Tests for Gram-Positive Cocci

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Catalase

converts 3% H2O2 to oxygen and water, resulting in immediate bubling. Staphylococci are positive. Streptococci and enterococci are negative.

Coagulase

causes coagulation (tube test) or agglutination (slide test) in rabbit plasma. S. aureus is positive.

Manitol salt agar

fermentation results in a color change from pink to yellow. Salt concentration of 7.5% inhibits most organisms other than staph. All staph can grow on MSA. Only S. aureus, and some S. saprophyticus, cause color change.

Novobiocin

Organisms that are resistant grow to the edge of the disk. S. Saprophyticus is resistant. Other coag-neg staph are susceptible. Test is performed on coag-neg staph from urine.

Bacitracin disk

Group A strep is susceptible. Also known as the A disk. No longer recommended because some groups C and G are also susceptible.

SXT

Group A and B strep are resistant. Group C and G strep are susceptible. Used in combination with bacitracin disk to differentiate group A strep from group C or G strep.

PYR

If PYR is hydrolyzed, a red color develops after addition of a color developer. Group A strep and enterococci are positive. More specific than bacitracin for group A strep. Group A strep is the only species of beta-hemolytic strep that is PYR-positive

CAMP test

Group B strep enhances hemolysis of beta-hemolytic S. aureus. An arrowhead formation is seen at the juncture of the two organisms.

Hippurate hydrolysis

If hippurate is hydrolyzed, a precipitate forms when ferric chloride is added to the medium. Group B strep is positive.

Bile esculin agar

When esculin is hydrolyzed, the product reacts with ferric ions in the medium to produce a dark brown color. Group D strep and enterococci are positive. Bile inhibits gram-pos bacteria other than group D strep and enterococci.

6.5% NaCl broth

Organisms that can grow in 6.5% NaCl produce turbidity. Enterococci grow in 6.5% NaCl. Group D streptococci do not.

Optochin disk

S. pneumoniae is susceptible. Also known as P disk.

Bile solubility

Bile salts cause lysis of some organisms. S. pneumoniae is positive.

Slide agglutination tests

Reagent antibodies react with bacterial antigens, causing agglutination. Organisms agglutinate in corresponding antisera. Tests are commercially available for rapid identification of S. aureus, streptococci (groups A, B, C, D, F, G), and S. pneumoniae.

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