Medical Bacteriology | MCQ Quiz | OmpathStudy Kenya

Question 1

The term "Staphylococcus" is derived from Greek meaning:

Accepted Answer

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.

Question 2

Which test is most useful for distinguishing staphylococci from streptococci?

Accepted Answer

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.

Question 3

Staphylococci can grow in salt concentrations up to:

Accepted Answer

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.

Question 4

The primary test used to clinically classify staphylococci is:

Accepted Answer

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.

Question 5

Which staphylococcal species is coagulase-positive?

Accepted Answer

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.

Question 6

Protein A in S. aureus cell wall has affinity for:

Accepted Answer

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.

Question 7

Which toxin is responsible for Staphylococcal Scalded Skin Syndrome (SSSS)?

Accepted Answer

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.

Question 8

Staphylococcal food poisoning is caused by:

Accepted Answer

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.

Question 9

The characteristic feature of Toxic Shock Syndrome occurring 1-2 weeks after acute illness is:

Accepted Answer

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.

Question 10

On Mannitol Salt Agar, S. aureus colonies appear:

Accepted Answer

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.

Question 11

Which enzyme helps S. aureus spread through tissues by destroying connective tissue?

Accepted Answer

Hyaluronidase — Hyaluronidase is called the "spreading factor" because it breaks down hyaluronic acid in connective tissues, facilitating bacterial spread through tissues.

Question 12

MRSA is resistant to methicillin due to:

Accepted Answer

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.

Question 13

The drug of choice for MRSA infections is:

Accepted Answer

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.

Question 14

S. epidermidis is particularly associated with infections of:

Accepted Answer

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.

Question 15

Which test distinguishes S. epidermidis from S. saprophyticus?

Accepted Answer

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.

Question 16

S. saprophyticus is most commonly associated with:

Accepted Answer

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.

Question 17

Which staphylococcal species is vancomycin-resistant?

Accepted Answer

S. haemolyticus — S. haemolyticus is naturally resistant to vancomycin and is highly multi-drug resistant, making it a challenging pathogen to treat.

Question 18

The most important virulence factor that distinguishes S. aureus from coagulase-negative staphylococci is:

Accepted Answer

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.

Question 19

Staphylococcal enterotoxin causes food poisoning by:

Accepted Answer

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.

Question 20

SSSS (Staphylococcal Scalded Skin Syndrome) is most common in:

Accepted Answer

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.

Question 21

The key test to differentiate Micrococcus from Staphylococcus is:

Accepted Answer

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.

Question 22

Micrococcus colonies on blood agar typically appear:

Accepted Answer

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.

Question 23

Which of the following is characteristic of all staphylococci?

Accepted Answer

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.

Question 24

The primary reservoir of S. aureus in healthy individuals is:

Accepted Answer

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.

Question 25

Which antibiotic is commonly used topically for staphylococcal skin infections?

Accepted Answer

Mupirocin — Mupirocin is a topical antibiotic highly effective against staphylococci, including MRSA. It's commonly used for localized skin infections and nasal decolonization.

Question 26

The mechanism by which β-lactamase confers antibiotic resistance is:

Accepted Answer

Breaking down β-lactam antibiotics — β-lactamase enzymes cleave the β-lactam ring in penicillins and cephalosporins, inactivating these antibiotics before they can inhibit cell wall synthesis.

Question 27

Toxic Shock Syndrome Toxin-1 (TSST-1) causes disease by:

Accepted Answer

Massive cytokine release — TSST-1 is a superantigen that causes massive, non-specific T-cell activation and cytokine release (IL-1, IL-2, TNF), leading to the systemic inflammatory response seen in toxic shock syndrome.

Question 28

S. aureus appears as colonies on blood agar with hemolysis:

Accepted Answer

Golden-yellow; beta — S. aureus characteristically produces golden-yellow, fairly large colonies that are beta-hemolytic (complete clearing around colonies) on blood agar. This appearance helps in preliminary identification.

Question 29

The DNAase test is positive in:

Accepted Answer

S. aureus and weakly in S. epidermidis — The DNAase test is strongly positive in S. aureus and weakly positive in S. epidermidis, but negative in S. saprophyticus. This helps differentiate between these species.

Question 30

Prevention of staphylococcal hospital-acquired infections is best achieved by:

Accepted Answer

Hand antisepsis and aseptic techniques — The most effective prevention methods are proper hand hygiene, aseptic handling of medical devices and catheters, and proper wound care. These basic infection control measures are more important than antibiotic prophylaxis or screening. ## Score Interpretation: - 26-30 correct: Excellent understanding - 21-25 correct: Good understanding, review weak areas - 16-20 correct: Fair understanding, needs more study - Below 16: Poor understanding, comprehensive review needed

Medical Bacteriology | MCQ Quiz | OmpathStudy Kenya

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