Streptococcus Questions and Answers

Q: Which of the following is the most effective drug therapy for the infections caused by Enterococcus faecalis?

Correct answer: Aminoglycoside with penicillinEnterococci are only inhibited by penicillin, not killed, and aminoglycosides alone cannot cross the cell wall in sufficient quantity. Given together the penicillin damages the wall enough for the aminoglycoside to enter, and the combination is bactericidal - a genuine synergy, and the reason endocarditis is treated with two drugs rather than one. Cephalosporins are inactive against enterococci altogether.

Q: Which of the following toxins cause damage to the tissue cells by its phospholipase action?

Correct answer: beta toxinStaphylococcal beta toxin is a sphingomyelinase C, an enzyme that hydrolyses the sphingomyelin of the cell membrane and so destroys the cell from its lipid outwards. Alpha toxin damages membranes by a different route, assembling into a pore; protein A binds the Fc region of immunoglobulin and blocks opsonisation; and coagulase clots plasma rather than attacking cells.

Q: Which of the following test(s) may establish retrospective diagnosis of streptococcal infection?

Correct answer: All of theseAll three detect antibody rather than the organism, so all can establish an infection that has already resolved - which is exactly what is needed when rheumatic fever or glomerulonephritis appears weeks later. The ASO test detects antibody to streptolysin O, anti-DNase B is more reliable after skin infection, and the streptozyme test screens for several streptococcal antibodies at once.

Q: Which of the following pathogen is the main cause of endocarditis in individuals having heart diseases?

Correct answer: Enterococcus faecalisEnterococci are among the leading causes of infective endocarditis, and they characteristically settle on valves already damaged by rheumatic or degenerative disease. They reach the bloodstream from the bowel or the urinary tract, which is why enterococcal bacteraemia in a patient with valve disease is taken seriously and treated as endocarditis until proved otherwise.

Q: Streptolysin O is

Correct answer: all of theseStreptolysin O is antigenic, which is why the antistreptolysin O titre can be measured and used to establish a recent infection. It is oxygen-labile, being inactivated by atmospheric oxygen, which is why the haemolysis it produces is best seen in a stab or under a coverslip. And it is heat-labile. Streptolysin S is the opposite on the first two counts: oxygen-stable and non-antigenic.

Q: Cell wall protein of Streptococcus pyogenes crossreacts with human

Correct answer: myocardiumThe M protein, the chief cell wall protein and the principal virulence factor, shares epitopes with cardiac muscle, so antibody directed against it also attacks the myocardium. Together with the cell wall carbohydrate’s cross-reaction with the valves, this accounts for the pancarditis of rheumatic fever - two different bacterial antigens attacking two different cardiac tissues.

Q: Which of the following is the complications of autoimmune reaction from pharyngitis?

Correct answer: Rheumatic feverRheumatic fever is the autoimmune sequel to streptococcal pharyngitis: antibody raised against the M protein and the cell wall carbohydrate cross-reacts with myocardium and cardiac valves, producing carditis, arthritis and chorea weeks after the throat infection has settled. Scarlet fever is a toxin-mediated feature of the acute illness, not an autoimmune complication.

Q: Crystal violet blood agar is a selective medium for

Correct answer: β-haemolytic streptococciCrystal violet inhibits staphylococci and most other gram-positive contaminants while permitting streptococci to grow, so crystal violet blood agar is used to recover beta-haemolytic streptococci from throat swabs and other specimens where the normal flora would otherwise overgrow them. The blood in the medium allows the haemolysis to be read at the same time.

Q: All of the following are the general characteristics of Micrococcus spp and Staphylococcus spp, Except?

Correct answer: None of the aboveAll three statements are correct, so there is no exception. Micrococcus colonies are smaller than staphylococcal ones and often pigmented, the genus is widely distributed in the environment and on skin, and both genera are catalase positive - which is what separates them from the streptococci rather than from each other.

31.

Viridans Streptococci are all, Except?

A.Most of them show alpha hemolysis on sheep blood agar
B.Found as a most common pathogen in dental caries
C.Not groupable according to Lancefield serology
D.Found only in the oral cavities as commensals

Answer: D

Correct answer: Found only in the oral cavities as commensals

Viridans streptococci are commensals of the mouth but also of the pharynx, the gastrointestinal tract and the female genital tract, so “only in the oral cavities” is too narrow. The other three statements hold: most are alpha-haemolytic, they are the principal agents of dental caries, and they carry no Lancefield group antigen.

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32.

Which of the following is the most effective drug therapy for the infections caused by Enterococcus faecalis?

A.Cephalosporin
B.Aminoglycoside with penicillin
C.Macrolide
D.Oral penicillin V with ampicillin

Answer: B

Correct answer: Aminoglycoside with penicillin

Enterococci are only inhibited by penicillin, not killed, and aminoglycosides alone cannot cross the cell wall in sufficient quantity. Given together the penicillin damages the wall enough for the aminoglycoside to enter, and the combination is bactericidal - a genuine synergy, and the reason endocarditis is treated with two drugs rather than one. Cephalosporins are inactive against enterococci altogether.

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33.

Which of the following toxins cause damage to the tissue cells by its phospholipase action?

A.alpha toxin
B.beta toxin
C.protein A
D.coagulase

Answer: B

Correct answer: beta toxin

Staphylococcal beta toxin is a sphingomyelinase C, an enzyme that hydrolyses the sphingomyelin of the cell membrane and so destroys the cell from its lipid outwards. Alpha toxin damages membranes by a different route, assembling into a pore; protein A binds the Fc region of immunoglobulin and blocks opsonisation; and coagulase clots plasma rather than attacking cells.

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34.

Which of the following test(s) may establish retrospective diagnosis of streptococcal infection?

A.Anti-DNase B test
B.ASO test
C.Streptozyme test
D.All of these

Answer: D

Correct answer: All of these

All three detect antibody rather than the organism, so all can establish an infection that has already resolved - which is exactly what is needed when rheumatic fever or glomerulonephritis appears weeks later. The ASO test detects antibody to streptolysin O, anti-DNase B is more reliable after skin infection, and the streptozyme test screens for several streptococcal antibodies at once.

Discuss in Forum

35.

Which of the following pathogen is the main cause of endocarditis in individuals having heart diseases?

A.Streptococcus intermedius
B.Enterococcus faecalis
C.Streptococcus suis
D.Micrococcus luteus

Answer: B

Correct answer: Enterococcus faecalis

Enterococci are among the leading causes of infective endocarditis, and they characteristically settle on valves already damaged by rheumatic or degenerative disease. They reach the bloodstream from the bowel or the urinary tract, which is why enterococcal bacteraemia in a patient with valve disease is taken seriously and treated as endocarditis until proved otherwise.

Discuss in Forum

36.

Streptolysin O is

A.antigenic
B.oxygen-labile
C.heat-labile
D.all of these

Answer: D

Correct answer: all of these

Streptolysin O is antigenic, which is why the antistreptolysin O titre can be measured and used to establish a recent infection. It is oxygen-labile, being inactivated by atmospheric oxygen, which is why the haemolysis it produces is best seen in a stab or under a coverslip. And it is heat-labile. Streptolysin S is the opposite on the first two counts: oxygen-stable and non-antigenic.

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37.

Cell wall protein of Streptococcus pyogenes crossreacts with human

A.synovial fluid
B.cardiac valves
C.myocardium
D.vascular intima

Answer: C

Correct answer: myocardium

The M protein, the chief cell wall protein and the principal virulence factor, shares epitopes with cardiac muscle, so antibody directed against it also attacks the myocardium. Together with the cell wall carbohydrate’s cross-reaction with the valves, this accounts for the pancarditis of rheumatic fever - two different bacterial antigens attacking two different cardiac tissues.

Discuss in Forum

38.

Which of the following is the complications of autoimmune reaction from pharyngitis?

A.Rheumatic fever
B.Scarlet fever
C.Fasciitis
D.None of the above

Answer: A

Correct answer: Rheumatic fever

Rheumatic fever is the autoimmune sequel to streptococcal pharyngitis: antibody raised against the M protein and the cell wall carbohydrate cross-reacts with myocardium and cardiac valves, producing carditis, arthritis and chorea weeks after the throat infection has settled. Scarlet fever is a toxin-mediated feature of the acute illness, not an autoimmune complication.

Discuss in Forum

39.

Crystal violet blood agar is a selective medium for

A.β-haemolytic streptococci
B.Staphylococcus aureus
C.Corynebacterium diphtheriae
D.Mycobacterium tuberculosis

Answer: A

Correct answer: β-haemolytic streptococci

Crystal violet inhibits staphylococci and most other gram-positive contaminants while permitting streptococci to grow, so crystal violet blood agar is used to recover beta-haemolytic streptococci from throat swabs and other specimens where the normal flora would otherwise overgrow them. The blood in the medium allows the haemolysis to be read at the same time.

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40.

All of the following are the general characteristics of Micrococcus spp and Staphylococcus spp, Except?

A.On blood agar Micrococcus spp appear as smaller colonies than Staphylococcus spp
B.Micrococcus is ubiquitous in nature
C.Both are catalase positive
D.None of the above

Answer: D

Correct answer: None of the above

All three statements are correct, so there is no exception. Micrococcus colonies are smaller than staphylococcal ones and often pigmented, the genus is widely distributed in the environment and on skin, and both genera are catalase positive - which is what separates them from the streptococci rather than from each other.

Discuss in Forum

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