Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1971: Genus Neisseria is characterized by which of the following morphological and staining properties?

A. Gram-positive diplococci
B. Gram-negative diplococci (Correct Answer)
C. Gram-negative coccobacilli
D. Gram-positive bacilli

Explanation: ### Explanation **Correct Answer: B. Gram-negative diplococci** **1. Why it is correct:** The genus *Neisseria* (primarily *N. meningitidis* and *N. gonorrhoeae*) consists of aerobic, non-motile, non-spore-forming bacteria. Morphologically, they are **Gram-negative** (staining pink/red due to a thin peptidoglycan layer and an outer membrane containing Lipooligosaccharide) and appear as **diplococci** (pairs). A classic descriptive feature is their **kidney-bean or coffee-bean shape**, with the adjacent sides being concave or flattened. **2. Why other options are incorrect:** * **A. Gram-positive diplococci:** This describes *Streptococcus pneumoniae*. Unlike *Neisseria*, *S. pneumoniae* is lancet-shaped and stains purple. * **C. Gram-negative coccobacilli:** This describes organisms like *Haemophilus influenzae*, *Bordetella*, or *Brucella*. These are intermediate in shape between cocci and bacilli. * **D. Gram-positive bacilli:** This describes genera such as *Bacillus*, *Clostridium*, *Corynebacterium*, and *Listeria*. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Oxidase and Catalase:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Sugar Utilization:** *N. meningitidis* ferments **M**altose and **G**lucose (**M**eningitidis = **M** & **G**), whereas *N. gonorrhoeae* ferments only **G**lucose (**G**onorrhoeae = **G** only). * **Intracellular Location:** In clinical samples (like urethral discharge or CSF), *Neisseria* are frequently found **polymorphonuclear leukocytes (neutrophils)**. * **Culture:** They are fastidious and grow best on **Chocolate Agar** or selective media like **Thayer-Martin Medium** (VPN medium: Vancomycin, Polymyxin, Nystatin).

Question 1972: Which bacterium possesses a polypeptide capsule?

A. Corynebacterium diphtheriae
B. Clostridium welchii
C. Staphylococcus aureus
D. Bacillus anthracis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Bacillus anthracis**. **1. Why Bacillus anthracis is correct:** In the world of microbiology, almost all bacterial capsules are composed of **polysaccharides**. *Bacillus anthracis* is the classic, high-yield exception. Its capsule is composed of a **polypeptide** (specifically **poly-D-glutamic acid**). This capsule is encoded by the **pXO2 plasmid** and is essential for virulence as it inhibits phagocytosis. Because it is a protein, it can be visualized using the **McFadyean reaction** (polychrome methylene blue stain), which shows a pink/purple capsule around blue bacilli. **2. Why the other options are incorrect:** * **Corynebacterium diphtheriae:** This is a non-capsulated, Gram-positive rod. Its primary virulence factor is the diphtheria toxin, not a capsule. * **Clostridium welchii (C. perfringens):** While it possesses a capsule, it is **polysaccharide** in nature. It is famously known for "double zone hemolysis" and gas gangrene. * **Staphylococcus aureus:** Most strains have a microcapsule, but like the majority of bacteria, it is made of **polysaccharides**. **3. NEET-PG High-Yield Pearls:** * **Capsule Rule:** All capsules are polysaccharides EXCEPT *Bacillus anthracis* (Polypeptide). * **Quellung Reaction:** Used to identify capsulated bacteria (e.g., *S. pneumoniae*, *H. influenzae*). * **India Ink:** Used specifically for *Cryptococcus neoformans* (a fungus with a polysaccharide capsule). * **Mnemonic for Capsulated Bacteria:** "**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies" (*S. pneumoniae, Klebsiella, H. influenzae, N. meningitidis, Salmonella typhi, B. anthracis*).

Question 1973: What is the diagnostic test for rickettsial infection?

A. Weil-Felix reaction (Correct Answer)
B. Widal test
C. Sabin test
D. Culture

Explanation: **Explanation:** The **Weil-Felix reaction** is the classic diagnostic test for Rickettsial infections. It is a **heterophile agglutination test** based on the principle of **cross-reactivity**. Certain strains of *Proteus vulgaris* (OX-19, OX-2) and *Proteus mirabilis* (OX-K) share common alkali-stable carbohydrate antigens with various Rickettsial species. When a patient is infected with Rickettsia, they produce antibodies that cross-react with these Proteus antigens, causing visible agglutination. **Analysis of Options:** * **Widal Test:** Used for the diagnosis of Enteric fever (Typhoid), detecting antibodies against *Salmonella typhi* O and H antigens. * **Sabin-Feldman Dye Test:** The gold standard serological test for *Toxoplasma gondii*. * **Culture:** Rickettsiae are **obligate intracellular pathogens** and do not grow on cell-free media. They require living systems (egg yolk sacs or cell lines), making culture difficult, hazardous, and rarely used for routine diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Specific Patterns:** * **Epidemic & Endemic Typhus:** OX-19 positive. * **Rocky Mountain Spotted Fever:** OX-19 and OX-2 positive. * **Scrub Typhus:** OX-K positive (caused by *Orientia tsutsugamushi*). * **Q Fever Exception:** *Coxiella burnetii* (Q fever) is **Weil-Felix negative**. * **Modern Gold Standard:** While Weil-Felix is historically important and frequently tested, the **Indirect Immunofluorescence Assay (IFA)** is now the modern gold standard for diagnosis.

Question 1974: The attachment of Actinomyces species to the tooth surface is facilitated by:

A. Fimbriae. (Correct Answer)
B. Cilia
C. Flagella.
D. Capsule.

Explanation: **Explanation:** **Actinomyces species**, particularly *Actinomyces viscosus* and *Actinomyces naeslundii*, are primary colonizers of the dental plaque. The attachment to the tooth surface (the acquired pellicle) is mediated by **Fimbriae** (pili). These filamentous surface appendages contain specific adhesins that recognize and bind to proline-rich proteins and statherins found in human saliva. This initial attachment is a critical step in the formation of dental biofilms and subsequent calculus development. **Analysis of Options:** * **Fimbriae (Correct):** These are proteinaceous, hair-like appendages used specifically for **adhesion** to surfaces and other cells. In *Actinomyces*, Type 1 fimbriae mediate attachment to the tooth, while Type 2 fimbriae are involved in co-aggregation with oral streptococci. * **Cilia:** These are complex organelles found only in **eukaryotic** cells (e.g., respiratory epithelium). Bacteria do not possess cilia. * **Flagella:** These are primarily organs of **motility**. While some bacteria use flagella for initial surface contact, *Actinomyces* species are characteristically **non-motile** and lack flagella. * **Capsule:** While capsules help in evading phagocytosis and can assist in biofilm stability, they are not the primary organelle for the specific attachment of *Actinomyces* to the dental pellicle. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *Actinomyces* are Gram-positive, non-acid-fast, anaerobic to microaerophilic branching filaments (often described as "ray fungus"). * **Clinical Presentation:** Causes "Lumpy Jaw" (Cervicofacial actinomycosis) characterized by chronic granulomatous lesions and abscesses. * **Diagnostic Hallmark:** Presence of **Sulfur granules** in the pus (yellowish colonies of bacteria). * **Drug of Choice:** Penicillin G (long-term therapy).

Question 1975: Which organism shows Nagler's reaction?

A. Clostridium tetani
B. Clostridium welchii (Correct Answer)
C. Mycobacterium tuberculosis
D. Mycobacterium leprae

Explanation: **Explanation:** **Nagler’s Reaction** is a biochemical test used for the rapid identification of **Clostridium welchii** (also known as *Clostridium perfringens*). 1. **Mechanism of the Correct Answer:** *Clostridium welchii* produces a potent exotoxin called **Alpha-toxin**, which is a **lecithinase** (phospholipase C). When the organism is grown on an agar medium containing egg yolk (which is rich in lecithin), the alpha-toxin breaks down the lecithin into insoluble diglycerides. This results in a characteristic **opalescence (opacity)** around the colonies. The reaction is confirmed as specific if the opalescence is inhibited by adding *C. perfringens* antitoxin to one half of the plate. 2. **Analysis of Incorrect Options:** * **Clostridium tetani:** Produces tetanospasmin (a neurotoxin) but lacks lecithinase activity; it is known for "drumstick" appearance and swarming growth. * **Mycobacterium tuberculosis & M. leprae:** These are Acid-Fast Bacilli (AFB). They do not produce lecithinase and are identified via Ziehl-Neelsen staining or culture (LJ medium), not Nagler’s reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Double Zone of Hemolysis:** On blood agar, *C. welchii* shows a target appearance—an inner zone of complete hemolysis (theta-toxin) and an outer zone of incomplete hemolysis (alpha-toxin). * **Stormy Fermentation:** In litmus milk medium, *C. welchii* produces acid and gas, leading to a "stormy" appearance of the clot. * **Clinical Condition:** It is the most common cause of **Gas Gangrene** and food poisoning. * **Morphology:** It is a Gram-positive, capsulated, non-motile bacillus with subterminal spores (though spores are rarely seen in clinical samples).

Question 1976: Which of the following is an important distinguishing characteristic of H. pylori as compared to Campylobacter species?

A. Oxidase production
B. Catalase production
C. Urease production (Correct Answer)
D. Curved shape

Explanation: **Explanation:** The correct answer is **C. Urease production.** While both *Helicobacter pylori* and *Campylobacter* species belong to the same family of epsilon-proteobacteria and share several morphological and biochemical traits, the **strong production of urease** is the hallmark feature that distinguishes *H. pylori*. 1. **Why Urease is the key:** *H. pylori* produces abundant urease, which hydrolyzes urea into ammonia and carbon dioxide. This creates a localized alkaline "cloud" that neutralizes gastric acid, allowing the bacteria to survive in the acidic stomach environment. This property is the basis for the **Urea Breath Test** and **Rapid Urease Test (RUT)** used in clinical diagnosis. *Campylobacter* species are typically urease-negative. 2. **Why other options are incorrect:** * **Oxidase and Catalase production (A & B):** Both *H. pylori* and *Campylobacter* are typically positive for oxidase and catalase. Therefore, these tests cannot be used to differentiate between them. * **Curved shape (D):** Both genera exhibit a characteristic curved, spiral, or "S-shaped" morphology (often described as "gull-wing" appearance for *Campylobacter*). **High-Yield Clinical Pearls for NEET-PG:** * **H. pylori:** Associated with Type B antral gastritis, Peptic Ulcer Disease (PUD), Gastric Adenocarcinoma, and MALToma. * **Culture Media:** *H. pylori* grows on Skirrow’s medium or Chocolate agar; *Campylobacter* requires selective media like Skirrow’s or Butzler’s medium. * **Temperature:** *Campylobacter jejuni* is thermophilic (grows best at 42°C), whereas *H. pylori* grows at 37°C. * **Motility:** Both are motile via polar flagella; *H. pylori* has multiple flagella at one pole (lophotrichous), while *Campylobacter* usually has a single flagellum (monotrichous).

Question 1977: Which group of atypical mycobacteria does Mycobacterium scrofulaceum, the causative agent of scrofula, belong to?

A. Photochromogens
B. Scotochromogens (Correct Answer)
C. Non-photochromogens
D. Rapid growers

Explanation: ### Explanation The classification of atypical mycobacteria (Nontuberculous Mycobacteria - NTM) is based on the **Runyon Classification**, which categorizes them according to their growth rate and pigment production. **Why Scotochromogens is correct:** *Mycobacterium scrofulaceum* belongs to **Runyon Group II (Scotochromogens)**. These organisms produce pigment (typically yellow-orange) regardless of whether they are grown in the light or the dark. Clinically, *M. scrofulaceum* is a classic cause of granulomatous cervical lymphadenitis (scrofula) in children. **Analysis of Incorrect Options:** * **Photochromogens (Group I):** These produce pigment only after exposure to light (e.g., *M. kansasii*, *M. marinum*). * **Non-photochromogens (Group III):** These produce little to no pigment regardless of light exposure (e.g., *M. avium-intracellulare* complex, *M. ulcerans*). * **Rapid Growers (Group IV):** These are characterized by fast growth (usually <7 days) and include *M. fortuitum*, *M. chelonae*, and *M. abscessus*. **High-Yield Clinical Pearls for NEET-PG:** * **Scrofula:** While *M. tuberculosis* is the most common cause of scrofula in adults, *M. scrofulaceum* and *M. avium* complex are significant causes in children. * **M. marinum:** Known as "Fish tank granuloma" or "Swimming pool granuloma" (Group I). * **M. ulcerans:** Causes **Buruli ulcer**, characterized by a painless necrotic ulcer (Group III). * **M. kansasii:** Most common NTM to cause a TB-like pulmonary disease (Group I). * **M. abscessus:** Often associated with post-injection abscesses and cystic fibrosis (Group IV).

Question 1978: A group of social activists cleaning sewers develop fever and headaches. On examination, a non-catalase, non-glucose fermenting organism is isolated from them. Which of the following organisms is most likely responsible?

A. Pseudomonas
B. Leptospira (Correct Answer)
C. Chlamydia
D. Actinomyces

Explanation: **Explanation:** The clinical scenario describes a classic presentation of **Leptospirosis**, an occupational hazard for sewer workers, farmers, and those in contact with water contaminated by animal urine (typically rats). **Why Leptospira is correct:** * **Occupational Exposure:** Sewer workers are at high risk due to exposure to rodent urine. * **Biochemical Profile:** *Leptospira interrogans* is a spirochete. Unlike many common bacteria, it is **catalase-negative** and **does not ferment glucose** (it utilizes long-chain fatty acids as its primary energy and carbon source). * **Clinical Presentation:** Fever and headache are the initial symptoms of the bacteremic phase. **Why the other options are incorrect:** * **A. Pseudomonas:** While common in moist environments and sewers, *Pseudomonas aeruginosa* is **catalase-positive** and **oxidase-positive**. It utilizes glucose via oxidative pathways (O/F test), not fermentation, but the catalase-positive status rules it out. * **C. Chlamydia:** These are obligate intracellular bacteria that do not grow on standard culture media and are not typically associated with sewer-related outbreaks of acute fever. * **D. Actinomyces:** These are Gram-positive anaerobic filamentous bacteria. They are part of the normal oral flora and typically cause chronic "lumpy jaw" abscesses with sulfur granules, not acute febrile illness in sewer workers. **High-Yield Clinical Pearls for NEET-PG:** * **Weil’s Disease:** The severe form of Leptospirosis characterized by the triad of **jaundice, renal failure, and hemorrhage**. * **Culture:** Grown on specialized media like **EMJH** (Ellinghausen-McCullough-Johnson-Harris) or **Fletcher’s medium**. * **Microscopy:** Cannot be seen under light microscopy; requires **Dark Ground Microscopy (DGM)**. * **Serology:** **Microscopic Agglutination Test (MAT)** is the gold standard for diagnosis.

Question 1979: Which of the following causes a dead end infection?

A. Clostridium tetani (Correct Answer)
B. Vibrio cholerae
C. Staphylococcus aureus
D. Haemophilus

Explanation: **Explanation:** A **dead-end infection** occurs when a pathogen enters a host but cannot be transmitted further to another susceptible host. This usually happens because the pathogen does not exit the body in sufficient quantities or via a route that allows for continued transmission. **Why Clostridium tetani is the correct answer:** *Clostridium tetani* spores typically enter the body through contaminated deep puncture wounds. Once inside, they germinate and produce the potent neurotoxin **tetanospasmin**. While the toxin causes severe clinical disease (Tetanus), the bacteria remain localized at the wound site and are not shed through respiratory droplets, feces, or blood. Consequently, an infected human cannot transmit tetanus to another human, making it a classic example of a dead-end infection. **Why the other options are incorrect:** * **Vibrio cholerae:** Transmitted via the fecal-oral route. Infected individuals shed massive amounts of bacteria in "rice-water stools," which can contaminate water supplies and infect others. * **Staphylococcus aureus:** Transmitted through direct skin contact, fomites, or respiratory droplets (in cases of pneumonia). It is highly contagious in both community and hospital settings. * **Haemophilus (e.g., H. influenzae):** Primarily transmitted via respiratory droplets from the nasopharynx of colonized or infected individuals. **High-Yield Clinical Pearls for NEET-PG:** * **Other Dead-end Hosts/Infections:** Rabies (in humans), Brucellosis (in humans), Legionella, and many zoonotic viral encephalitides (like West Nile Virus). * **Tetanus Mechanism:** Tetanospasmin acts by blocking the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells in the spinal cord, leading to spastic paralysis. * **Key Clinical Sign:** "Risus sardonicus" (characteristic grin due to facial muscle spasms) and "Opisthotonus" (archings of the back).

Question 1980: Scotochromogens are:

A. Mycobacterium Gordonae (Correct Answer)
B. Mycobacterium Marinum
C. Mycobacterium Intracellulare
D. Mycobacterium avium

Explanation: **Explanation:** The classification of Non-Tuberculous Mycobacteria (NTM) is based on the **Runyon Classification**, which categorizes species according to their growth rate and pigment production in response to light. **1. Why Mycobacterium gordonae is correct:** * **Scotochromogens (Runyon Group II):** These organisms produce pigment (usually yellow-orange) regardless of whether they are grown in the dark or light. * *M. gordonae* (the "tap water bacillus") is the classic example of a Scotochromogen. It is generally non-pathogenic and often encountered as a laboratory contaminant. **2. Why the other options are incorrect:** * **Mycobacterium marinum (Option B):** This is a **Photochromogen (Runyon Group I)**. These organisms are non-pigmented when grown in the dark but produce a photo-activated yellow-orange pigment after exposure to light. * **Mycobacterium intracellulare & Mycobacterium avium (Options C & D):** These belong to the **Mycobacterium Avium Complex (MAC)**, which are **Non-chromogens (Runyon Group III)**. They produce little to no pigment regardless of light exposure and appear as buff-colored colonies. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group I (Photochromogens):** *M. kansasii, M. marinum* (causes "Swimming pool granuloma"). * **Runyon Group II (Scotochromogens):** *M. gordonae, M. scrofulaceum* (causes cervical lymphadenitis in children). * **Runyon Group III (Non-chromogens):** *M. avium, M. intracellulare* (common in HIV/AIDS patients). * **Runyon Group IV (Rapid Growers):** Growth occurs within 7 days. Examples include *M. fortuitum, M. chelonae, M. abscessus*. * **Mnemonic:** **P**hoto-**S**coto-**N**on-**R**apid (**P**lease **S**ay **N**o **R**unyon).

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free