Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 321: Group A Streptococcus causes which of the following conditions EXCEPT?

A. Scarlet fever
B. Erysipelas
C. Impetigo
D. Epidermolysis bullosa (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Epidermolysis bullosa**. **Why it is the correct answer:** Epidermolysis bullosa is a group of rare **genetic connective tissue disorders** characterized by skin fragility and blister formation in response to minor mechanical trauma. It is caused by mutations in genes encoding structural proteins (like keratin, laminin, or collagen) that anchor the epidermis to the dermis. It is **not** an infectious disease and is not caused by *Streptococcus pyogenes* (Group A Streptococcus). **Why the other options are incorrect:** Group A Streptococcus (GAS) is a versatile pathogen known for causing various pyogenic and toxigenic infections: * **Scarlet Fever:** Caused by GAS strains that produce **Streptococcal Pyrogenic Exotoxins (SPE)**, typically following pharyngitis. It presents with a "strawberry tongue" and a sandpaper-like rash. * **Erysipelas:** A superficial bacterial skin infection involving the upper dermis and superficial lymphatics, characteristically presenting as a well-demarcated, fiery red, edematous plaque. GAS is the most common cause. * **Impetigo:** GAS is a leading cause of non-bullous impetigo (honey-colored crusts), though *Staphylococcus aureus* is also frequently implicated. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The chief virulence factor of GAS; it is anti-phagocytic and responsible for molecular mimicry leading to Rheumatic Fever. * **ASO Titre:** Used to diagnose prior GAS pharyngeal infections (Rheumatic Fever), while **Anti-DNase B** is the preferred test for prior skin infections (Post-Streptococcal Glomerulonephritis). * **Distinction:** Do not confuse *Epidermolysis bullosa* (genetic) with *Staphylococcal Scalded Skin Syndrome* (caused by *S. aureus* exfoliative toxins).

Question 322: Which of the following bacteria is transmitted by houseflies?

A. Leptospira
B. Listeria
C. Vibrio cholerae (Correct Answer)
D. Yersinia

Explanation: **Explanation:** **Vibrio cholerae** is the correct answer because it is primarily transmitted via the **fecal-oral route**. Houseflies (*Musca domestica*) act as significant **mechanical vectors** for cholera. They transport the bacteria from infected feces to human food and water sources through their legs, body hairs, and vomitus. In endemic areas, fly control is a critical public health measure to prevent outbreaks of cholera. **Analysis of Incorrect Options:** * **Leptospira:** These are spirochetes transmitted through contact with water or soil contaminated by the **urine of infected animals** (primarily rodents). Entry occurs through abraded skin or mucous membranes, not via houseflies. * **Listeria:** *Listeria monocytogenes* is typically a **food-borne pathogen** associated with unpasteurized dairy, deli meats, and refrigerated ready-to-eat foods. It is known for its ability to grow at psychrophilic (cold) temperatures. * **Yersinia:** *Yersinia pestis* (Plague) is transmitted by the **rat flea** (*Xenopsylla cheopis*), while *Yersinia enterocolitica* is usually transmitted through contaminated food (especially pork) or water. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanical Vectors:** Houseflies are also responsible for transmitting other enteric pathogens like *Shigella* (causing bacillary dysentery), *Salmonella typhi*, and *Entamoeba histolytica*. * **Vibrio cholerae Characteristics:** It is a comma-shaped, Gram-negative rod showing **darting motility** (monotrichous flagella). * **Culture Media:** Thiosulfate Citrate Bile Salts Sucrose (**TCBS**) agar is the selective medium of choice, where it produces yellow colonies due to sucrose fermentation.

Question 323: Which of the following tests is NOT used for the diagnosis of leprosy?

A. Lepromin test (Correct Answer)
B. Split skin smear
C. Fine needle aspiration cytology
D. Skin biopsy

Explanation: The correct answer is **A. Lepromin test**. ### **Explanation** The **Lepromin test** is a delayed hypersensitivity reaction (Type IV) used to assess the **cell-mediated immunity (CMI)** of a patient against *Mycobacterium leprae*. It is **NOT** a diagnostic test because it cannot confirm the presence of the disease; it is often negative in the most severe form (lepromatous leprosy). Instead, it is used for: 1. **Classification:** To distinguish between Tuberculoid (Positive) and Lepromatous (Negative) leprosy. 2. **Prognosis:** A positive test indicates a good immune response. ### **Why other options are used for diagnosis:** * **Split skin smear (B):** This is the standard screening tool. It involves taking smears from earlobes and skin lesions to look for Acid-Fast Bacilli (AFB) using the Ziehl-Neelsen stain. * **Skin biopsy (D):** This is the **Gold Standard** for diagnosis. It allows for histopathological examination to see nerve involvement, granuloma formation, and the presence of bacilli. * **Fine needle aspiration cytology (FNAC) (C):** FNAC of enlarged nerves or skin nodules is an effective, less invasive diagnostic tool used to demonstrate AFB and cytomorphological features of leprosy. ### **High-Yield Clinical Pearls for NEET-PG:** * **Bacteriological Index (BI):** Calculated from split skin smears; a BI of $\geq$ 1+ at any site defines **Multibacillary (MB)** leprosy. * **Fernandez Reaction:** Early reading of Lepromin test (at 48 hours). * **Mitsuda Reaction:** Late reading of Lepromin test (at 21 days); more clinically significant. * **First Sign:** The earliest clinical sign of leprosy is often a pale (hypopigmented) patch with loss of sensation.

Question 324: Which of the following is NOT true about diphtheria?

A. Incubation period is 2 to 5 days.
B. Chemoprophylaxis is done with rifampicin. (Correct Answer)
C. Previously immunized asymptomatic household contacts should receive a booster dose.
D. A child with diphtheria is given antitoxin.

Explanation: **Explanation:** **Why Option B is the correct answer (False statement):** The drug of choice for chemoprophylaxis in diphtheria is **Erythromycin** (oral) or a single dose of **Benzathine Penicillin G** (IM). Rifampicin is primarily used for chemoprophylaxis in *Meningococcal meningitis* and *H. influenzae* type b infections, not diphtheria. **Analysis of other options:** * **Option A:** The incubation period for *Corynebacterium diphtheriae* is typically short, ranging from **2 to 5 days**, though it can occasionally extend to 10 days. * **Option C:** Management of close contacts is a high-yield topic. Asymptomatic household contacts who were previously immunized should receive a **booster dose** of a diphtheria toxoid-containing vaccine to ensure immediate protective immunity. * **Option D:** Diphtheria Antitoxin (DAT) is the cornerstone of treatment. It must be administered **immediately** based on clinical suspicion to neutralize circulating toxins before they bind to tissues (myocardium and nerves), as it cannot neutralize toxin already fixed to cells. **High-Yield Clinical Pearls for NEET-PG:** * **Schick Test:** Used to demonstrate immunity/susceptibility to diphtheria. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Morphology:** Gram-positive bacilli with "Chinese letter" or cuneiform arrangement; contains **Volutin/Metachromatic granules** (Albert’s stain). * **Toxin:** It is an A-B toxin that inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 325: Several weeks following a severe streptococcal sore throat, a 7-year-old boy develops acute glomerulonephritis with edema, hypertension, and hematuria. Which of the following best describes the properties of the most likely causative organism?

A. Alpha hemolytic, lysed by optochin
B. Alpha hemolytic, not lysed by optochin
C. Beta hemolytic, can grow in 40% bile and 6.5% sodium chloride
D. Beta hemolytic, inhibited by bacitracin (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The clinical presentation of edema, hypertension, and hematuria (nephritic syndrome) following a sore throat is diagnostic of **Post-Streptococcal Acute Glomerulonephritis (PSAGN)**. This is a Type III hypersensitivity reaction caused by nephritogenic strains of **Group A Beta-hemolytic Streptococcus (GABS)**, also known as *Streptococcus pyogenes*. In the microbiology laboratory, *S. pyogenes* is characterized by: * **Beta-hemolysis:** Complete clearing of RBCs on blood agar. * **Bacitracin Sensitivity:** It is uniquely inhibited by low concentrations of bacitracin (Taxo A disc), which distinguishes it from other beta-hemolytic streptococci like Group B Strep. **2. Why Incorrect Options are Wrong:** * **Option A:** Describes *Streptococcus pneumoniae*. While it causes respiratory infections, it is alpha-hemolytic and does not cause PSAGN. * **Option B:** Describes Viridans group streptococci (e.g., *S. mutans*). These are common causes of subacute bacterial endocarditis, not glomerulonephritis. * **Option C:** Describes *Enterococcus* species (formerly Group D Strep). These are typically gamma or alpha-hemolytic and are associated with UTIs and biliary tract infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Latent Period:** PSAGN occurs 1–3 weeks after pharyngitis or 3–6 weeks after skin infections (impetigo). Note that Rheumatic Fever *only* follows pharyngitis. * **Diagnosis:** Look for elevated **ASO titers** (after pharyngitis) or **Anti-DNase B** (after skin infections). Low C3 complement levels are a classic finding. * **M Protein:** The major virulence factor of *S. pyogenes* that is anti-phagocytic and responsible for molecular mimicry. * **PYR Test:** *S. pyogenes* is PYR positive, another rapid diagnostic marker.

Question 326: An enterococcus (E. faecium) was isolated from a urine specimen (100,000 cfu/mL). Treatment of the patient with ampicillin and gentamicin failed. What is the most clinically appropriate action?

A. Do no further clinical workup
B. Suggest to the laboratory that low colony counts may reflect infection
C. Determine if fluorescent microscopy is available for the diagnosis of actinomycosis
D. Consider vancomycin as an alternative drug (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** The patient has a significant urinary tract infection (UTI) caused by *Enterococcus faecium* (colony count $\geq 10^5$ CFU/mL). The failure of ampicillin and gentamicin suggests **High-Level Aminoglycoside Resistance (HLAR)** or intrinsic resistance common in *E. faecium*. Unlike *E. faecalis*, *E. faecium* is frequently resistant to ampicillin due to altered penicillin-binding proteins (PBP5). When first-line synergistic therapy (Beta-lactam + Aminoglycoside) fails, **Vancomycin** is the drug of choice, provided the strain is not Vancomycin-Resistant Enterococci (VRE). **2. Analysis of Incorrect Options:** * **Option A:** Incorrect. A colony count of 100,000 CFU/mL in a symptomatic patient is clinically significant and requires treatment modification if the initial therapy fails. * **Option B:** Incorrect. 100,000 CFU/mL is the standard threshold for "significant bacteriuria." Suggesting low colony counts is irrelevant here as the count is already high. * **Option C:** Incorrect. Actinomycosis is caused by *Actinomyces* species (anaerobic Gram-positive branching bacilli), not *Enterococcus*. It typically presents with sulfur granules and chronic abscesses, not acute UTIs. **3. NEET-PG High-Yield Pearls:** * **Enterococci Characteristics:** Catalase negative, PYR positive, grow in 6.5% NaCl and 40% bile (Bile Esculin Agar). * **Resistance Patterns:** *E. faecium* is generally more antibiotic-resistant than *E. faecalis*. * **Synergy:** Aminoglycosides alone cannot penetrate the cell wall of Enterococci; they must be combined with a cell-wall active agent (Ampicillin/Vancomycin) for bactericidal effect. * **VRE Treatment:** If Vancomycin resistance occurs (VanA/VanB genes), the next-line drugs are **Linezolid, Daptomycin, or Quinupristin-Dalfopristin.**

Question 327: A 55-year-old female patient presents with duodenal ulcer. What is the most sensitive test to detect Helicobacter pylori?

A. Urea breath test (Correct Answer)
B. Culture
C. Histology
D. Serology

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, spiral-shaped bacterium strongly associated with peptic ulcer disease and gastric carcinoma. **1. Why Urea Breath Test (UBT) is correct:** The **Urea Breath Test (UBT)** is considered the **most sensitive and specific non-invasive test** for detecting active *H. pylori* infection. It relies on the organism's potent **urease enzyme**, which hydrolyzes orally administered labeled urea ($^{13}C$ or $^{14}C$) into ammonia and labeled $CO_2$. The labeled $CO_2$ is then detected in the patient's expired breath. Its high sensitivity (>95%) and ability to reflect the "global" infection status of the stomach make it the gold standard for both initial diagnosis and confirming eradication. **2. Why other options are incorrect:** * **Culture:** While it is the **most specific** (100%) method, it is technically demanding, slow, and has **low sensitivity** because the organism is fastidious and fragile during transport. It is usually reserved for antibiotic sensitivity testing in refractory cases. * **Histology:** This is the "Gold Standard" for **invasive** diagnosis (via biopsy). While highly sensitive, it is subject to sampling error if the biopsy is taken from an area with low bacterial density. * **Serology (ELISA):** This detects IgG antibodies. It cannot distinguish between a past and current infection, making it the least useful for confirming eradication or active disease. **High-Yield Clinical Pearls for NEET-PG:** * **Invasive Gold Standard:** Histology (Warthin-Starry or Giemsa stain). * **Non-invasive Gold Standard:** Urea Breath Test. * **Fastest Test:** Rapid Urease Test (RUT/CLO test) on biopsy material. * **Test of choice for Eradication:** UBT (performed 4 weeks after completing therapy). * **Note:** Proton Pump Inhibitors (PPIs) and antibiotics must be stopped 2 weeks prior to UBT/RUT to avoid false negatives.

Question 328: Shanghai fever is a mild febrile illness caused by which of the following organisms?

A. Pseudomonas (Correct Answer)
B. Burkholderia
C. Acinetobacter
D. Stenotrophomonas

Explanation: **Explanation:** **Shanghai Fever** is a rare clinical syndrome traditionally associated with **Pseudomonas aeruginosa** sepsis, primarily occurring in infants and young children. 1. **Why Option A is Correct:** * *Pseudomonas aeruginosa* is the causative agent. The illness is characterized by a triad of **fever, diarrhea, and a characteristic skin rash** (often presenting as *Ecthyma Gangrenosum*—necrotic ulcers with a central black eschar). * Unlike typical *Pseudomonas* infections which are opportunistic/nosocomial, Shanghai fever can occur in previously healthy, community-dwelling children, leading to necrotizing enterocolitis and sepsis. 2. **Why Other Options are Incorrect:** * **B. Burkholderia:** *B. pseudomallei* causes Melioidosis (Vietnamese Time Bomb), while *B. cepacia* is associated with cystic fibrosis. Neither causes Shanghai fever. * **C. Acinetobacter:** Known for multidrug-resistant hospital-acquired pneumonia and VAP (Ventilator-Associated Pneumonia), but not associated with this specific pediatric febrile syndrome. * **D. Stenotrophomonas:** *S. maltophilia* is an opportunistic pathogen often seen in patients on long-term carbapenems, but it does not cause Shanghai fever. **High-Yield Clinical Pearls for NEET-PG:** * **Ecthyma Gangrenosum:** This is the pathognomonic skin lesion of *Pseudomonas* septicemia. It is a non-pyogenic, necrotic lesion caused by perivascular invasion and exotoxin release. * **Pseudomonas Characteristics:** Gram-negative bacilli, oxidase positive, non-fermenter, produces pigments like **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent). * **Other Key Associations:** Hot tub folliculitis, Swimmer’s ear (Otitis externa), and puncture wounds leading to osteomyelitis (especially through sneakers).

Question 329: A patient presents with pneumonia. His sputum was sent for culture. The bacterium obtained was gram-positive cocci in chains and formed alpha-hemolytic colonies on sheep agar. Which of the following tests will help in confirming the diagnosis?

A. Novobiocin susceptibility testing
B. Optochin susceptibility testing (Correct Answer)
C. Bacitracin susceptibility testing
D. Oxacillin susceptibility testing

Explanation: ### Explanation The clinical presentation of pneumonia combined with the laboratory finding of **Gram-positive cocci in chains** and **alpha-hemolysis** narrows the differential diagnosis to two primary organisms: *Streptococcus pneumoniae* and Viridans group streptococci. **1. Why Optochin Susceptibility is the Correct Answer:** To differentiate between these two, the **Optochin (Ethylhydrocupreine hydrochloride) susceptibility test** is used. *Streptococcus pneumoniae* is **exquisitely sensitive** to optochin, showing a zone of inhibition ≥14 mm. Conversely, Viridans streptococci are resistant. Additionally, *S. pneumoniae* is bile soluble, while Viridans streptococci are not. **2. Analysis of Incorrect Options:** * **Novobiocin susceptibility (A):** Used to differentiate coagulase-negative staphylococci. *S. saprophyticus* is resistant, while *S. epidermidis* is sensitive. * **Bacitracin susceptibility (C):** Used to identify **Group A Beta-hemolytic Streptococci** (*S. pyogenes*), which are sensitive, from other beta-hemolytic streptococci. * **Oxacillin susceptibility (D):** Used to screen for penicillin resistance in *S. pneumoniae* or methicillin resistance in *S. aureus* (MRSA), but it is not a primary diagnostic test for species identification. **3. NEET-PG High-Yield Pearls:** * **S. pneumoniae Morphology:** Classically described as "Flame-shaped" or "Lancet-shaped" diplococci. * **Quellung Reaction:** The gold standard for serotyping *S. pneumoniae* based on capsular swelling. * **Alpha-hemolysis:** Caused by partial reduction of hemoglobin to methemoglobin (greenish discoloration). * **Draughtsman/Checkers appearance:** Older colonies of *S. pneumoniae* undergo autolysis, leading to a central depression.

Question 330: Which of the following is a method for acquiring infection by Legionella pneumophila?

A. Consumption of contaminated water with protozoa of Legionella
B. Breathing aerosols from environmental water sources such as air conditioning units (Correct Answer)
C. Consumption of contaminated meat and meat products
D. Close contact with carriers of Legionella

Explanation: **Explanation:** **Legionella pneumophila** is a Gram-negative, aerobic, pleomorphic bacillus that causes **Legionnaires' disease** (severe pneumonia) and **Pontiac fever** (a mild, flu-like illness). 1. **Why Option B is Correct:** The primary mode of transmission for *Legionella* is the **inhalation of contaminated aerosols** or the aspiration of contaminated water. These bacteria thrive in man-made water systems where water is maintained at temperatures between 20°C and 50°C. Common sources include **air conditioning cooling towers**, nebulizers, showerheads, hot tubs, and decorative fountains. The bacteria survive within these systems by forming biofilms or parasitizing free-living amoebae. 2. **Why Other Options are Incorrect:** * **Option A:** While *Legionella* lives inside protozoa (like *Acanthamoeba*) in the environment, infection occurs via the respiratory route (inhalation), not by ingestion (consumption) of water. * **Option C:** *Legionella* is not a foodborne pathogen. Contaminated meat is associated with organisms like *Salmonella*, *Listeria*, or *E. coli*. * **Option D:** A critical high-yield fact is that there is **no person-to-person transmission** of *Legionella*. Therefore, close contact with carriers or infected patients does not spread the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with L-cysteine and iron. * **Clinical Clue:** Pneumonia associated with **hyponatremia**, diarrhea, and confusion (neurological symptoms). * **Diagnosis:** The **Urinary Antigen Test** is the most common rapid diagnostic method (detects Serogroup 1). * **Staining:** Poorly visualized on Gram stain; **Silver stains (Dieterle)** or Direct Fluorescent Antibody (DFA) are preferred.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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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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