The CAMP test is a biochemical assay used in diagnostic microbiology to identify certain types of bacteria. It relies on synergistic hemolysis, which involves the combined action of two different toxins that break down red blood cells. The test is a classic example of how bacterial products can interact to produce a unique and visible effect on a culture medium. This technique provides a rapid and relatively simple way to distinguish between closely related bacterial species based on their metabolic output.
The Core Biological Mechanism
The foundation of the CAMP test is the interaction between two distinct, diffusible proteins secreted by two different bacterial species. The test organism, typically the one being identified, produces the CAMP factor, a heat-stable, extracellular protein toxin.
The CAMP factor’s activity requires the beta-lysin, or sphingomyelinase, produced by the indicator organism, Staphylococcus aureus. The S. aureus beta-lysin is an enzyme that acts on the sphingomyelin lipids in the red blood cell membrane. This initial action modifies the membrane but usually causes only incomplete damage at the incubation temperature.
The CAMP factor then binds to the altered membrane structure, specifically targeting glycosylphosphatidylinositol (GPI)-anchored proteins. The factor enhances the damage initiated by the beta-lysin, leading to the complete and rapid breakdown (lysis) of the red blood cells in a localized area. This cooperative destruction is termed synergistic hemolysis.
The CAMP factor is a pore-forming toxin, meaning it creates holes in the cell membrane. This molecular partnership results in a clear, visible zone of enhanced lysis on the blood agar plate, which constitutes a positive test result.
Laboratory Procedure and Result Interpretation
Performing the standard CAMP test involves inoculating a sheep blood agar plate with two different bacterial strains. The procedure begins by streaking a known Staphylococcus aureus strain down the center of the agar plate, which serves as the source of the indicator enzyme, the beta-lysin.
Next, the unknown organism is streaked perpendicular to the S. aureus line. The two streak lines must come into close proximity but not physically touch, maintaining a separation of about one to two millimeters. This small gap ensures the visible reaction results from the diffusion of secreted toxins, not physical contact between the colonies.
The inoculated plate is incubated at 35°C to 37°C for 18 to 24 hours, typically in ambient air. Incubation conditions are important, as certain anaerobic or carbon dioxide-enriched environments can sometimes cause false-positive results with other bacterial species. After incubation, the plate is examined for zones of hemolysis around the streaks.
A positive result is identified by a distinct, enhanced zone of complete red blood cell lysis. This clear, wedge-shaped area forms an arrowhead or flame shape at the intersection point of the two streaks, confirming synergistic action between the two toxins. A negative result shows no such enhanced hemolysis at the junction, only the narrow, background hemolysis that may be produced by the individual organisms themselves.
Clinical Importance of Identification
The primary clinical application of the CAMP test is the presumptive identification of Streptococcus agalactiae, also known as Group B Streptococcus (GBS). The test is highly effective for quickly distinguishing GBS from other beta-hemolytic Streptococcus species that do not produce the CAMP factor.
GBS is a major concern in obstetrics because it causes serious, life-threatening infections in newborns, including sepsis and meningitis. Rapid identification through the CAMP test allows for prompt clinical action, such as screening pregnant women and administering antibiotics to prevent infant transmission.
While GBS is the most common target, other organisms also yield a positive CAMP reaction. Notably, Listeria monocytogenes, which causes severe foodborne illness, also produces a positive result. The reliability and simplicity of the CAMP test make it a valuable tool for initial screening in clinical laboratories.