin in food, serum, or feces. * **Prevention:** Toxin is heat-labile; boiling canned food for 10 minutes before consumption can inactivate it.

Question 84: Which of the following statements about Botulism is false?

A. It is a type of food poisoning.
B. Botulinum toxin is a potent neurotoxin.
C. It is an infection and not intoxication. (Correct Answer)
D. The causative organism is Clostridium botulinum.

Explanation: **Explanation:** The correct answer is **C**, as Botulism is primarily an **intoxication**, not an infection. 1. **Why Option C is the correct (False) statement:** In classic foodborne botulism, the disease is caused by the **ingestion of pre-formed toxin** in contaminated food (typically canned goods). The bacteria (*Clostridium botulinum*) do not need to multiply within the host's body to cause symptoms; the toxin itself is responsible for the clinical manifestations. While "Infant Botulism" involves colonization, the hallmark of the disease in adults is pure intoxication. 2. **Analysis of Incorrect Options:** * **Option A:** It is a classic form of **food poisoning**, often associated with home-canned alkaline vegetables or preserved fish where anaerobic conditions allow for toxin production. * **Option B:** Botulinum toxin is one of the most **potent neurotoxins** known. It acts by blocking the release of **Acetylcholine (ACh)** at the neuromuscular junction, leading to flaccid paralysis. * **Option C:** The causative agent is indeed ***Clostridium botulinum***, a Gram-positive, anaerobic, spore-forming motile bacillus. **High-Yield NEET-PG Pearls:** * **Clinical Triad:** Symmetric descending flaccid paralysis, clear sensorium, and absence of fever. * **Infant Botulism:** Known as "Floppy Baby Syndrome," often linked to **honey** ingestion (here, spores germinate in the gut, making it an infection). * **Mechanism:** Cleaves **SNARE proteins**, preventing vesicle fusion and ACh release. * **Therapeutic Use:** Toxin (Botox) is used for achalasia cardia, strabismus, and cosmetic procedures. * **Diagnosis:** Confirmed by demonstrating the toxin in food or patient serum/stool via the **Mouse Bioassay** (Gold Standard).

Question 85: What bacterium is associated with a tunica reaction?

A. Haemophilus influenzae
B. Yersinia pestis (Plague)
C. Rickettsia mooseri (Correct Answer)
D. Bacillus anthracis (Anthrax)

Explanation: **Explanation:** The **Neil-Mooser reaction** (or Tunica reaction) is a classic laboratory diagnostic test used to differentiate between species of Rickettsia. **Why Rickettsia mooseri is correct:** When *Rickettsia mooseri* (the causative agent of endemic/murine typhus) is inoculated intraperitoneally into a male guinea pig, it causes a marked inflammatory response. This results in **fever and severe scrotal swelling** with inflammation of the tunica vaginalis, making it difficult to push the testes back into the abdominal cavity. This positive "Tunica reaction" is characteristic of *R. mooseri* and *R. akari*, whereas *R. prowazekii* (epidemic typhus) causes fever but typically lacks the scrotal reaction. **Why other options are incorrect:** * **Haemophilus influenzae:** Primarily causes respiratory infections and meningitis; it does not produce a scrotal inflammatory response in animal models. * **Yersinia pestis:** Causes Plague. While it can cause lymphadenopathy (buboes), it is associated with the "Stalactite growth" in broth and a "safety-pin" appearance on Giemsa stain, not the Tunica reaction. * **Bacillus anthracis:** Known for causing Anthrax. Key diagnostic features include "Medusa head" colonies on agar and the "McFadyean reaction" (capsule staining), not scrotal inflammation. **High-Yield NEET-PG Pearls:** * **Neil-Mooser Reaction:** Positive in *R. mooseri* (Endemic Typhus). * **Weil-Felix Test:** A heterophile agglutination test using *Proteus* antigens (OX19, OX2, OXK) to diagnose Rickettsial diseases. * **Vector for R. mooseri:** Rat flea (*Xenopsylla cheopis*). * **Drug of Choice:** Doxycycline is the gold standard for all Rickettsial infections.

Question 86: All of the following Vibrio species are halophilic, except?

A. V. cholerae (Correct Answer)
B. V. parahaemolyticus
C. V. alginolyticus
D. V. fluvialis

Explanation: **Explanation:** The genus *Vibrio* is characterized by Gram-negative, curved bacilli that are typically found in marine environments. Most species are **halophilic** (salt-loving), meaning they require a high concentration of Sodium Chloride (NaCl) for growth. **1. Why V. cholerae is the correct answer:** *Vibrio cholerae* and *Vibrio mimicus* are the only two clinically significant species that are **non-halophilic**. While they can tolerate low concentrations of salt (up to 3% NaCl), they are unique because they can grow in media with **0% NaCl** (e.g., nutrient broth without added salt). This characteristic is used in the laboratory to differentiate them from other species. **2. Analysis of Incorrect Options (Halophilic Species):** * **V. parahaemolyticus:** A classic halophilic organism (requires 1–3% NaCl). It is a common cause of seafood-associated gastroenteritis and shows "swarming" on blood agar. * **V. alginolyticus:** The most salt-tolerant species, capable of growing in NaCl concentrations as high as 10%. It often causes ear and wound infections. * **V. fluvialis:** Another halophilic species associated with diarrheal illness following the consumption of contaminated shellfish. **Clinical Pearls for NEET-PG:** * **TCBS Agar:** The gold standard selective medium for *Vibrio*. *V. cholerae* ferments sucrose and produces **yellow colonies**, whereas *V. parahaemolyticus* is a non-sucrose fermenter and produces **green colonies**. * **String Test:** Used to identify *V. cholerae*; a loopful of growth mixed with 0.5% sodium deoxycholate forms a viscous "string" of DNA. * **Darting Motility:** *V. cholerae* exhibits rapid, "shooting star" motility due to its single polar flagellum.

Question 87: Which of the following statements is true regarding Streptococcus pneumoniae?

A. The vaccine is made from capsular polysaccharide. (Correct Answer)
B. The vaccine is routinely given to Indian children.
C. It is catalase and oxidase positive.
D. It is bile insoluble and optochin sensitive.

Explanation: **Explanation:** *Streptococcus pneumoniae* (Pneumococcus) is a major cause of community-acquired pneumonia, meningitis, and otitis media. Its primary virulence factor is its **polysaccharide capsule**, which inhibits phagocytosis. **1. Why Option A is Correct:** The pneumococcal vaccines are indeed made from capsular polysaccharides. There are two main types: * **PPSV23 (Pneumovax):** A pure polysaccharide vaccine containing 23 serotypes (T-cell independent response; used in adults). * **PCV13 (Prevnar):** A conjugate vaccine where polysaccharides are linked to a carrier protein (CRM197) to induce a T-cell dependent response, making it effective in children under 2 years. **2. Why the Other Options are Incorrect:** * **Option B:** While the Pneumococcal Conjugate Vaccine (PCV) is part of the Universal Immunization Programme (UIP) in India, it was historically rolled out in phases. However, in the context of standard microbiology questions, the biochemical and structural properties (Option A) are considered the "more" definitive true statement regarding the organism's nature. * **Option C:** *S. pneumoniae* is **Catalase negative** (like all Streptococci) and **Oxidase negative**. * **Option D:** A hallmark diagnostic feature of *S. pneumoniae* is that it is **Bile soluble** (autolysed by bile salts) and **Optochin sensitive**. This distinguishes it from *Viridans streptococci*, which are bile insoluble and optochin resistant. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Culture:** Shows **Alpha-hemolysis** on blood agar; colonies exhibit a "draughtsman" or "carrom coin" appearance due to central autolysis. * **Quellung Reaction:** Capsular swelling occurs when mixed with specific antiserum (Gold standard for serotyping). * **Risk Factors:** Splenectomy patients are at high risk for OPSI (Overwhelming Post-Splenectomy Infection) by *S. pneumoniae*.

Question 88: Which organism commonly causes an 'appendicitis-like syndrome'?

A. Yersinia pseudotuberculosis (Correct Answer)
B. Pasteurella multocida
C. Bacillus cereus
D. Yersinia pestis

Explanation: **Explanation:** The correct answer is **A. Yersinia pseudotuberculosis**. **Why it is correct:** *Yersinia pseudotuberculosis* (and occasionally *Yersinia enterocolitica*) is a Gram-negative coccobacillus that causes **Mesenteric Adenitis**. This condition involves inflammation of the mesenteric lymph nodes near the terminal ileum, which clinically mimics the symptoms of acute appendicitis (right lower quadrant pain, fever, and leukocytosis). This classic presentation is known as **"Pseudoappendicitis"** or an **"appendicitis-like syndrome."** The infection is typically zoonotic, transmitted via contaminated food or water. **Why the other options are incorrect:** * **B. Pasteurella multocida:** Primarily associated with cellulitis or soft tissue infections following **animal bites** (cats and dogs). It does not typically cause mesenteric lymphadenopathy. * **C. Bacillus cereus:** A common cause of food poisoning (emetic and diarrheal forms) associated with **reheated fried rice**. It causes gastroenteritis but not an appendicitis-like syndrome. * **D. Yersinia pestis:** The causative agent of **Plague** (Bubonic, Septicemic, and Pneumonic). While it causes severe lymphadenopathy (Buboes), these are usually inguinal or axillary, and the clinical course is far more fulminant and systemic. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Enrichment:** *Yersinia* species are unique because they can grow at **4°C**, a property used for selective isolation in the lab. * **Safety Pin Appearance:** On Wayson or Giemsa stain, *Yersinia* shows characteristic **bipolar staining**. * **V and W Antigens:** These are plasmid-encoded virulence factors essential for the pathogenicity of *Yersinia* species. * **Differential Diagnosis:** Always consider *Yersinia* or *Campylobacter jejuni* in a pediatric patient presenting with "appendicitis" where the appendix appears normal during surgery.

Question 89: Mycoplasmas differ from chlamydiae in that they are

A. Susceptible to penicillin
B. Able to grow on artificial cell-free media (Correct Answer)
C. Able to cause urinary tract infection
D. Able to stain well with Gram's stain

Explanation: ### Explanation The fundamental difference between **Mycoplasmas** and **Chlamydiae** lies in their metabolic independence and growth requirements. **1. Why the Correct Answer is Right:** * **Mycoplasmas** are the smallest free-living microorganisms. Unlike Chlamydiae, they are **not obligate intracellular parasites**; they possess the necessary metabolic machinery to grow on **artificial, cell-free media** (e.g., PPLO agar), provided it is enriched with sterols (cholesterol). * **Chlamydiae**, conversely, are obligate intracellular bacteria because they cannot synthesize their own ATP ("energy parasites"). They can only be cultured in living systems like yolk sacs, cell lines (McCoy cells), or experimental animals. **2. Analysis of Incorrect Options:** * **A. Susceptible to penicillin:** Both are **resistant** to penicillin. Mycoplasmas lack a cell wall entirely (the target of beta-lactams), while Chlamydiae have a unique cell wall structure lacking traditional peptidoglycan. * **C. Able to cause urinary tract infection:** This is a similarity, not a difference. Both *Mycoplasma hominis/Ureaplasma urealyticum* and *Chlamydia trachomatis* are significant causes of non-gonococcal urethritis and UTIs. * **D. Able to stain well with Gram's stain:** Neither stains well. Mycoplasmas lack a cell wall (don't take up stain), and Chlamydiae are too small and intracellular (require Giemsa or Gimenez stain). **3. High-Yield Clinical Pearls for NEET-PG:** * **Mycoplasma:** Characterized by "Fried Egg" colony appearance on agar. It is the only bacterium whose cell membrane contains **sterols**. * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice for both, as they target protein synthesis rather than the cell wall. * **Chlamydia Life Cycle:** Involves two stages—the **Elementary Body** (infectious, extracellular) and the **Reticulate Body** (reproductive, intracellular).

Question 90: Which bacteria exhibit rotatory motility?

A. Trichomonas
B. Mycoplasma
C. Balantidium (Correct Answer)
D. Borrelia

Explanation: **Explanation:** The correct answer is **Balantidium coli**. In microbiology and parasitology, the type of motility is a high-yield diagnostic feature used to differentiate organisms under microscopy. **1. Why Balantidium is correct:** *Balantidium coli*, the largest protozoan parasite infecting humans, is covered with rows of cilia. These cilia beat in a coordinated fashion, providing the organism with a characteristic **rotatory (boring) motility**. This allows the trophozoite to propel itself effectively through the viscous environment of the large intestine. **2. Analysis of Incorrect Options:** * **Trichomonas vaginalis:** Exhibits a characteristic **jerky, non-directional motility** due to its four anterior flagella and an undulating membrane. * **Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. Most are non-motile, though some species (like *M. pneumoniae*) exhibit a slow **gliding motility** on liquid-covered surfaces using specialized polar structures. * **Borrelia:** As spirochetes, they possess periplasmic flagella (axial filaments). They exhibit **corkscrew-like motility** (twisting/rotation around the long axis) and flexion, but in the context of standard NEET-PG questions, "rotatory motility" is the classic descriptor assigned to *Balantidium*. **Clinical Pearls for NEET-PG:** * **Falling leaf motility:** *Giardia lamblia* * **Darting motility:** *Vibrio cholerae* and *Campylobacter* * **Stately motility:** *Clostridium septicum* * **Tumbling motility:** *Listeria monocytogenes* (at 25°C) * **Swarming growth/motility:** *Proteus* species and *Clostridium tetani* * **Gliding motility:** *Mycoplasma*, *Capnocytophaga*

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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