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Clinical Microbiology > Gram-Positve Cocci > Flashcards

Gram-Positve Cocci Flashcards

1
Q

A swab of a superficial skin lesion is received for culture. A direct smear is prepared and gram-positive cocci are seen. The sulture shows a pure growth of moderate-sized, white, nonhemolytic colonies on blood agar, chocolate agar, and CNA. No growth is seen on MacConkey. What is the first biochemical test that should be performed?

A

Catalase, to determine if the isolate is Staphylococcus or Streptococcus. Staphylococcus is catalase-positive. Streptococcus is catalase-negative.

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2
Q

What is the reagent for the catalase test and how should it be stored?

A

3% hydrogen peroxide. It should be stored in the refrigerator in a dark bottle.

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3
Q

Describe a positive catalase reaction.

A

Immediate and rapid bubbling.

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4
Q

Which bacteria grow on mannitol salt agar (MSA)?

A

S. aureus, coagulase-negative staphylococci, and micrococci will grow. The high salt concentration inhibits most others. S. aureus ferments mannitol and causes a color change in the agar from pink to yellow. S. saprophyticus may also. MSA is not used much today since more rapid tests are available for the identification of S. aureus.

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5
Q

What does the tube coagulase test detect?

A

Free coagulase. The slide coagulase test detects cell-bound coagulase (clumping factor). S. aureus is coagulase-positive.

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6
Q

A gram-positive coccus resembling S. aureus is isolated. A slide test for coagulase is negative. What should be done next?

A

A tube coagulase. The slide test is only a screening test.

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7
Q

What type of plasma is best for the coagulase test?

A

Rabbit plasma.

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8
Q

Interpret the following results of a latex agglutination test for S. aureus.
Unknown: Agglutination
Positive Control: Agglutination
Negative Control: Agglutination

A

The test is invalid. Agglutination in the negative control indicates nonspecific agglutination.

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9
Q

Material was received from an abscess. The smear showed many gram-positive cocci in clusters with many polys. There was growth on the blood agar plates, but not on MacConkey. The colonies on blood agar were non-hemolytic and white. Catalase was positive and coagulase was negative. How should this organism be identified?

A

Coaguase-negative staphylococci.

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10
Q

Which Staphylococcus species causes urinary tract infections in sexually active young women and catheterized elderly men?

A

S. saprophyticus.

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11
Q

A urine culture from a 20-year-old female rows 100 medium sized, round, white, nonhemolytic colonies on BAP. There is no growth on MacConkey Agar. A Gram stain of the culture shows gram-positive cocci in clusters. The catalase test is positive. The latex test for S. aureus is negative. What is the next step?

A

A novobiocin disk to rule our S. saprophyticus. S. saprophyticus is resistant to novobiocin; other coagulase-negative staphylococci are sensitive.

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12
Q

When should coagulase-negative staphylococci be identified to the species level?

A

When isolated in urine, blood and other normally sterile body fluids, and surgical specimens.

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13
Q

A gram-positive coccus is isolated on blood agar. It is catalase-positive, modified oxidase (microdase)-positive, susceptible to bacitracin, and resistant to furazolidone and lysostaphin. What is it?

A

Micrococcus. this organism is rarely associated with infection. It is usually a contaminant that must be differentiated from Staphylococcus.

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14
Q

Some species of Staphylococcus produce an enterotoxin. What does it cause?

A

Food poisoning.

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15
Q

Compare the appearance of colonies of S. aureus and group A streptococci.

A

S, aureus colonies are larger, may be golden, and may produce a narrow zone of beta hemolysis compared to the diameter of the colony. Group A streptococci colonies are pinpoint, translucent, and produce a wide zone of beta hemolysis compared to the diameter of the colony.

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16
Q

Lancefield’s serological grouping of the streptococci is based on what cellular component?

A

The cell wall C carbohydrate.

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17
Q

To which Lancefield group does S. pyogenes belong?

A

Group A.

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18
Q

To which Lancefield group does S. agalactiae belong?

A

Group B.

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19
Q

To which Lancefield group do enterococci belong?

A

Group D.

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20
Q

What are the most common species of enterococci?

A

E. faecalis and E. faecium.

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21
Q

Name two streptococci that do not belong to any Lancefield group.

A

Streptococcus pneumonia and viridans streptococci.

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22
Q

Which Lancefield group causes most streptococcal infections?

A

Group A.

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23
Q

What is the only type of blood agar that should be used to determine hemolysis of streptococci?

A

Sheep blood (5%).

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24
Q

Describe alpha hemolysis, beta hemolysis, and gamma hemolysis on sheep blood agar.

A

Alpha hemolysis is partial hemolysis; the colony is surrounded by an olive-green zone. Beta hemolysis is complete hemolysis; the colony is surrounded by a clear zone. Gamma hemolysis means no hemolysis; there is no change in the agar around colony.

25
Q

Which Lancefield groups are usually beta-hemolytic?

A

Groups A, B, C, F, and G.

26
Q

Which streptococci are usually alpha-hemolytic?

A

S. pneumonia and viridans streptococci.

27
Q

Which streptococci are usually gamma-hemolytic?

A

Group D. Enterococci are also usually gamma-hemolytic.

28
Q

A sputum culture grows small glistening colonies with depressed centers and raised edges, surrounded by greenish zones of hemolysis on a blood agar plate. What would probably be seen on a Gram stain of these colonies.

A

The colonies resembles S. pneumonia, so gram-positive lancet-shaped diplococci would be expected.

29
Q

If one of these colonies referred to in the previous question was mixed with a drop of 3% hydrogen peroxide, what would be expected?

A

S. pneumonia is catalase-negative, so there would be no bubbles.

30
Q

Which Streptococcus is sensitive to optochin and positive for bile solubility?

A

S. pneumonia.

31
Q

An alpha=hemolytic colony is isolated on blood agar. It is catalase-negative. The Gram stain shows gram-positive cocci. An optochin test is performed and after overnight incubation, there is no inhibition of growth. Bile esculin is negative. What has been isolated?

A

Alpha streptococci (viridans group). Identification to species level is not required unless isolated from a critical specimen such as blood.

32
Q

What is the advantage of “stab streaking” or anaerobic incubation of blood agar plates when looking for beta hemolytic streptococci?

A

Both methods ensure detection of beta-hemolysis by those streptococci that only produce streptolysin O. Hemolysis by streptococci is due to two enzymes, streptolsyin O and streptolysin S. Most streptococci produce both hemolysins, but if the organism only produces streptolysin O, hemolysis will not be seen on surface colonies because streptolysin O is oxygen-labile. Stab streaking or anaerobic incubation preserves streptolysin O.

33
Q

Which beta-hemolytic streptococci are sensitive to bacitracin?

A

Group A. Bacitracin susceptibility is an unreliable indicator for group A strep.

34
Q

What organisms could be used as positive and negative controls for the bacitracin test.

A

Positive - S. pyogenes.

Negative - S. agalactiae.

35
Q

Why is the bacitracin disk alone not recommended for the identification of group A streptococci?

A

Groups C and G streptococci are also sensitive to bacitracin.

36
Q

Which beta-hemolytic streptococci streptococci are resistant to SXT (sulfamethoxazole and trimethoprim)?

A

Groups A and b, Groups C, F, and G are sensitive.

37
Q

Name test pereferred over bacitracin for the identification of group A streptococci.

A

The PYR test because it is more specific than bacitracin for S. pyogenes. S. pyogenes is the only species of beta-hemolytic streptococci that will give a positive PYR reaction. (Enterococci are also PYR positive, but are usually nonhemolytic.) The enzyme L-pyrroglutamyl-aminopeptidase can be detected using commercially available PYR disks. The test is a rapid test. Identification may also be made with streptococcal grouping reagents.

38
Q

A child complaining of fever and sore throat is examined in the physician’s office. A rapid group A streptococcal antigen test is performed in the office and result is negative. Control results were acceptable. What should be done and why?

A

A culture because , although specific, raid antigen tests are not sensitive.

39
Q

Which streptococcal are CAMP-positive?

A

Group B. A positive result is an area of enhanced hemolysis (arrowhead formation) at the junction of the Streptococcus and beta hemolytic S. aureus streaks. The CAMP test is rarely done now that more rapid identification methods are available.

40
Q

Which gram-positive cocci are most resistant to antibiotics and always requires sensitivity testing?

A

Enterococci

41
Q

In what part of the body are the enterococci considered normal flora?

A

In the GI tract.

42
Q

Prenatal anogenital cutlures are recommended in order to identify which group of streptococci?

A

Group B streptococci (GBS). GBS is the leading bacterial cause of neonatal illness and death. It asymptomatically colonizes the vaginal or rectal areas of 1-30% of pregnant women. If a pregnant women is colonized with GBS, antibiotics are administered during delivery to prevent infection in the neonate.

43
Q

A vaccine is available to confer immunity to which species of Streptococcus?

A

S. pneumonia. Immunization is recommended for the elderly and those with chronic respiratory disease.

44
Q

Which species of Streptococcus causes dental caries and endocarditis?

A

S. mutans.

45
Q

Name two diseases that follow untreated or partially treated streptococcal pharyngitis or skin infections.

A

Rheumatic fever and glomerulonephritis. The antistreptolysin O titer is increased with these diseases.

46
Q

What is the most common cause of subacute bacterial endocarditis?

A

Viridans strep.

47
Q

Which organisms may cause bacteremia following dental procedures?

A

Viridans strep.

48
Q

A throat specimen is inoculated on a selective streptococcal blood agar plate and a bacitracin disk is applied. After overnight incubation, beta-hemolytic colonies are observed but there is no growth next to the disk. Which Lancefield group has been isolated?

A

Group A. Both group A and group B streptococci grow on selective streptococcal blood agar containing SXT but only group A is sensitive to bacitracin.

49
Q

Gram-positive cocci in chains are isolated on blood agar. The colonies are beta-hemolytic and catalase-negative. What test(s) should be done next?

A

Tests for beta-hemolytic streptococci include bacitracin or PYR to identify group A streptococci, CAMP test or hippurate hydrolysis to identify group A streptococci, or a slide agglutination test which identifies groups A, B, C, F, and G streptococci.

50
Q

Which gram-positive cocci hydrolyze esculin?

A

Group D streptococci and enterococci.

51
Q

Which gram-positive cocci hydrolyze esculin, grow in 6.5% NaCl broth, and are PYR-positive?

A

Enterococci.

52
Q

Gram-positive cocci in chains are isolated on blood agar. The colonies are gamma-hemolytic and catalase-negative. What test(s) should be done next?

A

Bile esculin, PYR, and 6.5% NaCl to differentiate group D streptococci, enterococci, and viridans streptococci.

Group D strep is only positive for bile esculin.
Enterococci is positive for all three tests.
Viridans strep is negative for all three tests.

53
Q

The bacitracin test is only performed on colonies of which hemolytic type?

A

Beta.

54
Q

The optochin test is only performed on colonies of which hemolytic type?

A

Alpha.

55
Q

The CAMP test is only performed on colonies of which hemolytic type?

A

Beta.

56
Q

A blood culture is received from a patient suspected of having endocarditis. The blood culture instrument flags the culture as positive. A Gram stain is prepared and gram-positive cocci in chains are observed. The blood culture bottle is subcultured to aerobic and anaerobic blood agar and chocolate agar. No growth is observed on any of the plates at 24 or 48 hours. What should be done and why?

A

The broth should be subcultured again adding a pyridoxal disk to detect Abiotrophia spp., formerly known as nutritionally-variant streptococci. These opportunistic pathogens, which are normal flora in the moth, are gram-positive cocci that do not grow on blood or chocolate agars unless pyridoxal (vitamin B6) is added, either by a pyridoxal disk or by cross-streaking with Staphylococcus.

57
Q

Which Streptococcus has a distinctive Gram stain morphology that differentiates it from other streptococci?

A

S. pneumoniae is the only strep appears as lancet-shaped diplococcic (ovoid with pointed ends).

58
Q

The Gram stain of a sputum specimen shows many gram-positive cocci in chains and pairs. Many small alpha-hemolytic colonies are seen on primary sheep blood agar plate. Which tests could be done to determine if the organisms are S. pneumoniae or alpha streptococci?

A

A rapid slide agglutination test for S. pneumoniae or an optochin disk. S. pneumoniae is sensitive to optochin (P disk).

59
Q

What is the Quellung reaction?

A

A method for identifying and typing S. pneumoniae based on swelling of the capsule when the clinical specimen or isolated colony is mixed wit antiserum. The test is seldom used.

Clinical Microbiology (9 decks)

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