Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1391: Which transport medium is used for cholera?

A. LJ medium
B. Cary Blair medium (Correct Answer)
C. MYPA medium
D. Stewart medium

Explanation: **Explanation:** **Vibrio cholerae**, the causative agent of cholera, is highly sensitive to dehydration and acidic environments. Therefore, specialized transport media are required to maintain the viability of the organism during transit from the field to the laboratory. **1. Why Cary-Blair Medium is Correct:** Cary-Blair medium is the **gold standard** transport medium for *Vibrio cholerae*. It is a modified version of Stuart’s medium with a higher pH (8.4). Since *Vibrio* species are halophilic and prefer alkaline conditions, this high pH inhibits the overgrowth of competing intestinal flora (like Enterobacteriaceae) while preserving the pathogen. It lacks a carbon source, preventing the rapid multiplication of bacteria that could lead to a change in pH. **2. Analysis of Incorrect Options:** * **LJ (Löwenstein–Jensen) Medium:** This is a solid, egg-based **selective/enrichment medium** used specifically for the cultivation of *Mycobacterium tuberculosis*. * **MYPA (Mannitol Yolk Polymyxin Agar) Medium:** This is a **selective and differential medium** used for the isolation of *Bacillus cereus*. * **Stewart Medium:** A classic transport medium used primarily for delicate organisms like *Neisseria gonorrhoeae*. While it can be used for stool, Cary-Blair is superior for *Vibrio* due to its specific alkaline buffering. **Clinical Pearls for NEET-PG:** * **Other Transport Media for Cholera:** VR (Venkatraman-Ramakrishnan) medium and Autoclaved Sea Water. * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Selective Media:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) Agar—*V. cholerae* produces characteristic **yellow colonies** due to sucrose fermentation. * **Holding Medium:** Buffered Glycerol Saline (Note: This is **not** suitable for *Vibrio* as it inhibits its growth).

Question 1392: "Darting motility" is shown by:

A. Proteus
B. Serratia
C. Vibrio (Correct Answer)
D. E. coli

Explanation: **Explanation:** The correct answer is **Vibrio cholerae**. Motility in bacteria is determined by the presence and arrangement of flagella. *Vibrio cholerae* possesses a single polar flagellum (monotrichous) that acts like a powerful propeller, resulting in a rapid, jerky, and aggressive movement known as **"Darting Motility."** This is often compared to a "shooting star" appearance when viewed under dark-ground or phase-contrast microscopy. **Analysis of Options:** * **A. Proteus:** Exhibits **"Swarming Motility"** due to its peritrichous flagella. On solid media (like blood agar), it creates concentric ripples or waves rather than discrete colonies. * **B. Serratia:** Some species are motile via peritrichous flagella, but they are better known for producing a red pigment called **prodigiosin**. * **D. E. coli:** Shows simple **peritrichous motility** (non-directional swimming), which is much slower and less distinctive than the darting motion of Vibrio. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae:** Darting motility can be inhibited by adding specific **antisera** (the immobilization test), which is a diagnostic feature. * **Other Motility Patterns to Remember:** * **Tumbling Motility:** *Listeria monocytogenes* (at 25°C). * **Falling Leaf Motility:** *Giardia lamblia* (Parasitology). * **Corkscrew/Flexion-Extension Motility:** *Spirochetes* (e.g., *Treponema pallidum*). * **Stately Motility:** *Bacillus anthracis* (though usually non-motile, related species show this). * **Gliding Motility:** *Mycoplasma pneumoniae*.

Question 1393: Draughtsman appearance is seen with colonies of which bacterium?

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

Explanation: **Explanation:** **Pneumococcus (*Streptococcus pneumoniae*)** is the correct answer. The characteristic **"Draughtsman" or "Checkerboard" appearance** refers to the morphology of its colonies on blood agar after 24–48 hours of incubation. The underlying mechanism is **autolysis**. *S. pneumoniae* produces an intracellular autolytic enzyme called **LytA (amidases)**. As the culture ages, the central part of the colony undergoes autolysis, causing it to collapse or flatten, while the periphery remains raised. This results in a colony with a depressed center and raised edges, resembling the pieces used in a game of draughts (checkers). **Analysis of Incorrect Options:** * **Streptococcus (Viridans/Pyogenes):** These typically produce small, convex, or "matt/glossy" colonies. They do not possess the potent autolytic mechanism required to create the central depression seen in Pneumococci. * **Staphylococcus:** These form large, opaque, convex, "oil-paint" like colonies. *S. aureus* is known for its golden-yellow pigmentation, not a draughtsman appearance. * **Meningococcus:** *Neisseria meningitidis* forms medium-sized, convex, greyish-white, translucent, and mucoid colonies on chocolate agar. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Capsule:** The main virulence factor; identified by the **Quellung Reaction** (capsular swelling). * **Bile Solubility Test:** Pneumococci are bile-soluble (autolysis is enhanced by bile salts), which differentiates them from *S. viridans*. * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin (P-disk), whereas *S. viridans* is resistant. * **Hemolysis:** Shows **Alpha-hemolysis** (partial/greenish) under aerobic conditions.

Question 1394: Which of the following statements about Listeria monocytogenes is true?

A. A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, are True and C, E are False.
B. B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A is False.
C. B. Humans are probably accidental hosts for this microorganism and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A, C, D are False.
D. A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are all True. (Correct Answer)

Explanation: This question tests comprehensive knowledge of *Listeria monocytogenes*, a unique Gram-positive, non-spore-forming motile bacillus. The correct answer is **D** because all five statements accurately describe the pathogen's epidemiology, clinical presentation, and management. **Explanation of Statements:** * **A & B (Epidemiology):** *Listeria* is primarily a **foodborne pathogen** (found in unpasteurized dairy, deli meats, and coleslaw). It is ubiquitous in nature (soil/water); thus, humans are **accidental hosts** in the life cycle of the organism. * **C (Prognosis):** Despite appropriate antibiotic therapy, *Listeria* meningitis carries a high **case-fatality rate (15–26%)**, particularly in the elderly and immunocompromised. * **D (Neonatal Infection):** **Granulomatosis infantiseptica** is the classic "early-onset" neonatal listeriosis acquired *in utero*. It is characterized by widespread disseminated abscesses and granulomas, often leading to stillbirth or high mortality. * **E (Treatment):** **Ampicillin** is the drug of choice. Because *Listeria* is an intracellular pathogen and has low penicillin-binding protein affinity, high doses (e.g., 2g every 4 hours in adults) are required for CNS penetration. **Why other options are incorrect:** Options A, B, and C are incorrect because they falsely label one or more of these medically accurate statements as "False." In NEET-PG, *Listeria* is a "all-of-the-above" favorite due to its distinct characteristics. **High-Yield Clinical Pearls for NEET-PG:** * **Tumbling Motility:** Seen at 25°C (peritrichous flagella), but absent at 37°C. * **Cold Enrichment:** Can grow at 4°C (useful for isolation from contaminated food). * **CAMP Test Positive:** Shows "rectangular" hemolysis (unlike the arrowhead of Group B Strep). * **Cell-Mediated Immunity:** Essential for clearance; hence, it affects neonates, pregnant women, and transplant patients.

Question 1395: Which of the following bacterial toxins works by the same mechanism as Exotoxin A of Pseudomonas?

A. Tetanus toxin
B. Pertussis toxin
C. Cholera toxin
D. Diphtheria toxin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Diphtheria toxin**. Both **Pseudomonas Exotoxin A** and **Diphtheria toxin** (produced by *Corynebacterium diphtheriae*) share the exact same mechanism of action: they are **ADP-ribosyltransferases** that target **Elongation Factor-2 (EF-2)**. By attaching an ADP-ribose group to EF-2, they inhibit protein synthesis, leading to cell death. **Analysis of Options:** * **Diphtheria toxin (Correct):** Identical mechanism to Pseudomonas Exotoxin A (ADP-ribosylation of EF-2). * **Tetanus toxin:** Acts as a protease that cleaves **SNARE proteins** (specifically synaptobrevin). This prevents the release of inhibitory neurotransmitters (GABA and Glycine) from Renshaw cells, leading to spastic paralysis. * **Pertussis toxin:** Works by ADP-ribosylation of the **inhibitory G-protein (Gi)**. This leads to an increase in intracellular cAMP, interfering with immune cell signaling. * **Cholera toxin:** Works by ADP-ribosylation of the **stimulatory G-protein (Gs)**. This results in constitutive activation of adenylate cyclase, leading to massive increases in cAMP and subsequent secretory diarrhea. **High-Yield NEET-PG Pearls:** * **EF-2 Inhibitors:** Only two major toxins inhibit EF-2: Diphtheria toxin and Pseudomonas Exotoxin A. * **A-B Toxins:** All options listed (A, B, C, D) are A-B structural toxins, where the 'B' subunit binds to the cell surface and the 'A' subunit possesses the active enzymatic toxicity. * **Pseudomonas:** While Exotoxin A is its most important toxin, it also produces **Exoenzyme S**, which acts on the cytoskeleton. * **Diphtheria:** The toxin gene is encoded by a **lysogenic bacteriophage** (Beta-phage).

Question 1396: Which toxin of Streptococcus causes hemolysis around colonies?

A. Streptolysin O
B. Streptolysin S (Correct Answer)
C. Streptodornase
D. Hyaluronidase

Explanation: **Explanation:** The correct answer is **Streptolysin S**. *Streptococcus pyogenes* (Group A Strep) produces two main hemolysins that damage red blood cell membranes, leading to the characteristic zone of beta-hemolysis on blood agar. 1. **Streptolysin S (SLS):** This is a cell-bound, oxygen-stable hemolysin. Because it is not inactivated by oxygen, it is responsible for the **surface hemolysis** seen around colonies when incubated aerobically. It is non-antigenic. 2. **Streptolysin O (SLO):** This is an oxygen-labile hemolysin. It is inactivated by oxygen; therefore, it only contributes to hemolysis deep in the agar (subsurface hemolysis) where oxygen tension is low. Unlike SLS, it is highly antigenic. **Analysis of Incorrect Options:** * **A. Streptolysin O:** While it causes hemolysis, it does not cause the visible surface hemolysis around colonies under standard aerobic conditions. It is primarily used clinically for detecting recent infections (ASO titer). * **C. Streptodornase:** Also known as DNase, this enzyme liquefies thick pus by degrading DNA. it does not cause hemolysis. * **D. Hyaluronidase:** Known as the "spreading factor," it degrades hyaluronic acid in connective tissue to facilitate the spread of infection. **High-Yield Clinical Pearls for NEET-PG:** * **ASO Titer:** An Anti-Streptolysin O (ASO) titer >200 units is significant for diagnosing post-streptococcal sequelae like Rheumatic Fever. * **Dick Test:** Historically used to identify susceptibility to Scarlet Fever (caused by Erythrogenic toxin). * **CAMP Test:** Used to identify Group B Streptococci (*S. agalactiae*), which produces a factor that enhances the hemolysis of *Staphylococcus aureus*.

Question 1397: Elek's gel precipitation test is used for diagnosing which bacterium?

A. Corynebacterium jakeium
B. Corynebacterium diphtheriae (Correct Answer)
C. Clostridium tetani
D. Clostridium perfringens

Explanation: **Explanation:** **Elek’s gel precipitation test** is a specialized *in vitro* toxigenicity test used to detect the production of exotoxin by **Corynebacterium diphtheriae**. **1. Why Corynebacterium diphtheriae is correct:** The pathogenicity of *C. diphtheriae* depends on the production of the diphtheria toxin, encoded by the *tox* gene (introduced by a lysogenic β-phage). Since not all strains are toxigenic, Elek’s test is performed to confirm virulence. In this test, 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 toxin reacts with the antitoxin, forming visible **white precipitin lines** (forming an arrowhead pattern) at a 45-degree angle. **2. Why the other options are incorrect:** * **Corynebacterium jeikeium:** This is a multi-drug resistant opportunistic pathogen typically associated with prosthetic valve endocarditis and septicemia in immunocompromised patients; it does not produce diphtheria toxin. * **Clostridium tetani:** Diagnosis is primarily clinical. While it produces tetanospasmin, toxigenicity is usually demonstrated via *in vivo* animal inoculation (mouse lethality test), not Elek’s test. * **Clostridium perfringens:** Known for gas gangrene and food poisoning, it is identified by the **Nagler reaction** (detecting lecithinase activity) and the "stormy fermentation" of milk. **Clinical Pearls for NEET-PG:** * **Gold Standard:** While Elek’s test is classic, **PCR** for the *tox* gene is the rapid modern method. * **Culture Media:** *C. diphtheriae* grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Morphology:** Characterized by **Albert’s stain** showing metachromatic (volutin) granules and a "Chinese letter" arrangement.

Question 1398: Which of the following statements about Corynebacterium diphtheriae is false?

A. Toxin production is mediated by a lysogenic bacteriophage. (Correct Answer)
B. Organism may be identified by tests for toxigenicity.
C. Toxin acts by inhibiting protein synthesis.
D. Toxin may affect the heart and nerves.

Explanation: **Explanation** **Why Option A is the Correct (False) Statement:** The question asks for the **false** statement. Option A is actually a **true** statement: the virulence of *Corynebacterium diphtheriae* depends on the production of the diphtheria toxin (DT), which is encoded by the **tox gene**. This gene is introduced into the bacterium by a **lysogenic bacteriophage (Beta-phage)**. Only strains infected with this phage (lysogeny) cause clinical diphtheria. In the context of this specific question format, if the option is factually true, it is not the "false" statement required—however, in medical entrance exams, this is a classic "fact-check" question where all options provided are often true statements, but the student must identify the core mechanism. *Note: If this were a "Select the False" question and all options are true, it usually implies a technical error in the question paper or a request to identify the most fundamental characteristic.* **Analysis of Other Options:** * **Option B (True):** Diagnosis relies on demonstrating toxigenicity (e.g., **Elek’s gel precipitation test** or PCR for the *tox* gene), as non-toxigenic strains do not cause systemic disease. * **Option C (True):** The toxin is an A-B exotoxin. The 'A' subunit inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Option D (True):** While the infection is local (upper respiratory tract), the toxin enters the bloodstream (toxemia), causing distant complications like **myocarditis** and **demyelinating peripheral neuropathy**. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Use **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies). * **Morphology:** Gram-positive bacilli with "Chinese letter" or cuneiform arrangement; **Volutin/Metachromatic granules** (Albert’s stain). * **Schick Test:** Used to demonstrate the immune status of an individual against diphtheria. * **Treatment:** Immediate administration of **Antidiphtheritic Serum (ADS)** is critical to neutralize unbound toxin.

Question 1399: Which enterotoxin is responsible for most cases of food poisoning caused by Staphylococcus?

A. Type E
B. Type C
C. Type B
D. Type A (Correct Answer)

Explanation: **Explanation:** Staphylococcal food poisoning is caused by the ingestion of preformed enterotoxins produced by *Staphylococcus aureus*. These toxins act as **superantigens**, triggering massive cytokine release and stimulating the vagus nerve, which leads to the characteristic rapid onset of projectile vomiting and abdominal cramps. * **Correct Option (D): Type A** is the most common enterotoxin associated with staphylococcal food poisoning worldwide. It is highly potent and heat-stable (withstanding boiling for 30 minutes), which allows it to persist in food even after the bacteria have been killed by cooking. * **Option (B) & (C): Type B and C** are less common causes of food poisoning. Enterotoxin B is more frequently associated with staphylococcal enterocolitis and is also studied as a potential bioweapon due to its ability to cause severe systemic inflammation. * **Option (A): Type E** is a rare cause of foodborne outbreaks compared to Type A. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (1–6 hours) because the toxin is preformed in the food (e.g., creamy salads, processed meats, dairy). * **Heat Stability:** The toxin is heat-stable, whereas the organism itself is heat-labile. * **Mechanism:** Superantigen-mediated stimulation of the vomiting center via the vagus nerve. * **Diagnosis:** Primarily clinical; culture of the food is more useful than stool culture. * **TSST-1:** Do not confuse enterotoxins with Toxic Shock Syndrome Toxin-1, which causes systemic shock rather than food poisoning.

Question 1400: A boy presents with a skin ulcer on his leg. Culture from the ulcer reveals beta hemolysis. Cultures from school children with a sore throat from some days prior also revealed beta hemolysis. What is the similarity between the organisms isolated from both sources?

A. Mec A gene is related to it
B. M protein is the same
C. C carbohydrate antigen is the same (Correct Answer)
D. The strains causing both conditions are the same

Explanation: The clinical presentation describes two different infections: **Impetigo/Pyoderma** (skin ulcer) and **Pharyngitis** (sore throat). Both are caused by *Streptococcus pyogenes*, also known as **Group A Streptococcus (GAS)**. ### Why Option C is Correct *Streptococcus pyogenes* is characterized by **Lancefield grouping**, which classifies Beta-hemolytic streptococci based on the specific **C-carbohydrate antigen** located in the cell wall. Regardless of the site of infection (skin or throat) or the specific strain, all *S. pyogenes* isolates share the **Group A** C-carbohydrate antigen. This is the fundamental similarity between the two isolates. ### Why Other Options are Incorrect * **Option A:** The *mecA* gene is associated with Methicillin-resistant *Staphylococcus aureus* (MRSA), not Streptococcus. * **Option B:** The **M protein** is the chief virulence factor of GAS. However, there are over 100 different M-protein serotypes. Generally, "pyodermal strains" (e.g., type 49) differ from "pharyngeal strains" (e.g., types 1, 3, 12). * **Option D:** The strains are usually different. Strains causing skin infections are typically "nephritogenic" (linked to PSGN), while those causing pharyngitis are "rheumatogenic" (linked to Rheumatic Fever). ### NEET-PG High-Yield Pearls * **Lancefield Grouping:** Based on C-carbohydrate (Group A to V). Note: *S. pneumoniae* and Viridans group lack this antigen. * **M Protein:** Inhibits phagocytosis by interfering with the alternate complement pathway (C3b). * **Complications:** * **Pharyngitis** can lead to both Acute Rheumatic Fever (ARF) and Post-Streptococcal Glomerulonephritis (PSGN). * **Skin infections** (Pyoderma) can lead to PSGN but **never** lead to Rheumatic Fever.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

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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