MLT GlobeBacteriology: Gram-positive cocci – page 2
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
A typical Staphylococcus cell measures approximately:
Most cocci are about 1 µm across, within the 0.2–5 µm range of common bacteria. 10 µm is closer to a red cell's size.
Correctly Gram-stained streptococci appear:
Streptococci are gram-positive and retain the crystal violet–iodine complex. Gram-negative cells appear red.
Enterococci carry which Lancefield group antigen?
Enterococci and the Streptococcus bovis group possess group D antigen. Enterococci are now a separate genus but keep this antigen.
Acute rheumatic fever follows pharyngitis caused by:
Antibodies against S. pyogenes M protein cross-react with heart tissue. Post-streptococcal glomerulonephritis can follow throat or skin infection.
Staphylococcus aureus grows in terms of oxygen requirement as a:
S. aureus grows with or without oxygen, using respiration or fermentation. Micrococci are obligate aerobes.
A catalase-positive gram-positive coccus in clusters that is coagulase positive is identified as:
Coagulase, which clots plasma, is the key marker of S. aureus among human staphylococci. S. epidermidis and S. saprophyticus are coagulase-negative.
A coagulase-negative staphylococcus from the urine of a sexually active 22-year-old woman is resistant to a 5 µg novobiocin disk. It is most likely:
S. saprophyticus is the novobiocin-resistant CoNS that causes UTI in young women. S. epidermidis and most other clinically important CoNS are novobiocin susceptible.
Swelling of the capsule of an organism when mixed with type-specific antiserum and methylene blue is called the:
The Quellung (Neufeld) reaction is capsular swelling with specific antiserum and is classically used to serotype S. pneumoniae. CAMP relates to group B streptococci.
On mannitol salt agar, a yellow halo around colonies means the organism:
Mannitol fermentation produces acid, turning phenol red indicator yellow; this is typical of S. aureus. The 7.5% salt is the selective agent.
Menstrual toxic shock syndrome linked to tampon use is mainly caused by:
Toxic shock syndrome toxin-1 is a superantigen of S. aureus and causes most menstrual TSS. Exfoliative toxins cause scalded skin syndrome.
Lancefield grouping of beta-hemolytic streptococci is based on:
Lancefield groups (A, B, C, F, G) are defined by the cell wall carbohydrate (C substance). M protein is used for typing within group A, not for grouping.
Acute rheumatic fever after group A streptococcal pharyngitis is thought to result from antibodies to which bacterial antigen that cross-react with heart tissue?
Antibodies against streptococcal M protein cross-react with cardiac myosin and other heart proteins (molecular mimicry). The hyaluronic acid capsule resembles host tissue and is poorly immunogenic.
In a patient without prosthetic material, one of two blood culture sets grows a coagulase-negative staphylococcus in the aerobic bottle only after 3 days. The most likely interpretation is:
Coagulase-negative staphylococci in a single set, with long time to positivity and no risk devices, usually reflect skin contamination during collection. True bacteremia typically gives multiple positive sets.
Necrotizing pneumonia in a previously healthy young adult after influenza, caused by community-associated MRSA, is most strongly linked to which toxin?
Panton-Valentine leukocidin, a pore-forming toxin that destroys leukocytes, is carried by many community-associated MRSA strains and is linked to necrotizing pneumonia and skin abscesses.
The sandpaper-like rash and strawberry tongue of scarlet fever are caused by:
Scarlet fever is caused by streptococcal pyrogenic (erythrogenic) exotoxins, such as SpeA and SpeC, which are encoded by bacteriophages. Streptolysin O is a hemolysin, not the rash toxin.
Which viridans group streptococcus is most associated with dental caries because it makes sticky glucans (dextran) from sucrose?
S. mutans converts sucrose into adhesive glucans that form dental plaque and produces acid that damages enamel. S. anginosus is linked mainly to abscesses.
A patient with a damaged mitral valve has had low-grade fever for 6 weeks after a dental extraction. Blood cultures grow alpha-hemolytic, optochin-resistant, bile-insoluble streptococci. The most likely group is:
Viridans streptococci from the mouth enter blood during dental procedures and cause subacute endocarditis on abnormal valves. Optochin resistance and bile insolubility exclude S. pneumoniae.
A single blood culture bottle grows Staphylococcus aureus. How should this result be regarded?
S. aureus is rarely a blood culture contaminant, so even one positive bottle is treated as true bacteremia and reported promptly. Coagulase-negative staphylococci, not S. aureus, are the common contaminants.
Protein A of Staphylococcus aureus helps the organism avoid phagocytosis by binding:
Protein A binds the Fc region of IgG, so antibodies attach in the wrong orientation and cannot act as opsonins. Binding of fibrinogen is the role of clumping factor.
Staphylococcus epidermidis is most often a cause of infection in patients with:
S. epidermidis forms biofilms on plastic and metal, so it infects catheters, prosthetic joints and heart valves.