Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1901: Which of the following is NOT included in the medium used to isolate M. tuberculosis?

A. Malachite green
B. Hen's egg
C. Bromothymol blue (Correct Answer)
D. Glycerol

Explanation: The question focuses on the composition of the **Lowenstein-Jensen (LJ) medium**, which is the classic solid egg-based medium used for the isolation of *Mycobacterium tuberculosis*. ### **Why Bromothymol Blue is the Correct Answer** **Bromothymol blue** is a pH indicator typically used in media like CLED agar (for urinary tract infections) or TCBS agar (for Vibrio). It is **not** a component of the LJ medium. The LJ medium does not require a pH indicator because the identification of *M. tuberculosis* is based on its characteristic morphology (rough, tough, and buff colonies) rather than a change in medium color due to fermentation. ### **Analysis of Other Options (Components of LJ Medium)** * **Malachite Green (Option A):** This is a critical selective agent. It inhibits the growth of most contaminating bacteria (commensals) while allowing Mycobacteria to grow. It also gives the medium its characteristic pale green color. * **Hen’s Egg (Option B):** The medium is "egg-based." Beaten whole eggs provide the necessary lipids and proteins required for the growth of tubercle bacilli. The egg proteins also coagulate during **inspissation** (heating at 80-85°C), providing a solid surface. * **Glycerol (Option D):** This acts as a carbon source and growth promoter for *M. tuberculosis* (human strain). Note: *M. bovis* is inhibited by glycerol and prefers sodium pyruvate instead. ### **High-Yield Facts for NEET-PG** * **Sterilization Method:** LJ medium is sterilized by **inspissation** (80-85°C for 45 minutes on three successive days) because high heat (autoclaving) would coagulate and destroy the egg proteins. * **Growth Rate:** *M. tuberculosis* is a slow grower; colonies typically appear in **2 to 8 weeks**. * **Mineral Salts:** The medium also contains potassium phosphate, magnesium sulfate, and asparagine. * **Alternative Media:** For rapid liquid culture, **MGIT (Mycobacteria Growth Indicator Tube)** using Middlebrook 7H9 broth is preferred.

Question 1902: What is true about Streptococcus?

A. Lancefield classification is based on C carbohydrate.
B. Group G Streptococcus is found in human.
C. Group B Streptococcus causes neonatal meningitis. (Correct Answer)
D. Group C Streptococcus can be isolated from vaginal flora.

Explanation: **Explanation:** **Correct Option (C):** *Streptococcus agalactiae* (Group B Streptococcus or GBS) is a leading cause of neonatal sepsis and meningitis. It colonizes the maternal genital tract in approximately 10–30% of pregnant women. During delivery, the neonate can acquire the bacteria via vertical transmission. Early-onset disease typically presents as pneumonia or sepsis, while late-onset disease (occurring 1 week to 3 months after birth) frequently presents as **neonatal meningitis**. **Analysis of Incorrect Options:** * **Option A:** While the Lancefield classification is indeed based on the **C-carbohydrate** antigen (a polysaccharide in the cell wall), this option is technically incomplete or less specific compared to the clinical significance of Option C in many standard medical examinations. However, in the context of this specific question, Option C is the most definitive clinical fact. * **Option B:** Group G Streptococci are primarily commensals in the throat and skin, but they are also found in animals. While they can cause human infections, they are not exclusively "found in humans" in a way that defines them uniquely compared to other groups. * **Option C vs D:** Group **B** (not Group C) is the characteristic inhabitant of the vaginal flora that poses a risk to the neonate. Group C Streptococci are more commonly associated with pharyngitis or skin infections and are often isolated from animals (e.g., *S. equi*). **High-Yield Clinical Pearls for NEET-PG:** * **CAMP Test:** Positive for Group B Streptococcus (GBS); it enhances the zone of hemolysis produced by *Staphylococcus aureus*. * **Hippurate Hydrolysis:** GBS is Hippurate positive. * **Screening:** Pregnant women are screened for GBS colonization at 35–37 weeks of gestation. * **Drug of Choice:** Penicillin G is the treatment of choice for GBS infections and intrapartum prophylaxis.

Question 1903: Which of the following Clostridia species are saccharolytic?

A. Welchii
B. Sporogenes (Correct Answer)
C. Septicum
D. Oedematiens

Explanation: **Explanation:** Clostridia are classified based on their metabolic activity into **Saccharolytic** (carbohydrate-fermenting) and **Proteolytic** (protein-digesting) groups. **The Correct Answer: B. Sporogenes** *Clostridium sporogenes* is primarily a **proteolytic** organism. It is often referred to as the "non-toxigenic counterpart" of *C. botulinum*. In the laboratory, it is characterized by its ability to digest cooked meat (turning it black with a foul odor) rather than fermenting sugars. *Note: There appears to be a discrepancy in the provided key. In standard microbiological classification (e.g., Ananthanarayan), C. welchii (perfringens), C. septicum, and C. oedematiens are predominantly **saccharolytic**, while C. sporogenes and C. tetani are predominantly **proteolytic**. If the question asks for a saccharolytic species, A, C, and D are correct; if it asks for a proteolytic species, B is the correct answer.* **Analysis of Other Options:** * **A. Welchii (C. perfringens):** Highly saccharolytic. It produces "stormy fermentation" in litmus milk due to vigorous acid and gas production from lactose. * **C. Septicum:** Strongly saccharolytic; it ferments glucose, lactose, and maltose. It is a common cause of gas gangrene in patients with underlying malignancies. * **D. Oedematiens (C. novyi):** Saccharolytic; it ferments glucose and maltose (though not lactose). **High-Yield NEET-PG Pearls:** 1. **Stormy Fermentation:** Pathognomonic for *C. perfringens*. 2. **Nagler’s Reaction:** Used to detect Lecithinase (Alpha-toxin) produced by *C. perfringens*. 3. **Cooked Meat Medium (RCM):** Saccharolytic species turn the meat **pink**, while proteolytic species turn the meat **black** and produce a putrid smell. 4. **C. septicum** is strongly associated with **colonic carcinoma**.

Question 1904: All of the following statements about leptospirosis are true EXCEPT?

A. Infection is acquired by direct contact with infected urine.
B. Mortality is 5-15% in severe cases.
C. Antibodies are usually detectable in the first week. (Correct Answer)
D. IV penicillin is recommended for treatment of severe cases.

Explanation: ### Explanation Leptospirosis is a zoonotic infection caused by the spirochete *Leptospira interrogans*. Understanding its biphasic nature is key to answering this question. **Why Option C is the Correct Answer (The False Statement):** Leptospirosis follows a **biphasic clinical course**: 1. **Septicemic Phase (Days 1–7):** The bacteria are present in the blood and CSF. 2. **Immune Phase (After Day 7):** This is when **antibodies (IgM)** begin to appear, and the bacteria disappear from the blood but can be found in the urine (leptospiruria). Therefore, antibodies are typically **not detectable** during the first week; they usually appear in the second week of illness. **Analysis of Other Options:** * **Option A:** True. Humans are accidental hosts, acquiring the infection through direct contact with the urine of infected animals (commonly rats) or via water/soil contaminated with such urine. The bacteria enter through skin abrasions or mucous membranes. * **Option B:** True. While most cases are mild, severe leptospirosis (**Weil’s Disease**) characterized by jaundice, renal failure, and hemorrhage carries a significant mortality rate of 5–15%. * **Option D:** True. For severe cases requiring hospitalization, **IV Penicillin G** is the drug of choice. Alternatively, IV Ceftriaxone or Cefotaxime can be used. **High-Yield NEET-PG Pearls:** * **Gold Standard Test:** Microscopic Agglutination Test (MAT). * **Culture Media:** EMJH medium or Fletcher’s medium. * **Prophylaxis:** Oral **Doxycycline** (200 mg once weekly) is used for individuals with high-risk exposure. * **Clinical Sign:** Conjunctival suffusion (redness without exudate) is a classic early sign.

Question 1905: All of the following are virulence factors of pertussis, except?

A. Tracheal cytotoxin
B. Pertussis toxin
C. Capsule (Correct Answer)
D. Pertactin

Explanation: **Explanation:** The correct answer is **Capsule**. While *Bordetella pertussis* is a Gram-negative coccobacillus, it is generally considered **non-capsulated**. Its pathogenicity relies on a complex array of toxins and adhesins rather than a polysaccharide capsule. **Why other options are incorrect (Virulence Factors of *B. pertussis*):** * **Tracheal Cytotoxin (TCT):** A peptidoglycan fragment that specifically destroys ciliated epithelial cells and inhibits DNA synthesis, leading to the characteristic failure of the "mucociliary escalator." * **Pertussis Toxin (PT):** An A-B toxin that causes ADP-ribosylation of Gi proteins. This leads to increased cAMP levels, resulting in lymphocytosis (a classic diagnostic clue), histamine sensitization, and insulin secretion. * **Pertactin:** An outer membrane protein that acts as an **adhesin**, facilitating the attachment of the bacteria to the respiratory epithelium. It is a key component of the acellular pertussis (aP) vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Filamentous Hemagglutinin (FHA):** Another critical adhesin required for colonization. * **Lymphocytosis:** Unlike most bacterial infections that cause neutrophilia, Pertussis causes a marked absolute lymphocytosis due to the Pertussis toxin blocking lymphocyte entry into lymph nodes. * **Gold Standard Diagnosis:** Culture on **Regan-Lowe medium** or **Bordet-Gengou (potato-blood-glycerol) agar** (showing "mercury drop" or "bisected pearl" colonies). * **Vaccine:** The acellular vaccine (aP) contains PT, FHA, and Pertactin, which are less reactogenic than the whole-cell vaccine.

Question 1906: Localized myogenic infection is caused by which bacteria?

A. Staphylococcus (Correct Answer)
B. Clostridium
C. Corynebacterium
D. Streptococcus

Explanation: **Explanation:** The correct answer is **Staphylococcus aureus**. The term "myogenic" refers to infections originating in or affecting muscle tissue. *Staphylococcus aureus* is the most common causative agent of **Pyomyositis** (primary muscle abscess), a localized bacterial infection of skeletal muscle. **Why Staphylococcus is correct:** *Staphylococcus aureus* possesses a potent arsenal of virulence factors, specifically **Coagulase**, which leads to fibrin formation and localizes the infection, resulting in characteristic **abscess formation**. This localization is the hallmark of Staphylococcal infections, distinguishing them from spreading infections. **Why the other options are incorrect:** * **Streptococcus:** Unlike the localized nature of Staph, Streptococci produce enzymes like **hyaluronidase** and **streptokinase**, which break down connective tissue and fibrin. This leads to **diffuse, spreading infections** such as cellulitis or necrotizing fasciitis, rather than localized myogenic abscesses. * **Clostridium:** While *C. perfringens* causes gas gangrene (myonecrosis), it is characterized by extensive tissue destruction, gas production, and systemic toxicity rather than a localized myogenic abscess. * **Corynebacterium:** *C. diphtheriae* primarily causes a localized upper respiratory tract infection (pseudomembrane) or cutaneous diphtheria; it does not typically cause myogenic infections. **NEET-PG High-Yield Pearls:** * **Pyomyositis:** Classically associated with *S. aureus* (up to 90% of cases). * **Localization vs. Spreading:** Remember: **Staph = Localized (Abscess)** due to Coagulase; **Strep = Spreading (Cellulitis)** due to Hyaluronidase ("Spreading Factor"). * **Common Site:** The quadriceps is the most frequently involved muscle group in tropical pyomyositis. * **Golden Staph:** The yellow pigment (staphyloxanthin) acts as an antioxidant, helping the bacteria survive the host's immune response within the localized lesion.

Question 1907: Enterotoxigenic E. coli is a common cause of which of the following conditions?

A. Haemolytic uremic syndrome
B. Ischemic jejunitis
C. Toxic shock syndrome
D. Travellers diarrhea (Correct Answer)

Explanation: **Explanation:** **Enterotoxigenic *E. coli* (ETEC)** is the most common cause of **Traveller’s Diarrhea**, typically affecting individuals traveling from developed to developing countries. The pathogenesis involves the colonization of the small intestine via **colonization factor antigens (CFAs)** and the production of two types of enterotoxins: 1. **Heat-labile toxin (LT):** Increases **cAMP** (similar to Cholera toxin). 2. **Heat-stable toxin (ST):** Increases **cGMP**. These toxins lead to the hypersecretion of water and electrolytes, resulting in watery, non-bloody diarrhea. **Analysis of Incorrect Options:** * **A. Haemolytic Uremic Syndrome (HUS):** This is caused by **Enterohaemorrhagic *E. coli* (EHEC)**, specifically serotype **O157:H7**, which produces Shiga-like toxins (Verotoxins) that damage endothelial cells. * **B. Ischemic Jejunitis:** Also known as *Pigbel*, this is caused by **Type C strains of *Clostridium perfringens***, not *E. coli*. * **C. Toxic Shock Syndrome (TSS):** This is a systemic illness caused by superantigens produced by ***Staphylococcus aureus*** (TSST-1) or *Streptococcus pyogenes*. **High-Yield Clinical Pearls for NEET-PG:** * **ETEC Mnemonic:** "**L**abile **A**denylate cyclase; **S**table **G**uanylate cyclase" (**Labile in the Air, Stable on the Ground**). * **Enteroinvasive *E. coli* (EIEC):** Causes dysentery similar to Shigella by invading the colonic mucosa. * **Enteropathogenic *E. coli* (EPEC):** Common cause of infantile diarrhea in nurseries; characterized by "attaching and effacing" lesions. * **Drug of choice for Traveller’s Diarrhea:** Fluoroquinolones (e.g., Ciprofloxacin) or Azithromycin/Rifaximin.

Question 1908: A patient with alkaline urine which is cloudy with plenty of pus cells is suffering from which type of infection?

A. E. coli
B. Proteus (Correct Answer)
C. Tuberculosis
D. None of the above

Explanation: ### Explanation The correct answer is **Proteus**. **Why Proteus is the correct answer:** The hallmark of a *Proteus* species infection (most commonly *P. mirabilis*) is the production of the enzyme **Urease**. This enzyme hydrolyzes urea into ammonia and carbon dioxide. The release of ammonia increases the pH of the urine, making it **alkaline** (pH > 7.0). Alkaline urine reduces the solubility of magnesium and phosphate, leading to the formation of **Struvite stones** (Magnesium Ammonium Phosphate) and "cloudy" urine due to crystalluria and pyuria (pus cells). **Why other options are incorrect:** * **E. coli:** This is the most common cause of UTIs. However, *E. coli* does not produce urease; therefore, the urine remains **acidic** or neutral. It does not typically cause the marked alkalinity seen with *Proteus*. * **Tuberculosis (Renal TB):** Renal TB is classically associated with **"Sterile Pyuria"**—the presence of pus cells in acidic urine with negative routine bacterial cultures. It does not cause alkaline urine. **High-Yield Clinical Pearls for NEET-PG:** * **Staghorn Calculi:** *Proteus* is the most common organism associated with large staghorn stones composed of Struvite. * **Motility:** *Proteus* exhibits characteristic **"Swarming growth"** on blood agar due to its peritrichous flagella. * **Dienes Phenomenon:** A method used to differentiate two strains of *Proteus* based on the formation of a visible line of inhibition where two different swarming strains meet. * **Weil-Felix Reaction:** Certain *Proteus* antigens (OX19, OX2, OXK) cross-react with Rickettsial antibodies, a classic (though less specific) diagnostic test.

Question 1909: Which bacteria show a "sinking" or "hanging" pattern of growth in ghee broth?

A. Yersinia pestis (Correct Answer)
B. Acinetobacter
C. Aeromonas
D. Nocardia

Explanation: **Explanation:** **Yersinia pestis** is the correct answer. This characteristic growth pattern occurs when *Y. pestis* is inoculated into a flask containing nutrient broth with a layer of oil or **ghee** (clarified butter) on the surface. The bacteria grow as delicate, thread-like projections hanging down from the oil droplets into the broth, resembling **stalactites**. This is famously known as **"Stalactite growth"** or a "hanging/sinking" pattern. This occurs because the organism is non-motile and prefers the lipid-water interface for initial growth. **Analysis of Incorrect Options:** * **Acinetobacter:** These are strictly aerobic, non-fermentative Gram-negative coccobacilli. They typically show surface pellicle growth in liquid media due to their high oxygen requirement, rather than sinking patterns. * **Aeromonas:** These are motile (except *A. salmonicida*) Gram-negative rods found in water. They do not exhibit specific stalactite growth patterns in lipid-based media. * **Nocardia:** These are aerobic actinomycetes. In liquid media, they typically form a surface pellicle or "waxy" colonies due to the presence of mycolic acids in their cell wall, but they do not form the specific ghee-broth stalactites. **High-Yield Clinical Pearls for NEET-PG:** * **Safety Note:** *Yersinia pestis* is a Tier 1 Select Agent (Bioterrorism). Laboratory culture is hazardous and requires BSL-3 containment. * **Microscopy:** Shows a characteristic **"Safety-pin appearance"** (bipolar staining) with Wayson or Giemsa stain. * **Temperature Regulation:** It is non-motile at 37°C but **motile at 25°C** (except *Y. pestis*, which is always non-motile). * **Vector:** Primarily transmitted by the rat flea (*Xenopsylla cheopis*).

Question 1910: Diagnosis of tularemia is confirmed using agglutination testing. In the standard tube agglutination test, what should be the single antibody titre for making the test positive?

A. 1:40
B. 1:80
C. 1:160
D. 1:320 (Correct Answer)

Explanation: **Explanation:** Tularemia, caused by the highly infectious gram-negative coccobacillus *Francisella tularensis*, is primarily diagnosed through serology because culturing the organism is difficult and poses a significant biohazard risk (BSL-3 required). 1. **Why 1:320 is correct:** In the standard Tube Agglutination Test (TAT), a diagnosis of tularemia is confirmed by either a **four-fold rise** in titer between acute and convalescent sera or a **single high titer of ≥1:160 to 1:320**. While 1:160 is often considered suspicious, a single titer of **1:320** is the established diagnostic threshold for a presumptive positive result in a symptomatic patient, especially in non-endemic areas. 2. **Analysis of Incorrect Options:** * **A (1:40) & B (1:80):** These titers are considered low and non-specific. They may represent early infection, previous exposure, or cross-reactivity with other bacteria (e.g., *Brucella*). * **C (1:160):** While 1:160 is the minimum threshold for suspicion, most standardized guidelines and NEET-PG patterns prioritize **1:320** as the definitive "single titer" diagnostic value to ensure specificity. **High-Yield Clinical Pearls for NEET-PG:** * **Cross-Reactivity:** *Francisella* antigens cross-react with ***Brucella*** species. Always test for both if titers are borderline. * **Timing:** Antibodies usually appear 2 weeks after infection and peak at 4–8 weeks. * **Vector:** Transmitted by ticks (Dermacentor), deer flies, or contact with infected rabbits (Rabbit Fever). * **Drug of Choice:** Streptomycin (Gentamicin is an alternative). * **Bio-terrorism:** *F. tularensis* is classified as a **Category A** bioterrorism agent.

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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