Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 61: 2x10^2 Streptococcus viridans are inoculated into a flask containing one liter of enriched broth. If the lag time is 30 minutes and the generation time is 20 minutes, how many bacteria will there be in the culture after two and a half hours?

A. 6 x 10^2
B. 1.2 x 10^3
C. 1.6 x 10^3
D. 1.28 x 10^4 (Correct Answer)

Explanation: ### Explanation To solve this problem, we must apply the principles of bacterial growth kinetics. Bacterial growth follows a predictable mathematical progression: **$N_t = N_0 \times 2^n$**, where $N_t$ is the final population, $N_0$ is the initial population, and $n$ is the number of generations. **Step-by-Step Calculation:** 1. **Total Time:** 2.5 hours = 150 minutes. 2. **Effective Growth Time:** Bacteria do not divide during the **Lag Phase**. Subtract the lag time from the total time: $150\text{ mins} - 30\text{ mins} = 120\text{ minutes}$ of active division (Log Phase). 3. **Number of Generations ($n$):** Divide the growth time by the generation time: $120 / 20 = 6\text{ generations}$. 4. **Final Count:** $N_t = (2 \times 10^2) \times 2^6$. * $2^6 = 64$. * $200 \times 64 = 12,800$, which is **$1.28 \times 10^4$**. --- ### Analysis of Options * **Option D (Correct):** Correctly accounts for the lag phase and 6 doubling cycles. * **Option A & B:** These represent significant underestimations, likely failing to apply the exponential growth formula ($2^n$). * **Option C:** This would be the result if the lag phase was ignored and the bacteria divided for the full 150 minutes ($7.5$ generations), which is biologically incorrect. --- ### NEET-PG Clinical Pearls * **Lag Phase:** Period of intense metabolic activity and enzyme synthesis, but **no increase in cell number**. * **Log (Exponential) Phase:** Period of rapid growth where generation time is determined. Bacteria are **most sensitive to antibiotics** (e.g., Beta-lactams) during this phase. * **Stationary Phase:** Growth rate equals death rate due to nutrient exhaustion and toxin accumulation. **Sporulation** typically occurs here. * **Viridans Streptococci:** Common commensals of the oral cavity; the most common cause of **Subacute Bacterial Endocarditis (SBE)** following dental procedures.

Question 62: Salmonella infection is most commonly caused by which route?

A. Infected water (Correct Answer)
B. Infected vegetables
C. Aerosol inhalation
D. Skin contact

Explanation: **Explanation:** **Salmonella** species (specifically *S. Typhi* and *S. Paratyphi*) are the causative agents of Enteric Fever. The primary mode of transmission for these organisms is the **fecal-oral route**. 1. **Why "Infected Water" is correct:** Contaminated water is the most common vehicle for large-scale outbreaks of Typhoid fever. In areas with poor sanitation, human feces containing the bacteria contaminate water sources. Since *Salmonella* has a relatively high infectious dose ($10^3$ to $10^6$ organisms), water serves as the most efficient medium for widespread ingestion and transmission. 2. **Why other options are incorrect:** * **Infected Vegetables:** While vegetables washed with contaminated water or grown in soil fertilized with human waste can transmit the disease, this is secondary to the primary source (water). * **Aerosol Inhalation:** Salmonella is an enteric pathogen; it does not spread via the respiratory route. * **Skin Contact:** The bacteria cannot penetrate intact skin and must be ingested to reach the intestinal mucosa to cause infection. **High-Yield Clinical Pearls for NEET-PG:** * **The Carrier State:** The **Gallbladder** is the most common site of chronic colonization (especially in patients with gallstones). * **Diagnosis:** Use the **BASU** mnemonic for timing of cultures: **B**lood (1st week), **A**ntibody/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Clinical Signs:** Look for "Rose spots" on the trunk and "Pea-soup diarrhea." * **Drug of Choice:** Ceftriaxone is currently the preferred treatment due to widespread multidrug resistance (MDR) and fluoroquinolone resistance.

Question 63: A neonate developed a side effect following BCG vaccination. Which of the following side effects is the neonate most likely to develop after vaccination?

A. Fever
B. Ulceration (Correct Answer)
C. Osteomyelitis
D. Regional lymphadenitis

Explanation: **Explanation:** The BCG (Bacillus Calmette-Guérin) vaccine is a live attenuated vaccine derived from *Mycobacterium bovis*. Following intradermal administration, a predictable sequence of local reactions occurs, which are considered normal responses rather than adverse events. **Why Ulceration is the Correct Answer:** After vaccination, a papule forms at 2–3 weeks, which increases in size and eventually undergoes **ulceration** by 4–8 weeks. This ulcer is the most common "side effect" or expected reaction. It typically heals spontaneously within 6–12 weeks, leaving behind the characteristic permanent circular scar. **Analysis of Incorrect Options:** * **A. Fever:** Systemic reactions like fever are extremely rare following BCG vaccination. It is primarily a localized reaction. * **C. Osteomyelitis:** This is a very rare, late complication (1 in 1,000,000 doses) occurring due to hematogenous spread, usually seen in immunocompromised infants. * **D. Regional lymphadenitis:** While common (suppurative or non-suppurative), it occurs less frequently than the standard ulceration. It is considered a complication if the lymph nodes exceed 1.5 cm or become fluctuant. **High-Yield Clinical Pearls for NEET-PG:** * **Strain used in India:** Danis 1331 strain. * **Dose:** 0.05 ml for neonates (below 1 month) and 0.1 ml for infants above 1 month. * **Site/Route:** Left upper arm (deltoid region), strictly **intradermal** (using a tuberculin/Omega syringe). * **Protective Effect:** Highly effective against TB meningitis and miliary TB in children, but has variable efficacy against adult pulmonary TB. * **Contraindication:** BCG is contraindicated in symptomatic HIV-infected infants or those with known immunodeficiency.

Question 64: What enzyme produced by Staphylococcus aureus dissolves fibrin clots?

A. Hyaluronidase
B. Catalase
C. Staphylokinase (Correct Answer)
D. Lipase

Explanation: **Explanation:** **Staphylokinase** (also known as Müller’s factor) is the correct answer. It is a fibrinolysin produced by *Staphylococcus aureus* that acts as a plasminogen activator. It converts plasminogen into plasmin, which then digests fibrin, effectively dissolving blood clots. This mechanism allows the bacteria to break free from localized clots and spread through tissues. **Analysis of Incorrect Options:** * **A. Hyaluronidase:** Also known as the "spreading factor," this enzyme breaks down hyaluronic acid in the connective tissue ground substance. While it aids bacterial spread, it does not act on fibrin. * **B. Catalase:** This enzyme breaks down hydrogen peroxide into water and oxygen. It is a diagnostic hallmark used to differentiate Staphylococci (Catalase positive) from Streptococci (Catalase negative). * **C. Lipase:** These enzymes break down lipids/fats on the skin surface, helping the bacteria colonize sebaceous glands and cause skin infections like boils and carbuncles. **High-Yield Clinical Pearls for NEET-PG:** * **Coagulase vs. Staphylokinase:** Coagulase *forms* a fibrin wall to wall off the infection (forming an abscess), while Staphylokinase *dissolves* it to facilitate spread. * **Therapeutic Use:** Like Streptokinase, Staphylokinase has been researched for its thrombolytic properties in treating myocardial infarction. * **Golden Staph:** The yellow pigment (Staphyloxanthin) of *S. aureus* acts as an antioxidant, protecting the bacteria from host reactive oxygen species. * **Protein A:** A key virulence factor that binds to the Fc portion of IgG, preventing opsonization and phagocytosis.

Question 65: Bejel is caused by which spirochete?

A. Treponema pallidum
B. Treponema endemicum (Correct Answer)
C. Treponema pertenue
D. Treponema carateum

Explanation: **Explanation:** The question tests your knowledge of the genus *Treponema*, which includes several morphologically identical but clinically distinct species. These are collectively known as the **Treponematoses**. **Correct Answer: B. Treponema endemicum** Bejel, also known as **Endemic Syphilis**, is caused by *Treponema pallidum* subspecies *endemicum*. It is a non-venereal disease primarily seen in children in arid climates (Middle East and North Africa). It is transmitted via contaminated drinking vessels or direct contact and typically presents with oral papules and mucosal patches, followed by gummatous lesions of the skin and bone. **Analysis of Incorrect Options:** * **A. Treponema pallidum:** Specifically *T. pallidum* subsp. *pallidum* causes **Venereal Syphilis**, a sexually transmitted infection characterized by primary chancres, secondary rashes, and tertiary systemic involvement. * **C. Treponema pertenue:** *T. pallidum* subsp. *pertenue* causes **Yaws**. It is found in humid, tropical regions and primarily affects the skin and bones (e.g., "Sabre shin"). The hallmark is the "mother yaw" (a large painless papule). * **D. Treponema carateum:** This species causes **Pinta**, found in Central and South America. It is the mildest form, strictly affecting the skin, leading to hyperpigmented or depigmented (leukodermic) lesions. **NEET-PG High-Yield Pearls:** 1. **Morphology:** All treponemes are thin, spiral-shaped organisms that cannot be cultured on artificial media and are too thin to be seen by light microscopy (require **Dark Ground Microscopy**). 2. **Serology:** All four diseases give **positive results** for both non-specific (VDRL/RPR) and specific (TPHA/FTA-ABS) treponemal tests. 3. **Treatment:** Long-acting **Benzathine Penicillin G** remains the drug of choice for all treponematoses.

Question 66: Verocytotoxin of E. coli acts by which mechanism?

A. Increasing cAMP
B. Decreasing cAMP
C. Decreasing protein synthesis (Correct Answer)
D. Decreasing cGMP

Explanation: **Explanation:** **Verocytotoxin** (also known as **Shiga-like toxin** or SLT), produced by Enterohemorrhagic *E. coli* (EHEC), specifically the O157:H7 strain, is the primary virulence factor responsible for hemorrhagic colitis and Hemolytic Uremic Syndrome (HUS). **Mechanism of Action (Why C is correct):** Verocytotoxin consists of an A subunit and five B subunits (A-5B toxin). The B subunits bind to the **Gb3 receptor** on host cells. Once internalized, the **A subunit** acts as an N-glycosidase that cleaves a specific adenine residue from the **28S rRNA of the 60S ribosomal subunit**. This irreversible damage prevents the binding of aminoacyl-tRNA to the ribosome, thereby **inhibiting protein synthesis**, leading to cell death. This mechanism is identical to the Shiga toxin produced by *Shigella dysenteriae* type 1. **Why other options are incorrect:** * **A & B (cAMP):** Toxins that increase cAMP include Heat-labile toxin (LT) of ETEC, *Vibrio cholerae* (Cholera toxin), and *Bordetella pertussis*. These cause secretory diarrhea by altering ion transport, not by inhibiting protein synthesis. * **D (cGMP):** Heat-stable toxin (ST) of ETEC acts by stimulating guanylate cyclase, leading to increased cGMP levels. **High-Yield Clinical Pearls for NEET-PG:** * **Target Organ:** Verocytotoxin has a high affinity for renal glomerular endothelial cells (rich in Gb3 receptors), explaining the pathogenesis of **HUS** (Triad: Microangiopathic hemolytic anemia, Thrombocytopenia, and Acute Renal Failure). * **Diagnosis:** EHEC O157:H7 is identified by its inability to ferment sorbitol on **Sorbitol MacConkey (SMAC) agar**. * **Treatment Note:** Antibiotics are generally avoided in EHEC infections as they may trigger increased toxin release, worsening the risk of HUS.

Question 67: Donovan bodies are seen in which of the following?

A. Leishmania donovani
B. Chlamydia trachomatis
C. Klebsiella
D. Calymmatobacterium granulomatis (Correct Answer)

Explanation: **Explanation:** **Donovan bodies** are the hallmark histopathological finding in **Granuloma Inguinale (Donovanosis)**, a chronic, progressively destructive genital ulcer disease. They represent intracellular, safety-pin-shaped organisms found within the vacuoles of large mononuclear cells (macrophages). 1. **Why Option D is correct:** The causative agent of Granuloma Inguinale is *Klebsiella granulomatis* (formerly known as ***Calymmatobacterium granulomatis***). When stained with Wright or Giemsa stain, these Gram-negative bacteria appear as bipolar-staining bodies (Donovan bodies) inside macrophages. 2. **Why Option A is incorrect:** *Leishmania donovani* causes Visceral Leishmaniasis (Kala-azar). While the name is similar, it is characterized by **LD bodies** (Leishman-Donovan bodies), which are amastigote forms of the parasite found in the reticuloendothelial system, not Donovan bodies. 3. **Why Option B is incorrect:** *Chlamydia trachomatis* (Serotypes L1-L3) causes Lymphogranuloma Venereum (LGV). Histologically, it is associated with "Stellate abscesses" in lymph nodes, not Donovan bodies. 4. **Why Option C is incorrect:** While the organism is now classified under the genus *Klebsiella*, the option "Klebsiella" usually refers to *K. pneumoniae*. In the context of NEET-PG, the specific historical name *Calymmatobacterium* or the full species *K. granulomatis* is preferred to distinguish it from common respiratory/urinary pathogens. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Beefy red, painless, velvety ulcers that bleed easily on touch (friable). * **Pseudobubo:** Inguinal swelling occurs due to granulation tissue (not true lymphadenopathy). * **Stain of choice:** Giemsa or Wright stain (shows "safety-pin" appearance). * **Treatment of choice:** Azithromycin (as per CDC/WHO guidelines).

Question 68: Actinomycetoma is caused by which type of microorganism?

A. Bacteria (Correct Answer)
B. Virus
C. Fungus
D. Protozoa

Explanation: **Explanation:** The term **Mycetoma** refers to a chronic, granulomatous infection of the subcutaneous tissue, characterized by a triad of localized swelling, multiple discharging sinuses, and the presence of grains in the exudate. It is classified into two types based on the causative agent: 1. **Actinomycetoma (Correct Answer):** Caused by **filamentous, Gram-positive bacteria** belonging to the order Actinomycetales. Common genera include *Nocardia*, *Actinomadura*, and *Streptomyces*. Despite the "myco" suffix (which implies fungus), these organisms are prokaryotic bacteria. 2. **Eumycetoma:** Caused by true **fungi** (e.g., *Madurella mycetomatis*). **Why other options are incorrect:** * **B & D (Virus/Protozoa):** These organisms do not form the characteristic sulfur granules or chronic subcutaneous tracks seen in mycetoma. * **C (Fungus):** While fungi cause "Eumycetoma," the question specifically asks for "Actinomycetoma," which is strictly bacterial. **NEET-PG High-Yield Pearls:** * **The "Sulfur Granule" Myth:** The grains found in actinomycetoma are often called "sulfur granules" (especially in *Actinomyces israelii*), but they contain no actual sulfur; they are masses of bacterial filaments. * **Diagnosis:** Actinomycetoma is diagnosed via Gram stain (showing Gram-positive branching filaments) and Modified Ziehl-Neelsen stain (for *Nocardia*, which is weakly acid-fast). * **Treatment:** Unlike Eumycetoma (which requires surgery/antifungals), Actinomycetoma is treated with **antibiotics**, most commonly the **Welsh Regime** (Amikacin and Co-trimoxazole). * **Clinical Sign:** The "Dot-in-a-circle" sign on MRI is a characteristic feature of mycetoma.

Question 69: Microbiologic studies on a gram-positive coccus isolated from a skin abscess demonstrate a positive catalase test and a positive coagulase test. This is most consistent with which of the following organisms?

A. S. aureus (Correct Answer)
B. S. epidermidis
C. S. pneumoniae
D. S. pyogenes

Explanation: ### Explanation The organism described is **Staphylococcus aureus**. The identification of Gram-positive cocci (GPC) follows a specific diagnostic algorithm crucial for NEET-PG: 1. **Catalase Test:** This test differentiates *Staphylococci* (Catalase positive) from *Streptococci* (Catalase negative). Since the isolate is catalase-positive, it belongs to the genus *Staphylococcus*. 2. **Coagulase Test:** This is the definitive test to differentiate *S. aureus* from Coagulase-Negative Staphylococci (CoNS). *S. aureus* produces the enzyme coagulase, which converts fibrinogen to fibrin, resulting in clot formation. **Analysis of Options:** * **S. aureus (Correct):** It is the only major human pathogen that is both catalase-positive and coagulase-positive. It is a leading cause of pyogenic infections like skin abscesses. * **S. epidermidis:** While it is a catalase-positive GPC, it is **coagulase-negative** (CoNS). It typically causes infections associated with prosthetic devices and catheters. * **S. pneumoniae:** This is a catalase-negative organism. Morphologically, it appears as Gram-positive lancet-shaped diplococci and is alpha-hemolytic. * **S. pyogenes (Group A Strep):** This is a catalase-negative organism. It is a major cause of skin infections (impetigo, erysipelas) but would not show positivity in the biochemical tests mentioned. **High-Yield Clinical Pearls for NEET-PG:** * **Golden Yellow Pigment:** *S. aureus* colonies often show a characteristic golden yellow color on nutrient agar due to carotenoids (staphyloxanthin). * **Mannitol Salt Agar (MSA):** *S. aureus* ferments mannitol, turning the pink medium yellow. * **Protein A:** A key virulence factor of *S. aureus* that binds to the Fc portion of IgG, inhibiting phagocytosis. * **Commonest cause** of osteomyelitis and septic arthritis in the general population.

Question 70: Ehrlich phenomenon is seen in:

A. Myobacterium Tuberculosis
B. Proteus
C. Staphylococcus
D. Corynebacterium (Correct Answer)

Explanation: **Explanation:** The **Ehrlich phenomenon** is a classic concept in immunology and toxicology related to the **Corynebacterium diphtheriae** toxin. It describes the observation that the amount of antitoxin required to neutralize a specific dose of diphtheria toxin (the $L+$ dose) is significantly greater than the amount required to neutralize a single minimal lethal dose (MLD) multiplied by that same factor. This occurs because diphtheria toxin broth contains not only the active toxin but also **toxoids** (nontoxic but antigenic) and **toxones** (weakly toxic but antigenic). These degraded forms still bind to the antitoxin. Therefore, when neutralizing a toxin-toxoid mixture, the antitoxin is "diverted" by these inactive components, requiring a higher dose for complete neutralization of the lethal effect. **Analysis of Options:** * **Corynebacterium (Correct):** The phenomenon was specifically described by Paul Ehrlich while standardizing diphtheria antitoxin. * **Mycobacterium tuberculosis:** Known for the **Koch phenomenon** (a hypersensitivity reaction to tubercle bacilli), not the Ehrlich phenomenon. * **Proteus:** Associated with the **Weil-Felix reaction** (cross-reactivity with Rickettsia) and "swarming" motility. * **Staphylococcus:** Known for various toxins (TSST-1, Exfoliatin), but its neutralization does not follow the Ehrlich phenomenon model. **High-Yield Clinical Pearls for NEET-PG:** * **L+ dose (Limes death):** The smallest amount of toxin which, when mixed with 1 unit of antitoxin and injected into a guinea pig, causes death in 4 days. * **Lo dose (Limes zero):** The largest amount of toxin that is completely neutralized by 1 unit of antitoxin. * **Schick Test:** An in-vivo test used to determine immunity against Diphtheria. * **Elek’s Gel Precipitation Test:** The standard in-vitro test for demonstrating the toxigenicity of *C. diphtheriae*.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

Practice Questions

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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