30 Year 2: Microbiology exam questions on Medical Bacteriology for medical students. Includes MCQs, answers, explanations and written questions. Sample: The ter
This MCQ set contains 30 questions on Medical Bacteriology in the Year 2: Microbiology unit. Each question includes the correct answer and a detailed explanation for active recall and exam preparation.
Correct answer: B – Bunch of grapes
The name Staphylococcus comes from Greek "staphyle" meaning "bunch of grapes" because under microscopic examination, these bacteria appear arranged in irregular clusters resembling bunches of grapes.
Correct answer: B – Catalase test
The catalase test is the key differentiating test. Staphylococci are catalase-positive (produce bubbles when H₂O₂ is added), while streptococci and enterococci are catalase-negative. This is the first test performed to distinguish these gram-positive cocci.
Correct answer: C – 7.5-10% NaCl
Staphylococci are facultative halophiles and can tolerate high salt concentrations of 7.5-10% NaCl. This property is exploited in selective media like Mannitol Salt Agar to isolate staphylococci from mixed cultures.
Correct answer: B – Coagulase test
The coagulase test divides staphylococci into two clinically important groups: coagulase-positive (mainly S. aureus) and coagulase-negative staphylococci (CoNS). This classification has major therapeutic implications.
Correct answer: B – S. aureus
S. aureus is the primary coagulase-positive staphylococcus of clinical importance. Coagulase enzyme helps S. aureus clot plasma, which interferes with phagocytosis and facilitates tissue spread.
Correct answer: B – Fc portion of IgG
Protein A binds to the Fc portion of IgG antibodies, which inhibits opsonization and phagocytosis. This is an important virulence mechanism that helps S. aureus evade the immune system.
Correct answer: C – Epidermolytic toxin
Epidermolytic (exfoliative) toxins A and B act as proteases that cleave desmoglein protein, which holds epidermal cells together. This results in separation and peeling of the epidermal layer, causing the scalded skin appearance.
Correct answer: B – Preformed enterotoxin in food
S. aureus produces heat-stable enterotoxins in contaminated food. Even if bacteria are killed by cooking, the preformed toxin remains active and causes rapid-onset food poisoning when ingested.
Correct answer: C – Desquamation of palms and soles
While fever, hypotension, and rash occur during the acute phase, the pathognomonic (characteristic) late sign is desquamation (skin peeling) of palms and soles occurring 1-2 weeks later.
Correct answer: B – Yellow
S. aureus ferments mannitol, producing acid that turns the phenol red indicator yellow. S. epidermidis doesn't ferment mannitol, so colonies remain pink/red.
Correct answer: B – Hyaluronidase
Hyaluronidase is called the "spreading factor" because it breaks down hyaluronic acid in connective tissues, facilitating bacterial spread through tissues.
Correct answer: B – Altered penicillin-binding proteins
MRSA produces an additional penicillin-binding protein (PBP2a) with low affinity for β-lactam antibiotics, allowing cell wall synthesis to continue despite antibiotic presence.
Correct answer: C – Vancomycin
Vancomycin remains the first-line treatment for MRSA infections as it has a different mechanism of action (inhibiting cell wall synthesis at a different step) that bypasses β-lactam resistance.
Correct answer: D – Medical devices and catheters
S. epidermidis produces glycocalyx (biofilm) that allows it to adhere strongly to medical devices like catheters, shunts, and prosthetic materials, making it a major cause of device-related infections.
Correct answer: C – Novobiocin sensitivity
Both are coagulase-negative, but S. epidermidis is novobiocin-sensitive while S. saprophyticus is novobiocin-resistant. This is the key differentiating test between these two species.
Correct answer: B – Urinary tract infections
S. saprophyticus is the second most common cause of UTIs in young, sexually active women (after E. coli). It inhabits the skin around the genitourinary tract.
Correct answer: C – S. haemolyticus
S. haemolyticus is naturally resistant to vancomycin and is highly multi-drug resistant, making it a challenging pathogen to treat.
Correct answer: B – Coagulase enzyme
Coagulase is the key virulence factor that defines S. aureus. It helps the bacteria clot plasma, creating a protective barrier against phagocytosis and facilitating tissue invasion.
Correct answer: B – Stimulating cytokine release
Enterotoxin stimulates release of IL-1 and IL-2, which activate the enteric nervous system and stimulate the vomiting center in the brain, causing the rapid onset of nausea and vomiting.
Correct answer: C – Children and patients with renal failure
SSSS occurs mainly in children (due to immature kidney function) and adults with renal failure because they cannot effectively clear the epidermolytic toxin from their system.
Correct answer: C – Modified oxidase test
Micrococcus species are oxidase-positive while Staphylococcus species (except S. sciuri, S. lentus, S. vutulus) are oxidase-negative. This is the most reliable test to differentiate these genera.
Correct answer: C – Yellow or red-pigmented
Micrococcus species characteristically produce yellow or red pigments on blood agar, which helps distinguish them from staphylococci that typically produce golden-yellow (S. aureus) or white colonies.
Correct answer: C – Catalase positive
All staphylococci are catalase-positive. This is a defining characteristic of the genus, while other properties like coagulase production, mannitol fermentation, and hemolysis vary among species.
Correct answer: B – Skin and nasal passages
S. aureus is part of the normal flora of skin and nasal passages in about 20-30% of healthy individuals. The anterior nares (nostrils) are the primary carriage site.
Correct answer: C – Mupirocin
Mupirocin is a topical antibiotic highly effective against staphylococci, including MRSA. It's commonly used for localized skin infections and nasal decolonization.