CAMP test

CAMP test is used for the presumptive identification of Group B Streptococcus (group B strep) or Streptococcus agalactiae and Listeria monocytogenes ¹. CAMP test is used to distinguish the species Streptococcus agalactiae from other species of beta-hemolytic Streptococcus ². Streptococcus agalactiae also known as group B strep, is a bacterium that commonly cause of severe infections in newborns during the first week of life. More recently, experts recognized the increasing impact invasive group B Streptococcus disease has on adults. Streptococcus agalactiae is the only beta-hemolytic Streptococcus which yields a positive CAMP test.

CAMP test is an acronym for the authors of this test , Christie, Atkins, and Munch-Peterson, who described it in 1944.

Uses of CAMP test

1. CAMP test is used to distinguish the species Streptococcus agalactiae from other species of beta-hemolytic Streptococcus.
2. CAMP test is used to identify Listeria monocytogenes which also produces a positive CAMP reaction.

Procedure of CAMP test

1. Streak a beta-lysin–producing strain of aureus down the center of a sheep blood agar plate.
2. The streptococcal streak should be 3 to 4 cm long.
3. Streak test organisms across the plate perpendicular to the aureus streak within 2 mm. (Multiple organisms can be tested on a single plate).
4. Incubate at 35°-37°C in ambient air for 18-24 hours.
5. Group B streptococci and a few other beta-streptococci produce an enhancement of the beta-lysin activity of the aureus strain.

CAMP test limitation

• A small percentage of group A streptococci may have a positive CAMP reaction.
• Some Group A Streptococcal will be CAMP test positive if the plate is incubated in a candle jar in an atmosphere or under anaerobic conditions. Therefore, ambient air incubation should be done.
• CAMP test should be limited to colonies with the characteristic group B streptococci morphology and narrow zone beta-hemolysis on sheep blood agar.
• Extended incubation times or elevated incubation temperatures may give false-positive results.
• 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.

CAMP test principle

CAMP test detects the CAMP factor, which is a diffusible, heat-stable, extracellular protein produced by Group B Streptococcus that acts synergistically with the beta-lysin of Staphylococcus aureus to cause enhanced lysis of sheep or bovine red blood cells but not human, rabbit or horse red blood cells. A known hemolytic strain of Staphylococcus aureus (ATCC 25923) is streaked in a straight line across the center of the sheep blood agar plate. Test inoculum is streaked in a straight line (2-3 cms in length) perpendicular to Staphylococcus aureus streak but without touching it. A known Group B Streptococcus may also be streaked similarly as a positive control. Four-five test organisms may be tested per plate. The plate is incubated at 37 °C for 18-24 hours. A positive test for CAMP factor appears as “arrowhead” hemolysis between the junction of growth of Staphylococcus aureus and Group B Streptococcus. There is no enhanced or “arrowhead” hemolysis if the test isolate is not Group B Streptococcus.

The hemolytic activity of the beta-hemolysin produced by most strains of Staphylococcus aureus is enhanced by extracellular protein (CAMP factor) 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.

A similar test has been described for Listeria ivanovii, where an “arrowhead” hemolysis occurs appear between streaks of Listeria ivanovii and Rhodococcus equi.

CAMP test results

• CAMP test Positive: Enhanced hemolysis is indicated by an arrow head-shaped zone of beta-hemolysis at the junction of the two organisms.
• CAMP test Negative: No enhancement of hemolysis.

Quality control for CAMP test

• CAMP test positive: Streptococcus agalactiae (ATCC13813)—enhanced arrowhead hemolysis.
• CAMP test negative: Streptococcus pyogenes (ATCC19615)—beta-hemolysis without enhanced arrowhead formation.

Figure 1. CAMP test results

Reverse CAMP test

Reverse CAMP test can be used for differentiation of Clostridium perfringens from other Clostridium species. Here, a CAMP test positive Group B Streptococcus is streaked in the center of sheep blood agar, and Clostridium perfringens is streaked perpendicular to it. Following incubation at 37oC for 24-48 hours in anaerobic conditions, an “arrowhead” hemolysis is seen between growth of Clostridium perfringens and Group B Streptococcus. This is because of alpha toxin produced by Clostridium perfringens interacts with CAMP factor and produce synergistic hemolysis.

References

1. Savini, V., Paparella, A., Serio, A., Marrollo, R., Carretto, E., & Fazii, P. (2014). Staphylococcus pseudintermedius for CAMP-test. International journal of clinical and experimental pathology, 7 4, 1733-4.
2. Al-mohanna, Moshtaq. (2016). camp test for identification. 10.13140/RG.2.2.25876.71046