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Flashcards in Gram-Positve Cocci Deck (59)
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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?

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


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

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


Describe a positive catalase reaction.

Immediate and rapid bubbling.


Which bacteria grow on mannitol salt agar (MSA)?

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.


What does the tube coagulase test detect?

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


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

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


What type of plasma is best for the coagulase test?

Rabbit plasma.


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

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


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?

Coaguase-negative staphylococci.


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

S. saprophyticus.


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


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

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


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?

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


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

Food poisoning.


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

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.


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

The cell wall C carbohydrate.


To which Lancefield group does S. pyogenes belong?

Group A.


To which Lancefield group does S. agalactiae belong?

Group B.


To which Lancefield group do enterococci belong?

Group D.


What are the most common species of enterococci?

E. faecalis and E. faecium.


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

Streptococcus pneumonia and viridans streptococci.


Which Lancefield group causes most streptococcal infections?

Group A.


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

Sheep blood (5%).


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

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.


Which Lancefield groups are usually beta-hemolytic?

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


Which streptococci are usually alpha-hemolytic?

S. pneumonia and viridans streptococci.


Which streptococci are usually gamma-hemolytic?

Group D. Enterococci are also usually gamma-hemolytic.


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.

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


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

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


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

S. pneumonia.