The CAMP Test - Scope, Principle, Test Procedure, Results and Interpretation

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Scope

The presumptive identification of group B Streptococci can be done with the CAMP test.

Principle

The hemolytic phenomenon was first described in 1944 by Christie, Atkins, and Munch–Petersen, and it is their names that provide the acronym (CAMP) for the test. The hemolytic activity of the β-hemolysin produced by most strains of Staphylococcus aureus is enhanced by an extracellular protein produced by group B streptococci. Interaction of the β-hemolysin with this factor causes “synergistic hemolysis,” which is easily observed on a blood agar plate. This phenomenon is seen with both hemolytic and nonhemolytic isolates of group B streptococci. Other bacteria, such as Listeria monocytogenes, Rhodococcus equi, and certain strains of Vibrio cholera, are also CAMP-positive.

Materials

β-hemolysin-producing strain of Staphylococcus aureus

Sheep Blood agar plate

Quality Control

Positive control: Group B Streptococcus

Negative control: Group A Streptococcus

Test Procedure

Down the center of a blood agar plate, and make a single straight line streak of β-hemolysin-producing S. aureus.

Taking care not to intersect the staphylococcal streak, inoculate a streak of the β-hemolytic streptococcus to be identified perpendicular to the staphylococcal streak. Make these streaks so that, after incubation, the growth of the two organisms will not be touching.

The streptococcal streak should be 3–4 cm long. Known group A and B streptococcal strains should be similarly inoculated on the same plate as negative and positive controls, respectively.

Incubate the plate at 35°C in ambient air for 18–24 hours.

Results and Interpretation

Positive: Enhanced hemolysis is indicated by an arrow head-shaped zone of beta-hemolysis at the junction of the two organisms.

The CAMP test

Negative: No enhancement of hemolysis.

As illustrated below, the area of increased hemolysis occurs where the β-hemolysin secreted by the Staphylococcus and the CAMP factor secreted by the group B Streptococcus intersect. Any bacitracin resistant, trimethoprim-sulfamethoxazole resistant, CAMP test-positive, β-hemolytic Streptococcus can be reported as “β-hemolytic Streptococcus, presumptive group B by CAMP test.”

Limitations

Some group A streptococci will be CAMP test-positive if the test plate is incubated in a candle jar, in a CO2 atmosphere, or under anaerobic conditions. Therefore, ambient-air incubation should be used.

Sheep blood agar plates are only used. Human, horse, rabbit, or guinea pig blood plates will not give a proper reaction.

Colonies of Listeria monocytogenes have a narrow zone of beta-hemolysis on sheep blood agar and may be confused with group B beta-hemolytic streptococci, if catalase and gram stain are not performed

Reference:

Koneman’s Color Atlas and Text book of Diagnistic
Microbiology.

Bailey and Scott’s Diagnostic Microbiology.

Mackie and McCartney Practical Medical Microbiology.

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