Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 811: A cook prepares sandwiches for 10 people going for a picnic. Eight of them develop severe gastroenteritis within 4-6 hours of consumption. Investigations are likely to reveal that the cook is the carrier of which organism?

A. Salmonella typhi
B. Vibrio cholerae
C. Entamoeba histolytica
D. Staphylococcus aureus (Correct Answer)

Explanation: ### Explanation The correct answer is **Staphylococcus aureus**. **1. Why Staphylococcus aureus is correct:** The key to this question lies in the **short incubation period (1–6 hours)**. *Staphylococcus aureus* food poisoning is caused by the ingestion of **pre-formed enterotoxins** (Type A–E) produced in food contaminated by a carrier (often a food handler with a skin lesion or asymptomatic nasal colonization). Because the toxin is already present in the food, the onset of symptoms (nausea, projectile vomiting, and abdominal cramps) is rapid. Common vehicles include protein-rich or creamy foods like sandwiches, salads (mayonnaise), and pastries. **2. Why the other options are incorrect:** * **Salmonella typhi:** Causes Enteric Fever, which has a much longer incubation period (7–14 days). It presents with high-grade fever and "pea-soup" diarrhea, not rapid-onset vomiting. * **Vibrio cholerae:** Causes "rice-water" stools. The incubation period is typically 1–3 days. It is transmitted via contaminated water or seafood, not usually through sandwiches handled by a carrier. * **Entamoeba histolytica:** Causes amoebic dysentery. The incubation period is long (2–4 weeks), and it presents with bloody mucoid stools and chronic progression. **3. Clinical Pearls for NEET-PG:** * **Shortest Incubation Periods:** *S. aureus* (1–6 hours) and *Bacillus cereus* (emetic type: 1–5 hours). * **Heat Stability:** Staphylococcal enterotoxins are **heat-stable** (resist boiling for 30 minutes); reheating food does not prevent illness. * **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. * **Diagnosis:** Primarily clinical; culture of the food or the carrier’s nasal/skin swab can confirm the source.

Question 812: Which organism did Griffith use to demonstrate transformation?

A. H. influenzae
B. E. coli
C. Proteus
D. Pneumococcus (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pneumococcus (*Streptococcus pneumoniae*)**. **1. Why Pneumococcus is correct:** In 1928, Frederick Griffith conducted the "Griffith's Experiment," which provided the first evidence of bacterial transformation. He used two strains of *S. pneumoniae*: the **Smooth (S) strain** (virulent due to a polysaccharide capsule) and the **Rough (R) strain** (non-virulent, lacking a capsule). Griffith observed that when heat-killed S-strain bacteria were injected into mice along with live R-strain bacteria, the mice died. He concluded that the live R-strain had been "transformed" into the virulent S-strain by picking up a "transforming principle" (later identified as DNA) from the dead S-strain. **2. Why other options are incorrect:** * **H. influenzae:** While *Haemophilus influenzae* was the first bacterium to have its entire genome sequenced and is naturally competent for transformation, it was not the organism used in Griffith’s landmark study. * **E. coli:** This is the most common model organism in molecular biology and is used for artificial transformation in labs (using $CaCl_2$ or electroporation), but it does not naturally undergo transformation as Pneumococcus does. * **Proteus:** Known for its "swarming motility" and urease production, it is not historically associated with the discovery of transformation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Avery, MacLeod, and McCarty (1944):** Proved that the "transforming principle" in Griffith's experiment was **DNA**. * **Natural Competence:** Only certain bacteria are naturally capable of transformation (e.g., *S. pneumoniae*, *H. influenzae*, *Neisseria* species). * **Pneumococcal Capsule:** The capsule is the primary virulence factor; non-capsulated strains are non-pathogenic. * **Transformation Definition:** The process of horizontal gene transfer where a bacterium takes up "naked" DNA from the surrounding environment.

Question 813: You are examining a specimen taken from a child with Otitis. On microscopy, you see capsulated lanceolate diplococci, and the culture of the specimen shows a 'draughtsman appearance'. What is the diagnosis?

A. Meningococcus
B. Gonococcus
C. Pneumococcus (Correct Answer)
D. Enterococcus

Explanation: **Explanation:** The clinical presentation and morphological features described are classic for **Streptococcus pneumoniae (Pneumococcus)**. **1. Why Pneumococcus is correct:** * **Morphology:** Under microscopy, *S. pneumoniae* typically appears as Gram-positive, **lanceolate (flame-shaped)** diplococci. It is characteristically **capsulated**, which is its primary virulence factor. * **Culture Characteristics:** On Blood Agar, it produces small, translucent colonies. As the culture ages (after 24–48 hours), the central part of the colony undergoes **autolysis**, leading to a collapsed center with a raised periphery. This specific morphology is known as the **'draughtsman' or 'checkerboard' appearance**. * **Clinical Context:** It is the most common cause of Otitis Media in children. **2. Why other options are incorrect:** * **Meningococcus (*N. meningitidis*):** These are Gram-negative, **lenticular (kidney-bean shaped)** diplococci. They do not show autolysis or draughtsman colonies. * **Gonococcus (*N. gonorrhoeae*):** Also Gram-negative kidney-shaped diplococci; primarily associated with STIs and ophthalmia neonatorum, not typically otitis media. * **Enterococcus:** These appear as oval cocci in pairs or short chains. They are relatively resistant to autolysis and do not exhibit the draughtsman appearance. **High-Yield NEET-PG Pearls:** * **Quellung Reaction:** Swelling of the capsule when treated with specific antiserum (Gold standard for identification). * **Bile Solubility Test:** Pneumococci are **bile soluble**, which differentiates them from *S. viridans* (bile insoluble). * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin, while other alpha-hemolytic streptococci are resistant. * **Hemolysis:** Shows **Alpha-hemolysis** (partial/greenish) on blood agar under aerobic conditions.

Question 814: Initiation of caries by Streptococcus mutans is by the production of which of the following?

A. Dextranase and soluble dextran
B. Insoluble dextran and glucosyl transferase (Correct Answer)
C. Soluble dextran and glycosyl transferase
D. None of the above

Explanation: **Explanation:** The initiation of dental caries by *Streptococcus mutans* is a classic example of bacterial biofilm formation. The process relies on the metabolism of dietary sucrose through a specific enzymatic pathway. **Why Option B is Correct:** *Streptococcus mutans* produces an extracellular enzyme called **Glucosyltransferase (GTF)**. This enzyme breaks down sucrose into glucose and fructose. It then polymerizes the glucose units into **Insoluble Dextrans (Glucans)**. * **Role of Insoluble Dextran:** Unlike soluble forms, insoluble dextrans act as a "biological glue." They allow *S. mutans* to adhere tenaciously to the tooth enamel (forming dental plaque) and provide a matrix that traps other bacteria and organic acids. These acids eventually demineralize the enamel, leading to caries. **Why Other Options are Incorrect:** * **Option A:** **Dextranase** is an enzyme that *breaks down* dextran. While some oral bacteria produce it to mobilize energy, it does not initiate caries; it would actually oppose the formation of the sticky plaque matrix. * **Option C:** **Soluble dextrans** are easily washed away by saliva and do not contribute significantly to the stable adherence required for plaque formation. The key to pathogenicity is the *insolubility* of the glucan polymer. **High-Yield Clinical Pearls for NEET-PG:** * **Substrate Specificity:** Sucrose is the only sugar that *S. mutans* can use to synthesize these sticky glucans. This is why sucrose is the most cariogenic sugar. * **Acidogenesis:** *S. mutans* is "acidogenic" (produces lactic acid) and "aciduric" (survives in low pH), both of which are critical for enamel erosion. * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci, which are also the most common cause of **Subacute Bacterial Endocarditis (SBE)** following dental procedures.

Question 815: About rickettsiae, all are true except -

A. These are transmitted by arthropod vectors
B. Eschar is not seen in RMSF
C. Weil-Felix reaction may be diagnostic
D. Cephalosporins are drugs of choice (Correct Answer)

Explanation: ### Explanation **Why Option D is the correct answer (The "Except" statement):** Rickettsiae are **obligate intracellular bacteria**. Cephalosporins (and other $\beta$-lactams) are ineffective because they target cell wall synthesis and have poor intracellular penetration. Furthermore, Rickettsiae are inherently resistant to these agents. The **drug of choice for all Rickettsial infections is Doxycycline**, regardless of the patient's age. Chloramphenicol is the alternative, especially in pregnant women (though Doxycycline is still preferred in many guidelines due to the severity of the disease). **Analysis of Incorrect Options:** * **Option A:** Most Rickettsial diseases are **zoonoses** transmitted by arthropod vectors like ticks (Rocky Mountain Spotted Fever), lice (Epidemic typhus), fleas (Endemic typhus), and mites (Scrub typhus). *Note: Q fever (Coxiella) is the exception as it is usually transmitted via aerosol.* * **Option B:** An **eschar** (a necrotic, blackened lesion at the site of the bite) is a characteristic feature of Scrub typhus and Rickettsialpox but is **notably absent in Rocky Mountain Spotted Fever (RMSF)**. * **Option C:** The **Weil-Felix reaction** is a heterophile agglutination test using *Proteus* antigens ($OX_{19}, OX_2, OX_K$) that cross-react with Rickettsial antibodies. While lacking specificity and being replaced by IFA (Immunofluorescence Assay), it is still used in resource-limited settings for presumptive diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Scrub Typhus:** Caused by *Orientia tsutsugamushi*; Vector: Trombiculid mite (chiggers); Weil-Felix: $OX_K$ positive. * **Epidemic Typhus:** Caused by *R. prowazekii*; Vector: Body louse; Brill-Zinsser disease is its recrudescent form. * **Triad of Rickettsial diseases:** Fever, headache, and rash (usually starting on wrists/ankles and spreading centripetally). * **Culture:** They do not grow on cell-free media; they require living cells (e.g., yolk sac of embryonated eggs).

Question 816: Which of the following is NOT true regarding Chlamydia psittacosis?

A. Acquired from bird's droppings
B. Causes urethritis (Correct Answer)
C. Causes pneumonia
D. Treatment is tetracycline

Explanation: The correct answer is **B. Causes urethritis.** ### **Explanation** *Chlamydia psittaci* is an obligate intracellular bacterium primarily associated with avian species. The reason it does **not** cause urethritis is that it is a respiratory pathogen, not a sexually transmitted one. Urethritis is characteristically caused by *Chlamydia trachomatis* (Serotypes D-K). **Why the other options are incorrect (True statements about C. psittaci):** * **Option A (Acquired from bird’s droppings):** This is the primary mode of transmission. Humans become infected by inhaling aerosolized dust from dried feces, urine, or feather dust of infected birds (parrots, pigeons, poultry). * **Option C (Causes pneumonia):** *C. psittaci* causes **Psittacosis** (Parrot Fever), which clinically manifests as atypical pneumonia. It often presents with high fever, dry cough, and a characteristic "Horder’s spots" (rose-colored macules). * **Option D (Treatment is tetracycline):** Tetracyclines (specifically **Doxycycline**) are the first-line treatment for all Chlamydial infections, including Psittacosis. Macrolides are used as alternatives. ### **High-Yield NEET-PG Pearls** 1. **Occupational Hazard:** Always look for a history of a pet shop owner, poultry worker, or bird fancier in the clinical stem. 2. **Diagnosis:** Serology (MIF - Microimmunofluorescence) is the gold standard. 3. **Differential for Atypical Pneumonia:** Along with *C. psittaci*, consider *Mycoplasma pneumoniae* and *Legionella pneumophila*. 4. **C. trachomatis vs. C. psittaci:** Remember that *C. trachomatis* affects the eyes and urogenital tract, while *C. psittaci* and *C. pneumoniae* affect the respiratory tract.

Question 817: Corynebacterium diphtheriae is typically arranged in which pattern?

A. Bamboo stick pattern
B. Cuneiform pattern (Correct Answer)
C. Fish in stream pattern
D. Rail road track pattern

Explanation: **Explanation:** **Corynebacterium diphtheriae** is a Gram-positive, pleomorphic bacillus. The correct answer is **Cuneiform pattern** (Option B) because of the unique way these bacteria divide. They undergo "snapping division," where the daughter cells remain attached at angles, resembling Chinese letters or the V/L shapes of cuneiform script. They also possess metachromatic (Volutin) granules, which give them a "club-shaped" appearance. **Analysis of Incorrect Options:** * **A. Bamboo stick pattern:** Characteristic of **Bacillus anthracis**. The long chains of bacilli with squared-off ends and central spores resemble the joints of a bamboo stick. * **C. Fish in stream pattern:** Characteristic of **Vibrio cholerae**. When viewed in a mucus flake from "rice water stools," the comma-shaped bacilli align in parallel, mimicking fish swimming in a stream. * **D. Rail road track pattern:** Typically associated with **Hemophilus ducreyi** (the causative agent of Chancroid). The small Gram-negative coccobacilli arrange themselves in parallel rows or chains, also described as a "school of fish" appearance. **High-Yield Clinical Pearls for NEET-PG:** * **Special Stains:** Use **Albert’s, Neisser’s, or Ponder’s stain** to visualize metachromatic granules (which appear bluish-black against a green cytoplasm). * **Culture Media:** **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (selective; colonies appear grey-black). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity. * **Clinical Sign:** Presence of a tough, leathery **pseudo-membrane** on the tonsils/pharynx that bleeds upon attempted removal.

Question 818: The catalase test is used to differentiate which of the following groups of bacteria?

A. Staphylococcus aureus from Staphylococcus epidermidis
B. Staphylococci from Streptococci (Correct Answer)
C. Streptococcus pyogenes from other streptococci
D. None of the above

Explanation: **Explanation:** The **Catalase Test** is a fundamental biochemical tool used in microbiology to differentiate Gram-positive cocci. **1. Why Option B is Correct:** The test detects the presence of the enzyme **catalase**, which neutralizes toxic hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen ($O_2$). When a colony is mixed with $H_2O_2$, the rapid evolution of oxygen bubbles indicates a positive result. * **Staphylococci** are **Catalase-positive** (produce bubbles). * **Streptococci** (and Enterococci) are **Catalase-negative** (no bubbles). This is the primary step in the laboratory algorithm for identifying Gram-positive cocci. **2. Why Other Options are Incorrect:** * **Option A:** Both *S. aureus* and *S. epidermidis* belong to the genus *Staphylococcus* and are therefore both catalase-positive. To differentiate them, the **Coagulase test** is used (*S. aureus* is positive; *S. epidermidis* is negative). * **Option C:** All members of the genus *Streptococcus* are catalase-negative. To differentiate *S. pyogenes* (Group A) from other streptococci, tests like **Bacitracin sensitivity** or **PYR test** are employed. **Clinical Pearls for NEET-PG:** * **Mechanism:** Catalase protects bacteria from the "oxidative burst" of phagocytes (neutrophils). * **Clinical Correlation:** Patients with **Chronic Granulomatous Disease (CGD)** have a deficiency in NADPH oxidase and are specifically susceptible to **Catalase-positive** organisms (e.g., *S. aureus*, *Aspergillus*, *Serratia*), as these bacteria neutralize the small amount of $H_2O_2$ the host produces. * **False Positive Caution:** Never perform a catalase test on **Blood Agar**, as red blood cells contain their own catalase, which can lead to a false-positive result. Use Nutrient Agar instead.

Question 819: Which of the following mycobacteria are non-pathogenic to humans?

A. M. bovis
B. M. kansii
C. M. avium intracellular
D. M. phlei (Correct Answer)

Explanation: **Explanation:** The genus *Mycobacterium* is broadly classified into the *M. tuberculosis* complex, *M. leprae*, and Non-Tuberculous Mycobacteria (NTM). NTM are further categorized by the **Runyon Classification** based on growth rate and pigment production. **M. phlei** (Option D) is a **saprophytic**, rapidly growing mycobacterium (Runyon Group IV). It is commonly found in soil and dust and is considered **non-pathogenic to humans**. It is frequently used in laboratory settings for biochemical testing (e.g., as a control in the urease test) because it does not cause disease. **Analysis of Incorrect Options:** * **M. bovis (Option A):** A member of the *M. tuberculosis* complex. It causes bovine tuberculosis but is highly pathogenic to humans, typically transmitted via unpasteurized milk, leading to extrapulmonary TB. * **M. kansasii (Option B):** A photochromogen (Runyon Group I). It is a significant human pathogen that causes a chronic pulmonary disease clinically indistinguishable from tuberculosis. * **M. avium-intracellulare (Option C):** Also known as MAC (Runyon Group III). It is a major opportunistic pathogen, especially in HIV/AIDS patients with low CD4 counts, causing disseminated infection. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group IV (Rapid Growers):** Includes *M. phlei* (saprophyte), *M. smegmatis* (commensal), and *M. fortuitum/chelonae* (potential pathogens causing skin/soft tissue abscesses). * **M. smegmatis:** Found in smegma; must be differentiated from *M. tuberculosis* in urine samples. * **Buruli Ulcer:** Caused by *M. ulcerans* (produces mycolactone toxin). * **Swimming Pool Granuloma:** Caused by *M. marinum*.

Question 820: ASO titre is useful in the diagnosis of which bacterium?

A. S. bovis
B. S. pyogenes (Correct Answer)
C. S. agalactiae
D. S. pneumonia

Explanation: **Explanation:** **S. pyogenes (Group A Streptococcus)** is the correct answer because it produces **Streptolysin O**, a potent oxygen-labile exotoxin that causes hemolysis. The body responds by producing **Anti-Streptolysin O (ASO) antibodies**. A rising or high ASO titre (typically >200 units) is a diagnostic marker indicating a recent infection. It is clinically indispensable for diagnosing non-suppurative post-streptococcal complications, specifically **Acute Rheumatic Fever** and **Acute Post-Streptococcal Glomerulonephritis (PSGN)**. **Analysis of Incorrect Options:** * **S. bovis (Group D):** Associated with endocarditis and colorectal cancer; it does not produce Streptolysin O. * **S. agalactiae (Group B):** A leading cause of neonatal sepsis and meningitis; diagnosis relies on culture and CAMP test, not ASO titres. * **S. pneumoniae:** While it produces a similar toxin called *Pneumolysin*, it does not produce Streptolysin O. Diagnosis is usually via sputum culture, Gram stain (lanceolate diplococci), or urinary antigen tests. **High-Yield Clinical Pearls for NEET-PG:** * **ASO vs. Anti-DNase B:** ASO titres are highly sensitive for Rheumatic Fever following pharyngitis but are often **low or absent in Streptococcal pyoderma (skin infections)**. For PSGN following a skin infection, **Anti-DNase B** is the more reliable marker. * **Todd Units:** ASO titres are expressed in Todd units. * **Mechanism:** Streptolysin O is antigenic and oxygen-labile, whereas **Streptolysin S** is non-antigenic and oxygen-stable (responsible for the surface hemolysis on blood agar).

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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