Bacteriology Practice Questions

Q: What enzyme produced by Staphylococcus aureus dissolves fibrin clots?

Staphylokinase. **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.

Q: Verocytotoxin of E. coli acts by which mechanism?

Decreasing protein synthesis. **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.

Q: Actinomycetoma is caused by which type of microorganism?

Bacteria. **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.

Q: 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?

S. aureus. ### 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.

Q: Ehrlich phenomenon is seen in:

Corynebacterium. **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*.

Q: A 30-year-old woman with a history of poor obstetric outcomes presents with fever. Blood culture reveals gram-positive, small to medium, pleomorphic coccobacilli occurring in short chains. Direct microscopy of the culture shows tumbling motility. What is the most likely organism?

Listeria monocytogenes. ### Explanation **Correct Option: A. Listeria monocytogenes** The clinical presentation and laboratory findings are classic for *Listeria monocytogenes*. * **Morphology:** It is a Gram-positive, non-spore-forming, pleomorphic coccobacillus often appearing in short chains or pairs (resembling *S. pneumoniae*). * **Motility:** The pathognomonic feature is **"tumbling motility"** observed in hanging drop preparations at 20–25°C (due to peritrichous flagella). At 37°C, motility is absent or sluggish. * **Clinical Context:** In adults, it typically affects pregnant women and the immunocompromised. It is a significant cause of **recurrent abortions** and stillbirths (as hinted by "poor obstetric outcomes") and can cause neonatal meningitis. **Why other options are incorrect:** * **B. Corynebacterium sp.:** While Gram-positive and pleomorphic, they are non-motile and typically show a "Chinese-letter" or cuneiform arrangement. * **C. Enterococcus sp.:** These are Gram-positive cocci in pairs/chains. While some species show motility, they do not exhibit the characteristic "tumbling" motion and are not typically associated with recurrent obstetric loss in this manner. * **D. Erysipelothrix rhusiopathiae:** This is a Gram-positive rod associated with skin infections (Erysipeloid) in handlers of animals/fish. It is **non-motile** and produces H₂S on TSI agar. **High-Yield NEET-PG Pearls for Listeria:** 1. **Cold Enrichment:** It can grow at 4°C, a property used for selective isolation from contaminated samples. 2. **Culture:** Shows **"Umbrella-shaped"** growth in semi-solid motility medium. 3. **Hemolysis:** Produces a narrow zone of **beta-hemolysis** on blood agar (CAMP test positive). 4. **Intracellular Pathogen:** It moves cell-to-cell via "actin rockets" (actin polymerization). 5. **Treatment:** Drug of choice is **Ampicillin**. It is inherently resistant to all Cephalosporins.

Q: Which of the following is NOT associated with endocarditis?

ESKAPE group. **Explanation:** In Infective Endocarditis (IE), the causative agents are typically organisms capable of adhering to damaged endocardium or prosthetic valves. The **ESKAPE group** is the correct answer because it is a classification based on **antimicrobial resistance patterns** rather than a specific association with endocarditis. * **Why ESKAPE is the correct answer:** The ESKAPE acronym (*Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa,* and *Enterobacter* species) represents a group of pathogens that "escape" the effects of antibiotics. While individual members (like *S. aureus*) cause IE, the group as a whole is defined by its clinical challenge in nosocomial infections and multi-drug resistance, not as a standard cluster for endocarditis. * **Why other options are incorrect:** * **Staphylococcus aureus:** The most common cause of acute IE globally, especially in IV drug users and those with prosthetic valves. * **Enterococcus:** A common cause of subacute IE, often following urinary tract or GI manipulations. * **HACEK group:** (*Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella*) This group consists of fastidious Gram-negative bacilli known for causing **culture-negative endocarditis**. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of IE (Overall):** *Staphylococcus aureus*. * **Most common cause of Subacute IE:** *Viridans group Streptococci*. * **Most common cause in IV drug users:** *S. aureus* (affects the Tricuspid valve). * **Culture-negative IE:** Most commonly due to prior antibiotic use, but classically associated with HACEK, *Coxiella burnetii* (Q fever), and *Bartonella*. * **Streptococcus bovis (S. gallolyticus):** If found in IE, always screen the patient for **Colorectal Carcinoma**.

Q: A 32-year-old man presented with a painless ulcer with a black eschar on his leg. Smears of exudate show non-motile organisms which are in chains. Spores are absent in the smear. What is the most probable organism?

Bacillus anthracis. ### Explanation The clinical presentation of a **painless ulcer with a black eschar** is a classic description of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. **Why Bacillus anthracis is correct:** * **Morphology:** *B. anthracis* is a Gram-positive, **non-motile** rod that typically appears in long chains (often described as "bamboo-stick" appearance). * **The "No Spore" Rule:** While *B. anthracis* is a spore-former, it **never forms spores in living tissue** or clinical samples (exudate). Sporulation only occurs under aerobic conditions outside the host (e.g., on culture media or in the environment). * **Clinical Sign:** The "Malignant Pustule" or eschar is characteristic, resulting from the action of the Lethal Toxin (LF) and Edema Toxin (EF). **Why other options are incorrect:** * **Proteus:** These are Gram-negative bacilli known for being **highly motile** (swarming growth). They do not cause eschars. * **Diphtheroids:** These are commensal Gram-positive bacilli that are pleomorphic (club-shaped) and usually arranged in "Chinese letter" patterns, not long chains. * **Clostridium:** While these are Gram-positive rods, they are typically **motile** (except *C. perfringens*) and are **obligate anaerobes**. They cause gas gangrene or tetanus, not a dry black eschar. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to visualize the capsule of *B. anthracis* using polychrome methylene blue. * **Medusa Head Appearance:** Characteristic morphology of colonies on agar. * **String of Pearls Reaction:** Seen when grown on agar containing penicillin. * **Selective Media:** PLET Medium (Polymyxin, Lysozyme, EDTA, Thallous acetate).

Q: Pus discharge (cervicitis/urethritis) in a female is most commonly caused by which of the following?

Chlamydia. **Explanation:** **1. Why Chlamydia is Correct:** *Chlamydia trachomatis* (Serotypes D-K) is the most common cause of **mucopurulent cervicitis** and non-gonococcal urethritis worldwide. In females, the primary site of infection is the endocervical columnar epithelium. It typically presents with a yellow or green mucopurulent discharge from the external os and friability (bleeding on contact). It is often "silent" or asymptomatic, making it a major cause of Pelvic Inflammatory Disease (PID) and subsequent tubal factor infertility. **2. Why the Other Options are Incorrect:** * **A. Papilloma (HPV):** Human Papillomavirus causes genital warts (condyloma acuminata) or cervical dysplasia/cancer. It does not cause an acute purulent discharge. * **C. Candida:** *Candida albicans* causes vulvovaginal candidiasis, characterized by a thick, white, **"curdy" or "cottage-cheese"** like discharge with intense pruritus. It is a fungal infection of the vagina, not a primary cause of cervicitis. * **D. Bacterial Vaginosis (BV):** Caused by an overgrowth of *Gardnerella vaginalis* and anaerobes, BV presents with a thin, homogenous, **fishy-smelling** greyish-white discharge. It is a vaginal dysbiosis rather than a true inflammatory cervicitis/urethritis. **3. NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the investigation of choice for Chlamydia. * **Treatment:** The current CDC recommendation for Chlamydial cervicitis is **Doxycycline 100 mg BID for 7 days**. (Azithromycin 1g stat is now an alternative). * **Co-infection:** Always screen for *Neisseria gonorrhoeae* when cervicitis is suspected, as the two frequently coexist. * **Reiter’s Syndrome:** Remember the triad of "Urethritis, Conjunctivitis, and Arthritis" often triggered by Chlamydia.

Question 1

What enzyme produced by Staphylococcus aureus dissolves fibrin clots?

Accepted Answer

Staphylokinase

Answer

Hyaluronidase

Answer

Catalase

Answer

Lipase

Question 2

Verocytotoxin of E. coli acts by which mechanism?

Accepted Answer

Decreasing protein synthesis

Answer

Increasing cAMP

Answer

Decreasing cAMP

Answer

Decreasing cGMP

Question 3

Actinomycetoma is caused by which type of microorganism?

Accepted Answer

Bacteria

Answer

Virus

Answer

Fungus

Answer

Protozoa

Question 4

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?

Accepted Answer

S. aureus

Answer

S. epidermidis

Answer

S. pneumoniae

Answer

S. pyogenes

Question 5

Ehrlich phenomenon is seen in:

Accepted Answer

Corynebacterium

Answer

Myobacterium Tuberculosis

Answer

Proteus

Answer

Staphylococcus

Question 6

A 30-year-old woman with a history of poor obstetric outcomes presents with fever. Blood culture reveals gram-positive, small to medium, pleomorphic coccobacilli occurring in short chains. Direct microscopy of the culture shows tumbling motility. What is the most likely organism?

Accepted Answer

Listeria monocytogenes

Answer

Corynebacterium sp.

Answer

Enterococcus sp.

Answer

Erysipelothrix rhusiopathiae

Question 7

Group A Streptococcus is the causative agent of pharyngitis. What is the typical mode of transmission for Group A Streptococcus pharyngitis?

Accepted Answer

Direct contact or droplet spread

Answer

Hematogenous spread

Answer

Lymphatic spread

Answer

None of the above

Question 8

Which of the following is NOT associated with endocarditis?

Accepted Answer

ESKAPE group

Answer

Staphylococcus aureus

Answer

Enterococcus

Answer

HACEK group

Question 9

A 32-year-old man presented with a painless ulcer with a black eschar on his leg. Smears of exudate show non-motile organisms which are in chains. Spores are absent in the smear. What is the most probable organism?

Accepted Answer

Bacillus anthracis

Answer

Proteus

Answer

Diphtheroids

Answer

Clostridium

Question 10

Pus discharge (cervicitis/urethritis) in a female is most commonly caused by which of the following?

Accepted Answer

Chlamydia

Answer

Papilloma

Answer

Candida

Answer

Bacterial Vaginosis

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?