Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 521: Which of the following bacteria acts by increasing cAMP?

A. Staphylococcus aureus
B. E. coli heat stable toxin
C. Vibrio cholera (Correct Answer)
D. Salmonella

Explanation: **Explanation:** The correct answer is **Vibrio cholerae**. The primary virulence factor of *V. cholerae* is the **Cholera Toxin (Choleragen)**, which belongs to the A-B subunit family of toxins. **Mechanism of Action:** The 'A' subunit of the toxin catalyzes the **ADP-ribosylation** of the **Gs (stimulatory) protein**. This locks the Gs protein in its active state, leading to the permanent activation of **Adenylate Cyclase**. This results in a massive increase in intracellular **cyclic AMP (cAMP)** levels. High cAMP levels stimulate the secretion of chloride ions (via CFTR channels) and inhibit the absorption of sodium, leading to the characteristic "rice-water" diarrhea. **Analysis of Other Options:** * **B. E. coli heat-stable toxin (ST):** Unlike the heat-labile toxin (LT) which increases cAMP, the **ST toxin increases cGMP** by activating Guanylate Cyclase. * **A. Staphylococcus aureus:** Its primary toxins include Enterotoxins (acting as superantigens) and Alpha-toxin (pore-forming), which do not primarily function via the cAMP pathway. * **D. Salmonella:** While some strains produce an enterotoxin similar to cholera toxin, it is not the primary or classic mechanism associated with the genus in standard medical examinations. **NEET-PG High-Yield Pearls:** * **Mnemonic for cAMP-increasing toxins:** "**cAMP** is increased by **C**holera, **A**nthrax (Edema factor), **M**onocytogenes (not classic), and **P**ertussis (via Gi inhibition)." * **E. coli toxins:** Remember **"Labile-AMP, Stable-GMP"** (LT acts like Cholera toxin; ST acts on Guanylate cyclase). * **Bacillus anthracis:** The Edema Factor (EF) is itself a Calmodulin-dependent adenylate cyclase.

Question 522: Which culture medium is used for the diagnosis of EHEC O157:H7?

A. O culture
B. Sorbitol MacConkey agar (Correct Answer)
C. XLD agar
D. Deoxycholate media

Explanation: **Explanation:** The correct answer is **Sorbitol MacConkey (SMAC) agar**. **1. Why Sorbitol MacConkey Agar is correct:** Most strains of *Escherichia coli* ferment lactose and sorbitol. However, **Enterohemorrhagic *E. coli* (EHEC) serotype O157:H7** is unique because it is a **non-sorbitol fermenter (NSF)**. On SMAC agar, the standard lactose is replaced with D-sorbitol. While commensal *E. coli* ferment sorbitol and produce pink colonies, EHEC O157:H7 produces **colorless (pale) colonies**, allowing for rapid screening and presumptive identification. **2. Why the other options are incorrect:** * **O culture:** This is not a standard microbiological medium; "O" usually refers to the somatic antigen used in serotyping. * **XLD (Xylose Lysine Deoxycholate) agar:** This is a selective and differential medium primarily used for the isolation of *Salmonella* and *Shigella* from stool samples. * **Deoxycholate media (DCA):** This is a selective medium used for the isolation of enteric pathogens like *Salmonella* and *Shigella* by inhibiting the growth of Gram-positive bacteria. **3. High-Yield Clinical Pearls for NEET-PG:** * **EHEC Pathogenesis:** Produces **Verocytotoxin (Shiga-like toxin)** which inhibits protein synthesis by targeting the 60S ribosomal subunit. * **Clinical Presentation:** Causes Hemorrhagic Colitis and is the leading cause of **Hemolytic Uremic Syndrome (HUS)** (Triad: Microangiopathic hemolytic anemia, Thrombocytopenia, and Acute Renal Failure). * **Treatment Caution:** Antibiotics are generally avoided in EHEC infections as they may increase the release of Shiga toxin, worsening the risk of HUS. * **Alternative Media:** Tellurite Cefixime-SMAC (CT-SMAC) is even more selective for O157:H7.

Question 523: Which of the following organisms lacks a cell wall?

A. Chlamydia
B. Mycoplasma (Correct Answer)
C. Staphylococcus
D. Clostridium

Explanation: **Explanation:** The correct answer is **Mycoplasma**. **1. Why Mycoplasma is correct:** Mycoplasmas are the smallest free-living prokaryotes. Their defining characteristic is the **complete absence of a peptidoglycan cell wall**. Instead, their cell membrane contains **sterols** (like cholesterol), which provide structural integrity and osmotic stability—a feature unique among bacteria. Because they lack a cell wall, they are naturally resistant to beta-lactam antibiotics (like Penicillins and Cephalosporins) which target cell wall synthesis. **2. Why other options are incorrect:** * **Chlamydia:** Although they are obligate intracellular pathogens and were once thought to lack peptidoglycan, they possess a cell wall structure (containing a modified peptidoglycan layer) and genes for its synthesis. * **Staphylococcus:** This is a classic Gram-positive coccus with a thick, multi-layered peptidoglycan cell wall. * **Clostridium:** This is a Gram-positive anaerobic spore-forming rod with a robust peptidoglycan cell wall. **3. NEET-PG High-Yield Pearls:** * **Pleomorphism:** Due to the lack of a rigid cell wall, Mycoplasma are highly pleomorphic (assuming various shapes) and can pass through bacterial filters (0.22 μm). * **Culture:** They produce characteristic **"Fried Egg" colonies** on PPLO agar. * **L-forms:** These are bacteria that *normally* have cell walls but have lost them (due to antibiotics or enzymes). Unlike Mycoplasma, L-forms can usually revert to their walled state. * **Clinical Correlation:** *Mycoplasma pneumoniae* causes "Walking Pneumonia" and is associated with **Cold Agglutinins** (IgM antibodies). * **Treatment:** Since they lack a cell wall, use protein synthesis inhibitors like **Macrolides** (Azithromycin) or Tetracyclines.

Question 524: Spores are formed by all the following bacteria except:

A. E. coli (Correct Answer)
B. B. anthracis
C. B. cereus
D. None of the above

Explanation: **Explanation:** The ability to form spores (sporulation) is a survival mechanism used by certain bacteria to withstand extreme environmental conditions like heat, desiccation, and disinfectants. In medical bacteriology, spore formation is a characteristic feature of two primary genera: **Bacillus** (aerobic) and **Clostridium** (anaerobic). **1. Why E. coli is the correct answer:** * *Escherichia coli* is a Gram-negative, non-spore-forming bacilli belonging to the family *Enterobacteriaceae*. It exists only in a vegetative state and is easily destroyed by heat (e.g., pasteurization) and standard disinfectants. **2. Analysis of incorrect options:** * **B. anthracis:** This is a Gram-positive, aerobic spore-forming rod. Its spores are highly resilient and are found in the soil. They are central to its pathogenesis (Anthrax) and its potential use as a biological weapon. * **B. cereus:** Like other members of the *Bacillus* genus, it produces spores. These spores often contaminate rice and can survive short periods of cooking, leading to food poisoning when the spores germinate into toxin-producing vegetative cells. **Clinical Pearls for NEET-PG:** * **Mnemonic for Spore-formers:** "**B**C **L**istens" (**B**acillus, **C**lostridium, **L**ineage/Sporosarcina). * **Sterilization Check:** Spores of *Geobacillus stearothermophilus* are used as biological indicators for autoclaves, while *Bacillus atrophaeus* is used for ethylene oxide and dry heat sterilization. * **Staining:** Spores do not take up routine Gram stains (appearing as clear areas). They are visualized using the **Schaffer-Fulton stain** (Malachite green) or Modified Ziehl-Neelsen stain. * **Key Property:** Spores contain **Calcium dipicolinate**, which provides heat resistance.

Question 525: Which of the following bacteria is catalase positive, coagulase negative, and beta-hemolytic?

A. Streptococcus pyogenes
B. Staphylococcus aureus
C. Coagulase-negative staphylococci (Correct Answer)
D. Enterococci

Explanation: ### Explanation This question tests your ability to differentiate common Gram-positive cocci using biochemical and hemolytic properties—a high-yield area for NEET-PG. **1. Why the Correct Answer is Right:** * **Catalase Test:** This test differentiates *Staphylococci* (positive) from *Streptococci* (negative). Since the organism is catalase-positive, it must belong to the genus *Staphylococcus*. * **Coagulase Test:** This differentiates *S. aureus* (positive) from **Coagulase-negative staphylococci (CoNS)** (negative). * **Hemolysis:** While many CoNS (like *S. epidermidis*) are non-hemolytic (gamma), certain species such as ***Staphylococcus haemolyticus*** are characteristically **beta-hemolytic**. Therefore, CoNS is the only option that fits all three criteria. **2. Why the Other Options are Wrong:** * **A. *Streptococcus pyogenes*:** While it is strongly beta-hemolytic, all *Streptococci* are **catalase-negative**. * **B. *Staphylococcus aureus*:** While it is catalase-positive and beta-hemolytic, it is **coagulase-positive**. * **D. Enterococci:** These are **catalase-negative** (or pseudocatalase) and typically show gamma (non-hemolytic) or alpha hemolysis. **3. Clinical Pearls for NEET-PG:** * **Most common CoNS:** *S. epidermidis* (associated with prosthetic valve endocarditis and biofilm on catheters). * **Novobiocin Sensitivity:** Used to differentiate CoNS. *S. saprophyticus* (UTI in young females) is **Resistant**, while *S. epidermidis* is **Sensitive**. * ***S. haemolyticus*** is the second most common CoNS isolated from clinical samples and is known for its multi-drug resistance (especially Vancomycin resistance).

Question 526: Oxidase test helps in the identification of which bacterium?

A. Brucella
B. Vibrio
C. Neisseria
D. E. coli (Correct Answer)

Explanation: **Explanation:** The **Oxidase Test** is a biochemical assay used to determine the presence of the enzyme **cytochrome c oxidase**. This enzyme is part of the bacterial electron transport chain. A positive result is indicated by the development of a deep purple/blue color when the organism is rubbed onto a reagent-soaked filter paper (using 1% tetramethyl-p-phenylenediamine dihydrochloride). **Why E. coli is the Correct Answer (in the context of this specific question):** The question asks which bacterium the test helps identify, often by its **negative** result. **E. coli** belongs to the family *Enterobacteriaceae*, all members of which are characteristically **Oxidase negative**. In clinical microbiology, the oxidase test is the primary tool used to differentiate the *Enterobacteriaceae* (Oxidase -ve) from other Gram-negative rods like *Pseudomonas* or *Vibrio* (Oxidase +ve). **Analysis of Incorrect Options:** * **Vibrio:** These are Gram-negative, curved bacilli that are strongly **Oxidase positive**. This test is a key step in differentiating *Vibrio* from *Enterobacteriaceae*. * **Neisseria:** All species of *Neisseria* (including *N. meningitidis* and *N. gonorrhoeae*) are strongly **Oxidase positive**. This is a hallmark diagnostic feature for the genus. * **Brucella:** These are small Gram-negative coccobacilli that are **Oxidase positive**. **NEET-PG High-Yield Pearls:** * **Oxidase Positive Organisms (Mnemonic: PVN BHAG):** **P**seudomonas, **V**ibrio, **N**eisseria, **B**rucella, **H**elicobacter/Haemophilus, **A**eromonas, **G**ampylobacter (Campylobacter). * **Reagent used:** Kovac’s reagent (1% tetramethyl-p-phenylenediamine dihydrochloride). * **Clinical Note:** Always use a wooden applicator stick or plastic loop; nichrome wires can cause a false-positive result due to surface oxidation.

Question 527: A sewer worker presents with high-grade fever, neck rigidity, and signs of meningismus. Laboratory findings are suggestive of renal failure and elevated liver enzymes. What is your diagnosis?

A. Leptospirosis (Correct Answer)
B. Borrelia recurrentis
C. Diphtheria
D. Plague

Explanation: **Explanation:** The clinical presentation of a **sewer worker** (occupational exposure) with high-grade fever, signs of meningitis (neck rigidity), and multi-organ involvement (**renal failure and jaundice/elevated liver enzymes**) is a classic description of **Weil’s Disease**, the severe form of **Leptospirosis**. 1. **Why Leptospirosis is correct:** *Leptospira interrogans* is transmitted through water contaminated with the urine of infected rodents (common in sewers). The disease is characteristically biphasic: the initial septicemic phase presents with fever and **conjunctival suffusion** (a high-yield sign), followed by an immune phase involving the liver (jaundice), kidneys (uremia), and CNS (aseptic meningitis). 2. **Why other options are incorrect:** * **Borrelia recurrentis:** Causes Relapsing Fever, characterized by recurring febrile episodes due to antigenic variation, but typically lacks the specific renal-hepatic failure triad seen here. * **Diphtheria:** Primarily presents as a pseudomembranous pharyngitis with potential bull-neck lymphadenopathy and myocarditis; it does not cause this systemic febrile-icteric-renal syndrome. * **Plague:** Caused by *Yersinia pestis*, it presents as painful lymphadenopathy (Buboes) or severe pneumonia, usually following a flea bite. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Microscopic Agglutination Test (MAT). * **Culture Media:** EMJH (Ellinghausen-McCullough-Johnson-Harris) or Fletcher’s medium. * **Morphology:** Tightly coiled spirochetes with "hooked ends" (Question-mark shape). * **Drug of Choice:** Doxycycline (prophylaxis/mild cases) or IV Penicillin G (severe cases).

Question 528: Which gram-positive bacilli exhibit swarming growth?

A. Clostridium tetani (Correct Answer)
B. Clostridium welchii
C. Proteus
D. All of the above

Explanation: **Explanation:** The phenomenon of **swarming growth** refers to the rapid, coordinated movement of bacteria across a solid agar surface, resulting in a thin, spreading film rather than discrete colonies. **1. Why Clostridium tetani is correct:** *Clostridium tetani* is a Gram-positive, anaerobic, motile bacillus. It is well-known for producing a thin, spreading film of growth (swarming) over the surface of agar plates. This occurs due to its peritrichous flagella. In the context of **Gram-positive bacilli**, *C. tetani* is the classic example tested in exams. **2. Why the other options are incorrect:** * **Clostridium welchii (C. perfringens):** This is a non-motile Gram-positive bacillus. Instead of swarming, it is characterized by "target hemolysis" (double zone of hemolysis) on blood agar. * **Proteus:** While *Proteus mirabilis* and *Proteus vulgaris* are the most famous examples of swarming bacteria, they are **Gram-negative bacilli**. The question specifically asks for a **Gram-positive** organism. * **All of the above:** Incorrect because only *C. tetani* fits both the Gram-stain and the morphological growth pattern requested. **High-Yield Clinical Pearls for NEET-PG:** * **Other Swarmers:** *Proteus* spp., *Vibrio parahaemolyticus* (on salt-free agar), *Bacillus subtilis*, and *Serratia marcescens*. * **C. tetani Morphology:** Often described as having a **"drumstick appearance"** due to terminal, spherical spores. * **Inhibition of Swarming:** In the lab, swarming can be inhibited by increasing agar concentration (6%), adding chloral hydrate, boric acid, or sodium azide. * **Toxin:** The clinical features of Tetanus (lockjaw, opisthotonus) are caused by **tetanospasmin**, a potent neurotoxin that blocks the release of inhibitory neurotransmitters (GABA and glycine).

Question 529: E. coli gives pink color on which medium?

A. Chocolate agar
B. L.J. medium
C. MacConkey's medium (Correct Answer)
D. Saline broth

Explanation: **Explanation:** The correct answer is **MacConkey’s medium**. This is a classic example of a **differential and selective medium** used primarily for the isolation of Gram-negative enteric bacteria. **Why MacConkey’s Medium is correct:** MacConkey agar contains **lactose** and a pH indicator, **neutral red**. *E. coli* is a **Lactose Fermenter (LF)**. It ferments lactose to produce organic acids, which lowers the pH of the medium. In an acidic environment, the neutral red indicator turns pink/red, resulting in the characteristic **pink-colored colonies**. **Analysis of Incorrect Options:** * **A. Chocolate agar:** This is an enriched medium (heated blood agar) used for fastidious organisms like *H. influenzae* and *Neisseria*. It does not contain lactose or indicators to differentiate *E. coli* by color. * **B. L.J. (Lowenstein-Jensen) medium:** This is a selective medium specifically used for the growth of *Mycobacterium tuberculosis*. It is egg-based and typically appears pale green. * **D. Saline broth:** This is a simple transport or diluent medium. It lacks the nutrients and indicators required to produce a color change or colonial morphology. **High-Yield Clinical Pearls for NEET-PG:** * **Lactose Fermenters (Pink on MacConkey):** *E. coli*, *Klebsiella* (mucoid colonies), *Enterobacter*. * **Non-Lactose Fermenters (Pale/Colorless):** *Salmonella*, *Shigella*, *Proteus*, *Pseudomonas*. * **E. coli on CLED Agar:** Produces **yellow colonies** (due to lactose fermentation). * **E. coli on EMBA (Eosin Methylene Blue):** Produces a characteristic **metallic green sheen**. * **IMViC Profile for E. coli:** Indole (+), Methyl Red (+), Voges-Proskauer (-), Citrate (-) [Mnemonic: **++--**].

Question 530: Eschar is formed in infections caused by which of the following organisms?

A. Bartonella henselae
B. Bordetella pertussis
C. Bacillus anthracis (Correct Answer)
D. Escherichia coli

Explanation: **Explanation:** The correct answer is **Bacillus anthracis**. An **eschar** is a characteristic clinical lesion defined as a painless, necrotic ulcer with a black, depressed center surrounded by edema and vesicles. **1. Why Bacillus anthracis is correct:** In **Cutaneous Anthrax** (the most common form of the disease), spores enter through skin abrasions. The organism produces **Anthrax Toxin**, specifically the **Lethal Factor (LF)** and **Edema Factor (EF)**. LF causes tissue necrosis, while EF causes significant localized "gelatinous" edema. The result is the classic **"Malignant Pustule,"** which is not actually a pustule but a painless black eschar. **2. Why other options are incorrect:** * **Bartonella henselae:** Causes Cat Scratch Disease. It typically presents with regional lymphadenopathy and a small papule or pustule at the site of inoculation, but not a necrotic black eschar. * **Bordetella pertussis:** Causes Whooping Cough, a respiratory infection characterized by paroxysmal cough. It does not cause skin lesions or eschars. * **Escherichia coli:** A common cause of UTIs, neonatal meningitis, and sepsis. While certain strains can cause skin infections (like cellulitis in diabetics), they do not characteristically form an eschar. **Clinical Pearls for NEET-PG:** * **Differential Diagnosis of Eschar:** Apart from Anthrax, eschars are high-yield features of **Scrub Typhus** (*Orientia tsutsugamushi*) and **Rickettsialpox**. * **Anthrax Key Features:** Look for keywords like "painless," "non-purulent," "gelatinous edema," and "occupational exposure" (e.g., wool sorters, tanners). * **Microscopy:** *B. anthracis* appears as large, Gram-positive, box-car shaped bacilli in chains (Bamboo-stick appearance) with a prominent polypeptide capsule (D-glutamate).

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

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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