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 required few manipulations.CAMP test  rarely give false positives with other Streptococcus. The 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 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. Known group A and B streptococcal strains should be similarly inoculated on the same plate as negative and positive controls respectively.
  4. Incubate the plate at 35oC  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 ).

CAMP test :
A. Streptococcus agalactiae (positive)
B. Streptococcus pyogenes (Negative)
Image Source: ASM

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

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 streptococcal 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.