Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 481: Under the Revised National Tuberculosis Control Programme (RNTCP), which method is used for drug susceptibility testing?

A. Resistance ratio method
B. Absolute concentration method
C. Proportional method (Correct Answer)
D. None of the above

Explanation: **Explanation:** The **Proportion Method** is the gold standard phenotypic drug susceptibility testing (DST) method used under the National Tuberculosis Elimination Programme (NTEP, formerly RNTCP). **1. Why the Proportion Method is correct:** This method determines the percentage of mutant drug-resistant bacilli within a bacterial population. It involves inoculating the specimen onto both drug-free media (control) and media containing a specific critical concentration of the antitubercular drug. If the number of colonies on the drug-containing medium is **≥1%** of the colonies on the control medium, the strain is reported as **resistant**. This 1% threshold is clinically significant as it correlates with treatment failure. **2. Why other options are incorrect:** * **Resistance Ratio Method:** This compares the Minimum Inhibitory Concentration (MIC) of the patient’s isolate with the MIC of a standard laboratory strain (H37Rv). A ratio of 4 or more indicates resistance. It is technically demanding and rarely used in routine programs. * **Absolute Concentration Method:** This involves inoculating the strain onto media containing several different concentrations of the drug. Resistance is defined as growth (usually >20 colonies) at a specific "cutoff" concentration. It is difficult to standardize across laboratories. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Media:** The proportion method is traditionally performed on **Lowenstein-Jensen (LJ) medium** (takes 6–8 weeks) or liquid media like **MGIT** (takes 1–2 weeks). * **Critical Concentration:** The lowest concentration of a drug that inhibits 95% of "wild-type" strains but allows growth of resistant mutants. * **Current Trend:** While the Proportion Method remains the phenotypic reference, NTEP has shifted towards **Universal Drug Susceptibility Testing (UDST)** using rapid molecular methods like **CBNAAT (GeneXpert)** and **Line Probe Assay (LPA)** for faster diagnosis.

Question 482: Blue-green pus in burns patients is indicative of which type of cellulitis?

A. Pseudomonas (Correct Answer)
B. Staphylococcus
C. Klebsiella
D. E.coli

Explanation: **Explanation:** The presence of **blue-green pus** in a burn wound is a classic clinical sign of infection by ***Pseudomonas aeruginosa***. This characteristic coloration is due to the production of two specific water-soluble pigments by the bacteria: 1. **Pyocyanin:** A blue-green pigment that also generates reactive oxygen species to damage host tissues. 2. **Pyoverdin:** A yellow-green fluorescent pigment that acts as a siderophore (iron-sequestering molecule). When these pigments mix, they produce the distinct "blue-green" appearance often accompanied by a characteristic **fruity, grape-like odor**. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** Typically produces **creamy, golden-yellow pus** (due to the pigment staphyloxanthin). It is a common cause of surgical site infections but does not produce green pigments. * **Klebsiella pneumoniae:** Usually associated with thick, mucoid colonies due to its prominent capsule. In respiratory infections, it causes "currant jelly" sputum, but not blue-green pus in wounds. * **E. coli:** A common gram-negative rod that typically causes fecal-smelling infections or purulent discharge without specific diagnostic coloration. **High-Yield Clinical Pearls for NEET-PG:** * **Ecthyma Gangrenosum:** A necrotic skin lesion characterized by a black eschar, highly specific for *Pseudomonas* septicemia in immunocompromised patients. * **Hot Tub Folliculitis:** Skin infection caused by *Pseudomonas* associated with contaminated swimming pools or saunas. * **Culture Characteristics:** *Pseudomonas* is a non-lactose fermenter (NLF), oxidase positive, and grows well at 42°C. * **Drug of Choice:** Antipseudonal penicillins (Piperacillin-Tazobactam), Ceftazidime (3rd gen), Cefepime (4th gen), or Carbapenems.

Question 483: What is the most common cause of bacteremia?

A. Dental extraction
B. Post sigmoidoscopy (Correct Answer)
C. Superficial skin infections
D. Elective surgery

Explanation: **Explanation:** Bacteremia refers to the presence of viable bacteria in the bloodstream. While many daily activities and procedures cause transient bacteremia, the frequency and magnitude vary significantly based on the microbial load of the site and the degree of trauma. **1. Why "Post sigmoidoscopy" is correct:** The large intestine contains the highest concentration of indigenous microflora in the human body (up to $10^{12}$ organisms per gram of feces). Any invasive procedure involving the colonic mucosa, such as **sigmoidoscopy**, carries a high risk of seeding these bacteria into the portal and systemic circulation. Studies indicate that sigmoidoscopy results in transient bacteremia in approximately **5–10%** of cases, often involving anaerobic organisms like *Bacteroides* or enteric gram-negative rods. **2. Analysis of Incorrect Options:** * **Dental extraction:** While dental procedures are a classic cause of transient bacteremia (often *Viridans streptococci*), the incidence is generally lower or more transient compared to the massive bacterial load encountered during lower GI manipulations in a clinical setting. * **Superficial skin infections:** These typically lead to localized inflammation or lymphangitis. Bacteremia only occurs if the infection becomes invasive (e.g., cellulitis or abscess) or if the patient is immunocompromised. * **Elective surgery:** The risk depends entirely on the site. Clean elective surgeries (e.g., hernia repair) have a negligible risk of bacteremia compared to procedures involving "dirty" mucosal surfaces like the colon. **Clinical Pearls for NEET-PG:** * **Transient Bacteremia:** Usually lasts 15–30 minutes and is cleared by the Reticuloendothelial System (RES). * **Highest Risk Procedure:** Periodontal surgery and tooth extraction are the most common *dental* causes, but **Surgical procedures on the colon/rectum** remain the most significant source due to bacterial density. * **Common Organisms:** *E. coli*, *Bacteroides fragilis*, and *Enterococci* are the most common isolates following lower GI procedures. * **Prophylaxis:** Antibiotic prophylaxis for endocarditis is no longer routinely recommended for GI procedures unless a high-risk cardiac condition is present and an active infection is being treated.

Question 484: What is the color of Actinomyces granules?

A. Black
B. Yellow (Correct Answer)
C. Blue
D. White

Explanation: **Explanation:** **Actinomyces** species (primarily *Actinomyces israelii*) are Gram-positive, anaerobic, branching filamentous bacteria. They are known for causing chronic granulomatous lesions characterized by the formation of abscesses and multiple draining sinus tracts. 1. **Why Yellow is Correct:** The hallmark of actinomycosis is the presence of **"Sulfur granules"** in the pus or tissue. These are not actually made of sulfur but are microcolonies of the bacteria held together by a calcium phosphate matrix. Macroscopically, these granules appear as small, hard, **yellowish** particles resembling grains of sand. Under a microscope, with Gram staining, they show a dense center with peripheral radiating filaments (the "Ray fungus" appearance). 2. **Why Other Options are Incorrect:** * **Black:** Black granules are characteristic of **Eumycetoma** (fungal mycetoma) caused by organisms like *Madurella mycetomatis*. * **White:** While some species of *Nocardia* or *Actinomadura* can produce white or cream-colored granules, "Sulfur yellow" is the classic diagnostic feature for *Actinomyces*. * **Blue:** Blue is not a standard macroscopic color for granules in clinical bacteriology; however, the center of the granule stains blue (basophilic) on H&E stain. **High-Yield NEET-PG Pearls:** * **Lumpy Jaw:** The most common clinical presentation is cervicofacial actinomycosis, often following dental trauma or poor oral hygiene. * **IUD Association:** *Actinomyces* is a known cause of pelvic inflammatory disease in women using intrauterine devices. * **Staining:** They are **Gram-positive** but **Non-acid fast** (unlike *Nocardia*, which is weakly acid-fast). * **Treatment:** The drug of choice is **Penicillin G** for a prolonged duration.

Question 485: Leptospirosis is transmitted by which route?

A. Fecal-oral route
B. Aerosol route
C. Contact with infected animal urine (Correct Answer)
D. Contaminated milk

Explanation: **Explanation:** **Leptospirosis** is a worldwide zoonotic infection caused by the spirochete *Leptospira interrogans*. 1. **Why Option C is Correct:** The primary reservoir for Leptospira is the proximal renal tubules of rodents (especially rats) and domestic animals. The bacteria are shed in the **urine** of these carrier animals. Human infection occurs through **direct contact** with infected urine or **indirect contact** with water, soil, or mud contaminated by such urine. The spirochetes enter the human body through abraded skin, mucous membranes (conjunctiva), or by ingestion of contaminated water. 2. **Why Other Options are Incorrect:** * **A & D (Fecal-oral/Milk):** While ingestion of contaminated water can cause infection, Leptospirosis is not a classic fecal-oral disease (like Cholera) nor is it primarily transmitted via milk (like Brucellosis or Bovine TB). The organism is sensitive to stomach acid and does not survive well in milk. * **B (Aerosol):** Respiratory transmission is not a recognized route for Leptospirosis. **High-Yield Clinical Pearls for NEET-PG:** * **Occupational Hazard:** Common in sewage workers, farmers, rice-field workers, and veterinarians. * **Biphasic Illness:** * *Septicemic phase:* Fever, chills, and characteristically **conjunctival suffusion** (redness without exudate). * *Immune phase:* Development of complications like meningitis. * **Weil’s Disease:** The severe form characterized by the triad of **Jaundice, Renal failure (Azotemia), and Hemorrhage**. * **Diagnosis:** **Microscopic Agglutination Test (MAT)** is the Gold Standard. Dark-field microscopy can visualize the "hooked-end" (question mark) morphology. * **Treatment:** Doxycycline is the drug of choice for prophylaxis and mild cases; IV Penicillin G for severe cases.

Question 486: Which drug disk is used to check for Streptococcus pyogenes?

A. Bacitracin (Correct Answer)
B. Polymyxin
C. Optochin
D. None of the above

Explanation: ### Explanation **Correct Option: A. Bacitracin** *Streptococcus pyogenes* (Group A Streptococcus or GAS) is characterized by its high sensitivity to low concentrations of **Bacitracin**. In the microbiology laboratory, a 0.04-unit Bacitracin disk (Taxo A) is placed on a blood agar plate inoculated with beta-hemolytic streptococci. Any zone of inhibition around the disk indicates a presumptive identification of *S. pyogenes*. This test is a classic biochemical marker used to differentiate GAS from other beta-hemolytic streptococci, such as *S. agalactiae* (Group B), which are typically resistant. **Analysis of Incorrect Options:** * **B. Polymyxin:** This antibiotic is primarily active against Gram-negative bacteria (like *Pseudomonas*). It is not used for the identification of Gram-positive cocci like *Streptococcus*. * **C. Optochin:** This disk (ethylhydrocupreine hydrochloride) is specifically used to identify ***Streptococcus pneumoniae***. *S. pneumoniae* is sensitive to Optochin, whereas other alpha-hemolytic viridans streptococci are resistant. **High-Yield Clinical Pearls for NEET-PG:** * **PYR Test:** While Bacitracin is a traditional test, the **PYR (L-pyrrolidonyl-α-naphthylamide) test** is now considered more specific for *S. pyogenes* (it turns positive/red). * **CAMP Test:** Used to identify *Streptococcus agalactiae* (Group B); it shows "arrowhead" hemolysis when streaked perpendicular to *Staphylococcus aureus*. * **ASO Titer:** A significant diagnostic marker for post-streptococcal sequelae like Rheumatic Fever and Glomerulonephritis. * **M Protein:** The chief virulence factor of *S. pyogenes* that inhibits phagocytosis.

Question 487: Modified Thayer-Martin medium is used for the isolation of which bacterium?

A. Neisseria gonorrhoeae (Correct Answer)
B. Legionella pneumophila
C. Helicobacter pylori
D. Campylobacter jejuni

Explanation: **Explanation:** **Modified Thayer-Martin (MTM) medium** is a selective enrichment medium specifically designed for the isolation of pathogenic *Neisseria* species (*N. gonorrhoeae* and *N. meningitidis*) from clinical specimens containing mixed microbial flora (e.g., endocervical or urethral swabs). The medium consists of a **Chocolate agar base** supplemented with specific antibiotics to inhibit the growth of commensal organisms: * **Vancomycin:** Inhibits most Gram-positive bacteria. * **Colistin:** Inhibits most Gram-negative bacteria (except *Neisseria*). * **Nystatin:** Inhibits fungi/yeasts. * **Trimethoprim:** Inhibits the swarming of *Proteus* species. **Analysis of Incorrect Options:** * **Legionella pneumophila:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar**, which provides essential L-cysteine and iron. * **Helicobacter pylori:** Typically isolated on **Skirrow’s medium** or specialized chocolate agar with antibiotics like vancomycin and polymyxin B. * **Campylobacter jejuni:** Requires thermophilic conditions (42°C) and selective media such as **Skirrow’s, Butzler’s, or Campy-BAP medium**. **High-Yield Clinical Pearls for NEET-PG:** * *Neisseria gonorrhoeae* is highly fastidious and sensitive to cold; specimens should never be refrigerated. * Other media for *Neisseria* include **Martin-Lewis medium** and **NYC (New York City) medium**. * On Gram stain, *N. gonorrhoeae* appears as **Gram-negative intracellular diplococci** (kidney-bean shaped) within polymorphonuclear leukocytes. * It is **Oxidase positive** and ferments **only Glucose** (unlike *N. meningitidis*, which ferments both Glucose and Maltose).

Question 488: A sputum sample was brought to the laboratory for analysis. Gram stain revealed rare epithelial cells, 8 to 10 polymorphonuclear leukocytes per high-power field, and pleomorphic Gram-negative rods. As the laboratory consultant, which of the following interpretations should you make?

A. The sputum specimen is too contaminated by saliva to be useful.
B. The appearance of the sputum is suggestive of Haemophilus pneumonia. (Correct Answer)
C. The patient has pneumococcal pneumonia.
D. There is no evidence of an inflammatory response.

Explanation: ### Explanation **1. Why Option B is Correct:** The diagnosis of *Haemophilus influenzae* pneumonia is supported by two key findings in the Gram stain: * **Morphology:** *H. influenzae* are characteristically described as **pleomorphic Gram-negative coccobacilli** or rods. * **Specimen Quality:** The presence of 8–10 polymorphonuclear leukocytes (PMNs) per high-power field (HPF) indicates an active inflammatory response, while "rare epithelial cells" suggest the sample is a deep respiratory specimen rather than superficial saliva. **2. Why Other Options are Incorrect:** * **Option A:** According to **Bartlett’s criteria** for sputum grading, a specimen is considered "contaminated" or poor quality if it contains >10–25 squamous epithelial cells per low-power field. "Rare" epithelial cells indicate a high-quality specimen. * **Option C:** *Streptococcus pneumoniae* (Pneumococcus) would appear as **Gram-positive, lancet-shaped diplococci**, not Gram-negative rods. * **Option D:** The presence of 8–10 PMNs per HPF is clear evidence of an **inflammatory exudate**, contradicting the claim that there is no inflammatory response. **3. NEET-PG High-Yield Pearls:** * **Bartlett’s Classification:** Used to assess sputum quality. A score of 0 or less indicates salivary contamination; a positive score indicates a reliable lower respiratory tract sample. * **Culture Requirements:** *H. influenzae* is fastidious and requires **Factor V (NAD)** and **Factor X (Hemin)** for growth. It grows on **Chocolate Agar** but not on Blood Agar (unless "Satellitism" occurs around *S. aureus*). * **Quellung Reaction:** While historically associated with *S. pneumoniae*, it is also positive for encapsulated *H. influenzae* type b (Hib). * **Clinical Context:** *H. influenzae* is a leading cause of pneumonia in patients with underlying **COPD**.

Question 489: The whiff test is done for which of the following conditions?

A. Gonococcal infections
B. Bacterial vaginosis (Correct Answer)
C. Trichomoniasis
D. Candidiasis

Explanation: **Explanation:** The **Whiff test** (also known as the Amine test) is a diagnostic procedure used to identify **Bacterial Vaginosis (BV)**. It involves adding a drop of 10% Potassium Hydroxide (KOH) to a sample of vaginal discharge. A "positive" result is the immediate release of a pungent, **fishy odor**. This occurs because the alkaline KOH volatilizes aromatic amines (like putrescine and cadaverine) produced by anaerobic bacteria (e.g., *Gardnerella vaginalis*, *Mobiluncus*) that overgrow in BV. **Analysis of Options:** * **Bacterial Vaginosis (Correct):** It is diagnosed using **Amsel’s Criteria** (3 out of 4 required): 1. Thin, homogenous discharge; 2. Vaginal pH >4.5; 3. Positive Whiff test; 4. Presence of **Clue cells** on wet mount. * **Gonococcal infections:** Diagnosed via Gram stain (Gram-negative intracellular diplococci) and NAAT. It typically causes endocervicitis rather than primary vaginitis. * **Trichomoniasis:** Caused by *Trichomonas vaginalis*. While it may produce a foul odor and elevated pH, the hallmark is a "strawberry cervix" and motile trichomonads on a wet mount. * **Candidiasis:** Characterized by a thick, "curdy/cottage-cheese" discharge and a **normal vaginal pH (<4.5)**. KOH is used here not for odor, but to dissolve cellular debris to better visualize pseudohyphae and spores. **NEET-PG High-Yield Pearls:** * **Nugent Scoring:** The "Gold Standard" for BV diagnosis (based on Gram stain morphotypes). * **Clue Cells:** Vaginal epithelial cells coated with bacteria, giving them a "shaggy" appearance; these are the most specific sign of BV. * **Treatment of Choice:** Oral or topical Metronidazole (Clindamycin is an alternative). Remember to treat the patient, but routine treatment of the male partner is not recommended for BV.

Question 490: Streptococcus is classified based on which of the following?

A. M protein
B. Cultural characteristics
C. Bile solubility
D. Cell wall carbohydrate (Correct Answer)

Explanation: ### Explanation The classification of *Streptococcus* is primarily based on the **Rebecca Lancefield classification** system. This system categorizes streptococci into groups (A–V) based on the antigenic differences in the **group-specific C-substance**, which is a **polysaccharide (carbohydrate)** located in the bacterial cell wall. #### Why the other options are incorrect: * **M protein:** This is a major virulence factor used for the **sub-typing** of Group A Streptococci (*S. pyogenes*), known as Griffith typing. It is not the primary basis for the broad classification of the genus. * **Cultural characteristics:** While hemolysis patterns on blood agar (Alpha, Beta, Gamma) are used for initial screening, they are not the definitive taxonomic classification. For instance, many different Lancefield groups can exhibit beta-hemolysis. * **Bile solubility:** This is a specific biochemical test used to differentiate *Streptococcus pneumoniae* (bile soluble) from other alpha-hemolytic viridans streptococci (bile insoluble). It is a diagnostic tool, not a classification system. #### High-Yield Clinical Pearls for NEET-PG: * **Lancefield Group A:** *S. pyogenes* (Bacitracin sensitive, PYR test positive). * **Lancefield Group B:** *S. agalactiae* (CAMP test positive, causes neonatal meningitis). * **Lancefield Group D:** Includes *Enterococcus* (now a separate genus) and *S. bovis* (associated with colon cancer). * **Exceptions:** *Streptococcus pneumoniae* and the Viridans group do not possess the specific cell wall carbohydrate and are therefore **non-groupable** under the Lancefield system.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

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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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