CAMP Test: Principle, Procedure and results

CAMP test is used for the presumptive identification of Group B beta-hemolytic streptococci, Streptococcus agalactiae.

CAMP test is effective for the “prompt and reliable” identification of Streptococcus agalactiae in the clinical lab, as results could be observed in as little as 18 hours and require few manipulations. CAMP test rarely gives false positives with other Streptococcus. This hemolytic phenomenon was first described in 1944 by Christie, Atkins, and Munch-Petersen, and CAMP test is an acronym of their names.

The hemolytic activity of the beta-hemolysin produced by most strains of Staphylococcus aureus is enhanced by an extracellular protein produced by group B streptococci. Interaction of the beta-hemolysin with this factor causes “synergistic hemolysis,” which is easily observed on a blood agar plate. This phenomenon is seen with both hemolytic and non-hemolytic isolates of group B streptococci.

Procedure for CAMP test:

1. Down the center of a blood agar plate (trypticase soy agar +5% sheep blood), make a single straight line streak of beta-hemolysin producing Staphylococcus aureus, taking care not to intersect the staphylococcal streak, inoculate a streak of the beta-hemolytic streptococcus to be identified perpendicular to the staphylococcal streak.
2. Make these streaks in such a way that, after incubation, the growth of the two organisms will not be touching.
3. The streptococcal streak should be 3 to 4 cm long.
4. Inoculate known group A and B streptococcal strains similarly on the same plate as negative and positive controls respectively.
5. Incubate the plate at 35°C in ambient air for 18-24 hours

Results and interpretations:

The area of increased hemolysis occurs where the beta hemolysin secreted by the Staphylococcus and the CAMP factor secreted by the group B Streptococcus intersect (See the figure).

Any bacitracin-resistant, trimethoprim-sulfomethoxazole-resistant, CAMP test-positive, beta-hemolytic streptococci can be reported as presumptive group B Streptococcus.

Quality control

1. Positive control: Group B streptococci (Streptococcus agalactiae)
2. Negative control: Group A streptococci (Streptococcus pyogenes)

Limitation of CAMP test

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.

Other CAMP test positive organisms

1. Listeria monocytogenes
2. Rhodococcus equi
3. Vibrio cholerae (certain strains)

References and further readings

• Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
• Koneman’s Color Atlas and Textbook of Diagnostic Microbiology