Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1341: Granules in Corynebacterium diphtheria are called by which of the following names, except?

A. Metachromatic granules
B. Retractile granules (Correct Answer)
C. Babes-Ernst granules
D. Volutin granules

Explanation: ### Explanation The characteristic granules found in *Corynebacterium diphtheriae* are intracellular storage bodies composed of **polymetaphosphate**. These granules serve as energy reserves for the bacteria. **Why "Retractile granules" is the correct answer:** Retractile granules (or "refractile bodies") are typically associated with bacterial **endospores** (like *Bacillus* or *Clostridium* species), which appear bright and shiny under light microscopy due to their dense structure. *C. diphtheriae* is a non-sporing bacterium and does not possess retractile granules. **Why the other options are incorrect:** * **Metachromatic granules:** This is a functional name. When stained with aniline dyes (like Methylene blue), these granules shift the color of the dye from blue to reddish-pink. This phenomenon is called metachromasia. * **Babes-Ernst granules:** This is the eponymous name given to these granules after the scientists who first described them. * **Volutin granules:** This is the biochemical name for these inclusions, referring to their composition of polymerized inorganic polyphosphates. ### NEET-PG High-Yield Pearls * **Staining Methods:** These granules are best demonstrated using **Albert’s stain**, Neisser’s stain, or Ponder’s stain. * **Appearance:** In Albert’s staining, the granules appear **bluish-black**, while the bacillary body appears green. * **Arrangement:** *C. diphtheriae* shows a characteristic **cuneiform** or "Chinese letter" arrangement due to incomplete separation during binary fission (snapping division). * **Culture Media:** Use **Loeffler’s Serum Slope** (rapid growth, enhances granules) or **Potassium Tellurite Agar** (selective media; colonies appear grey/black).

Question 1342: Which of the following species of Shigella does NOT ferment mannitol?

A. Shigella sonnei
B. Shigella flexneri
C. Shigella boydii
D. Shigella dysenteriae (Correct Answer)

Explanation: **Explanation:** The classification of *Shigella* species is based on both biochemical properties and antigenic structure (O-antigen). A fundamental biochemical marker used to differentiate these species is the ability to ferment **mannitol**. **1. Why Shigella dysenteriae is the correct answer:** *Shigella dysenteriae* (Serogroup A) is the only species among the four that is **mannitol-non-fermenting**. This is a high-yield diagnostic feature used in laboratory identification to distinguish it from other groups. Additionally, it is the most virulent species, producing the potent Shiga toxin, and is typically catalase-negative (unlike other *Shigella* species). **2. Why the other options are incorrect:** * **Shigella flexneri (Group B), Shigella boydii (Group C), and Shigella sonnei (Group D)** are all **mannitol fermenters**. * **Shigella sonnei** is further distinguished by being a **late lactose fermenter** and being indole negative, whereas *S. flexneri* and *S. boydii* are non-lactose fermenters. **Clinical Pearls for NEET-PG:** * **Mnemonic for Groups:** **D**irty **F**ingers **B**ring **S**higgella (**D**ysenteriae-A, **F**lexneri-B, **B**oydii-C, **S**onnei-D). * **Most common in India:** *Shigella flexneri*. * **Most common in developed countries:** *Shigella sonnei*. * **Most severe disease:** *Shigella dysenteriae* Type 1 (Shiga bacillus), which can lead to Hemolytic Uremic Syndrome (HUS). * **Key Lab Finding:** All *Shigella* are non-motile (lack H-antigen) and do not produce $H_2S$, distinguishing them from *Salmonella*.

Question 1343: All of the following statements about Vibrio cholera are true, except?

A. It is transported in alkaline pH.
B. It is an aerobic bacterium.
C. It is flagellated.
D. It is oxidase test negative. (Correct Answer)

Explanation: ### Explanation **Vibrio cholerae** is a Gram-negative, comma-shaped bacterium responsible for cholera. The correct answer is **Option D** because *Vibrio cholerae* is actually **oxidase positive**, not negative. #### Why Option D is the Correct Answer (The Exception): The **Oxidase Test** is a key biochemical reaction used to differentiate Gram-negative rods. *Vibrio* species are characteristically oxidase positive. This distinguishes them from members of the *Enterobacteriaceae* family (like *E. coli* or *Salmonella*), which are oxidase negative. #### Analysis of Other Options: * **Option A (Alkaline pH):** *Vibrio* is halophilic and highly acid-sensitive but acid-tolerant. It thrives in alkaline environments (pH 8.5–9.2). Transport media like **Venkataraman-Ramakrishnan (VR) medium** and **Alkaline Peptone Water** utilize this high pH to inhibit other commensal flora. * **Option B (Aerobic):** *Vibrio cholerae* is a **facultative anaerobe**, meaning it grows best under aerobic conditions but can survive without oxygen. In laboratory settings, it grows rapidly on standard aerobic media. * **Option C (Flagellated):** It possesses a **single polar flagellum**, which provides it with characteristic "darting motility" (often described as "shooting star" motility) when viewed under dark-ground microscopy. #### NEET-PG High-Yield Pearls: * **Culture Media:** **TCBS** (Thiosulfate Citrate Bile Salts Sucrose) agar is the selective medium of choice; *V. cholerae* produces **yellow colonies** due to sucrose fermentation. * **String Test:** Positive (colonies lose turbidity and form a "string" when mixed with 0.5% sodium deoxycholate). * **Cholera Toxin:** An enterotoxin (AB5 type) that increases **cAMP**, leading to the secretion of water and electrolytes, resulting in "rice-water stools." * **Hanging Drop Preparation:** Used to demonstrate the characteristic darting motility.

Question 1344: Treponema pallidum can be grown in which of the following hosts?

A. Mice
B. Armadillo
C. Rodent
D. Rabbit (Correct Answer)

Explanation: **Explanation:** *Treponema pallidum* subsp. *pallidum*, the causative agent of syphilis, is an **obligate intracellular pathogen** that cannot be cultured on artificial (cell-free) media. To maintain the organism for research or diagnostic purposes (like the TPI test), it must be grown in living systems. **1. Why Rabbit is Correct:** The **Rabbit** is the primary laboratory animal used for the propagation of *T. pallidum*. Specifically, the bacteria are inoculated into the **testes of rabbits** (intratesticular inoculation), where they multiply. This method is used to maintain the **Nichols strain**, which is the standard virulent strain used in laboratory research and serological testing. **2. Why Other Options are Incorrect:** * **Armadillo:** This is the specific animal model used for the cultivation of ***Mycobacterium leprae*** (Leprosy), as armadillos have a lower body temperature suitable for its growth. * **Mice/Rodents:** While mice are used in many immunological studies, they do not develop the characteristic syphilitic lesions required for the effective propagation and study of *T. pallidum* in the same way rabbits do. **High-Yield Clinical Pearls for NEET-PG:** * **Cultivation:** *T. pallidum* is "non-cultivable" on agar. It is maintained by serial passage in rabbit testes. * **Microscopy:** Since it cannot be stained by Gram stain (too thin), **Dark-ground microscopy (DGM)** is the gold standard for visualizing motile spirochetes from primary lesions. * **Silver Staining:** In tissue sections, they can be visualized using **Levaditi** or **Warthin-Starry** silver stains. * **Sensitivity:** The organism is extremely sensitive to heat, drying, and penicillin. Penicillin remains the drug of choice.

Question 1345: Salmonella species are characterized by all of the following EXCEPT:

A. Acid and gas production from fermenting glucose
B. Citrate synthesis (Correct Answer)
C. H2S production
D. Indole negative

Explanation: **Explanation:** The genus *Salmonella* belongs to the family *Enterobacteriaceae*. Understanding its biochemical profile is crucial for differentiating it from other enteric pathogens like *Shigella* and *E. coli*. **1. Why "Citrate synthesis" is the correct (EXCEPT) answer:** While most *Salmonella* species (like *S. Typhimurium*) are **Citrate positive** (meaning they can use citrate as a sole carbon source), they do not "synthesize" citrate. In biochemical testing, we look for **Citrate Utilization**, not synthesis. More importantly, the medically significant **Salmonella Typhi is Citrate negative**. Therefore, claiming citrate synthesis/utilization is a universal characteristic of all *Salmonella* species is incorrect. **2. Analysis of Incorrect Options:** * **Option A (Acid/Gas from Glucose):** Most *Salmonella* are aerogenic (produce acid and gas). *Note: S. Typhi is a notable exception as it is anaerogenic (acid only).* * **Option C (H2S production):** This is a hallmark of *Salmonella*. They produce hydrogen sulfide, visualized as black colonies on SSA (Salmonella-Shigella Agar) or Wilson-Blair medium. * **Option D (Indole negative):** *Salmonella* species do not produce tryptophanase; hence, they are Indole negative. **High-Yield NEET-PG Pearls:** * **IMViC Profile:** For *S. Typhi*, it is **(– + – +)** (Indole –, MR +, VP –, Citrate –). * **H2S Exception:** *S. Paratyphi A* is a rare *Salmonella* that does **not** produce H2S. * **Culture Media:** **Wilson-Blair Bismuth Sulfite Agar** is the most sensitive medium for *S. Typhi* (jet black colonies with a metallic sheen). * **Motility:** They are motile via **peritrichous flagella** (except *S. Gallinarum-Pullorum*).

Question 1346: Which of the following microorganisms resembles Streptococcus viridans in culture?

A. Staphylococcus
B. Streptococcus pyogenes
C. Pneumococcus (Correct Answer)
D. Streptococcus faecalis

Explanation: **Explanation:** The correct answer is **Pneumococcus (*Streptococcus pneumoniae*)**. Both *Streptococcus viridans* and *Streptococcus pneumoniae* are **alpha-hemolytic** (produce a greenish discoloration on blood agar due to partial hemolysis). This shared cultural characteristic often makes them difficult to distinguish on initial inspection. **Why Pneumococcus is correct:** Both organisms are Gram-positive cocci in chains/pairs and exhibit alpha-hemolysis. Differentiation is crucial and is based on: * **Optochin Sensitivity:** Pneumococcus is sensitive (inhibited), while Viridans is resistant. * **Bile Solubility:** Pneumococcus is bile-soluble (colonies dissolve), while Viridans is bile-insoluble. * **Morphology:** Pneumococcus is lancet-shaped (diplococci), whereas Viridans typically forms long chains. **Why other options are incorrect:** * **Staphylococcus:** These are Gram-positive cocci in clusters and are **Catalase positive**. Most species (like *S. aureus*) show beta-hemolysis or no hemolysis, not alpha-hemolysis. * **Streptococcus pyogenes (Group A Strep):** This organism exhibits **beta-hemolysis** (clear zone of complete hemolysis), distinguishing it clearly from the green alpha-hemolysis of Viridans. * **Streptococcus faecalis (Enterococcus):** These typically show **gamma-hemolysis** (no hemolysis), though some strains can be alpha or beta. They are distinguished by their ability to grow in 6.5% NaCl and bile esculin agar. **High-Yield Clinical Pearls for NEET-PG:** * **Pneumococcus:** Most common cause of Community-Acquired Pneumonia (CAP) and meningitis in adults. Look for "Rusty Sputum" and "Quellung Reaction positive." * **Viridans Group:** Most common cause of **Subacute Bacterial Endocarditis (SABE)** following dental procedures. * **Mnemonic:** **OVRPS** (Optochin: Viridans Resistant, Pneumococci Sensitive).

Question 1347: Which of the following is NOT a defense mechanism produced by bacteria in a biofilm?

A. Antibiotics are expelled from the surface (Correct Answer)
B. They adhere strongly to the surface
C. Reduced antibiotic penetration into the biofilm
D. Protection from inflammatory mediators

Explanation: ### Explanation **Biofilms** are complex, multicellular communities of bacteria embedded in a self-produced matrix of **Extracellular Polymeric Substances (EPS)**. This structure provides a significant survival advantage against the host immune system and antimicrobial therapy. **Why Option A is the Correct Answer:** The statement "Antibiotics are expelled from the surface" is incorrect because biofilms do not actively "expel" antibiotics from their outer surface. Instead, resistance is mediated by the **physical barrier** of the matrix and the **metabolic state** of the bacteria within. While individual bacteria may possess efflux pumps to expel drugs from their cytoplasm, this is a cellular mechanism, not a structural property of the biofilm surface itself. **Analysis of Other Options:** * **B. Strong Adherence:** Biofilms begin with the attachment of planktonic bacteria to a surface (e.g., prosthetic valves, catheters) via pili and adhesins, followed by the production of the EPS matrix which anchors them firmly. * **C. Reduced Penetration:** The thick EPS matrix (composed of polysaccharides, proteins, and DNA) acts as a physical and chemical diffusion barrier, significantly slowing the penetration of bulky or charged antibiotic molecules. * **D. Protection from Inflammatory Mediators:** The biofilm shield protects bacteria from host defenses such as phagocytosis, antibodies, and complement-mediated killing. Phagocytes often undergo "frustrated phagocytosis," releasing enzymes that damage host tissue instead of the bacteria. **NEET-PG High-Yield Pearls:** * **Quorum Sensing:** Bacteria in biofilms communicate via signaling molecules (e.g., Acyl-homoserine lactones) to coordinate gene expression. * **Persister Cells:** These are metabolically inactive (dormant) cells within the biofilm that are naturally resistant to antibiotics targeting active metabolism (like Beta-lactams). * **Clinical Examples:** *Pseudomonas aeruginosa* in Cystic Fibrosis lungs, *Staphylococcus epidermidis* on central lines, and dental plaque (*Streptococcus mutans*).

Question 1348: Negative staining technique is used for which of the following bacteria?

A. Mycobacteria
B. Pseudomonas
C. Bacillus (Correct Answer)
D. Clostridium

Explanation: **Explanation:** The correct answer is **Bacillus**. Negative staining is a technique used primarily to visualize bacterial **capsules**. In this method, acidic dyes like **Nigrosin or India ink** are used. These dyes carry a negative charge and are repelled by the negatively charged bacterial cell surface. Consequently, the background is stained dark, while the organism (and its capsule) remains clear and unstained, appearing as a "halo" against a dark background. * **Why Bacillus is correct:** Certain species of *Bacillus*, most notably ***Bacillus anthracis***, possess a prominent polypeptide capsule (composed of D-glutamic acid). Negative staining is the classic method to demonstrate this capsule, which is a key virulence factor. * **Why other options are incorrect:** * **Mycobacteria:** These are "Acid-fast" bacilli. Due to the high mycolic acid content in their cell walls, they require the **Ziehl-Neelsen (ZN) stain** rather than negative staining. * **Pseudomonas & Clostridium:** While some strains may produce biofilms or slime layers, they are typically identified using **Gram stain** (Gram-negative and Gram-positive respectively) and biochemical reactions. They are not the primary targets for diagnostic negative staining in a clinical or examination context. **High-Yield Clinical Pearls for NEET-PG:** * **India Ink Preparation:** Specifically used for identifying ***Cryptococcus neoformans*** (a fungus) in CSF samples to diagnose meningitis. * **McFadyean’s Reaction:** A specific staining technique (using polychrome methylene blue) used to visualize the capsule of *B. anthracis* in blood smears. * **Quellung Reaction:** The "gold standard" for capsule identification, involving capsule swelling when treated with specific antisera.

Question 1349: Which staphylococcal toxin is responsible for food poisoning?

A. Beta Exotoxin
B. Enterotoxin (Correct Answer)
C. Alpha Exotoxin
D. Toxic Shock Syndrome Toxin (TSST-1)

Explanation: **Explanation:** The correct answer is **Enterotoxin**. Staphylococcal food poisoning is caused by the ingestion of preformed **Staphylococcal Enterotoxins (SE)**, most commonly **Type A**. These toxins are produced by *Staphylococcus aureus* when contaminated food (typically protein-rich or salty foods like ham, custard, or potato salad) is left at room temperature. * **Mechanism:** Enterotoxins act as **superantigens**, stimulating T-cells to release cytokines, which act on the vagus nerve and the vomiting center in the brain. They are **heat-stable** (resisting boiling for 30 minutes) and resistant to gastric enzymes, meaning the toxin remains active even if the bacteria are killed by cooking. **Analysis of Incorrect Options:** * **Alpha Exotoxin:** This is the most potent membrane-damaging toxin of *S. aureus*. It forms pores in host cells (hemolysis) but is not associated with gastrointestinal symptoms. * **Beta Exotoxin:** Also known as Sphingomyelinase C, it degrades cell membrane lipids. It is used in the "CAMP test" for *S. agalactiae* but does not cause food poisoning. * **TSST-1:** This is a superantigen responsible for **Toxic Shock Syndrome (TSS)**, characterized by fever, rash, and multi-organ failure, usually associated with tampon use or wound infections. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (1–6 hours) because the toxin is preformed. * **Clinical Feature:** Violent vomiting is more prominent than diarrhea; usually resolves within 24 hours (self-limiting). * **Key Association:** "Picnic foods" or "creamy salads." * **Other Superantigens:** Both Enterotoxin and TSST-1 are superantigens, but they target different clinical syndromes.

Question 1350: Which transport medium is used for Neisseria?

A. Pike's medium
B. Amie's medium (Correct Answer)
C. VR medium
D. Autoclaved sea water

Explanation: **Explanation:** *Neisseria* species (including *N. gonorrhoeae* and *N. meningitidis*) are highly fastidious and extremely sensitive to drying, temperature changes, and fatty acids present in cotton swabs. Therefore, they require specialized transport media to maintain viability during transit to the laboratory. **Why Amie’s Medium is Correct:** **Amie’s medium** is an improved version of Stuart’s medium. It contains **charcoal**, which is the critical component for *Neisseria*. Charcoal neutralizes fatty acids and toxic metabolic products that are lethal to these delicate organisms. It provides an inorganic, non-nutritive environment that prevents overgrowth of commensal flora while preserving the pathogen. **Analysis of Incorrect Options:** * **Pike’s Medium:** A transport medium specifically used for **Streptococcus pyogenes** (Group A Strep). It contains inhibitory substances like crystal violet and sodium azide to suppress other throat flora. * **VR (Venkatraman-Ramakrishnan) Medium:** A high-pH (buffered saline) transport medium used for **Vibrio cholerae**. * **Autoclaved Sea Water:** Also used for the transport of **Vibrio cholerae**, as the organism thrives in saline, alkaline environments. **High-Yield Clinical Pearls for NEET-PG:** * **Stuart’s Medium:** The original transport medium for *Neisseria*, but Amie’s is preferred due to the addition of charcoal. * **Culture Media:** For *Neisseria*, the gold standard is **Thayer-Martin Medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin—VCN). * **Temperature:** Never refrigerate specimens suspected of containing *Neisseria*; they are cold-sensitive. * **Biochemicals:** *N. gonorrhoeae* ferments **G**lucose only; *N. meningitidis* ferments **G**lucose and **M**altose.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

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Mycobacteria

Practice Questions

Spirochetes

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