Quick Review Cards for Medical Laboratory Science

Tests for Identification of Beta-Hemolytic

Streptococci

Clinical Microbiology Review 165

TEST PRINCIPLE KEY REACTIONS OTHER

Bacitracin disk

Trimethoprim-sulfamethoxazole
(SXT) disk

Pyrrolidonase (PYR) test

CAMP test

Also known as A disk. Performed on

SBA. No longer recommended because

some groups C & G are susceptible.

Replaced by PYR & serogrouping by

latex agglutination.

Used in conjunction with bacitracin

disk to differentiate GAS from group C

or G. Requires overnight incubation.

More specific than bacitracin for GAS.

GAS is only beta-hemolytic strep that’s

PYR pos. Disk test only takes minutes.

Classic method: unknown streaked

perpendicular to streak of b-lysin pro-

ducing S. aureus. Incubated in ambient

air overnight. False pos in CO 2. Arrow-

head hemolysis where inoculum lines

meet. Disks containing b-lysin can be

used instead of S. aureus. Rapid test:

drop of b-lysin on colonies on SBA.

Only requires 20-min incubation.

Zone of inhibition after overnight

incubation = susceptibility to

bacitracin.

Organisms resistant to SXT grow

up to disk.

If PYR is hydrolyzed, red color

after addition of color developer.

GBS produces extracellular protein

that enhances hemolysis of beta-

hemolytic S. aureus on SBA.

GAS susceptible. GBS

resistant

GAS & GBS resistant.

Group C & G strep

susceptible

GAS pos (also

enterococci)

GBS pos

continued...