The Christie-Atkins-Munch-Peterson (CAMP) test is a straightforward biochemical assay widely used in clinical and food microbiology laboratories. This test provides a rapid method for characterizing bacterial isolates based on their hemolytic properties. Its primary application is to differentiate between harmless and pathogenic Listeria species. The CAMP test relies on observing an enhanced breakdown of red blood cells when two specific bacterial products interact.
Differentiating Pathogenic Listeria
The ability to quickly and accurately identify Listeria species is important for public health, particularly in food safety. While the genus Listeria contains several species, only Listeria monocytogenes is consistently associated with severe illness in humans. Other species, such as Listeria innocua or Listeria seeligeri, are considered non-pathogenic. Differentiation is necessary to avoid overreacting to the presence of benign bacteria in food or clinical samples.
Listeriosis, the disease caused by L. monocytogenes, is rare but carries a high fatality rate, often approaching 20 to 30%. This bacterium is dangerous for vulnerable groups, including pregnant women, newborns, the elderly, and those with weakened immune systems. L. monocytogenes is psychrotrophic, meaning it can grow and multiply even at refrigeration temperatures as low as 0°C. Accurate detection of this specific pathogen is necessary for clinical diagnostics and monitoring the food supply chain.
The Scientific Principle of Synergistic Hemolysis
The underlying mechanism of the CAMP test is a synergistic interaction between two different bacterial hemolysins, which are toxins that cause the breakdown of red blood cells. Listeria monocytogenes produces an exotoxin known as Listeriolysin O (LLO). LLO is a pore-forming protein and is a major factor in the pathogen’s ability to survive and replicate inside host cells.
The test relies on LLO from L. monocytogenes reacting with a specific toxin produced by an indicator organism, typically a beta-hemolysis producing strain of Staphylococcus aureus. S. aureus secretes beta-lysin, a protein that damages red blood cell membranes. While both LLO and beta-lysin cause some hemolysis on their own, the key observation is the synergistic effect.
When the two toxins diffuse into the agar medium and meet, they significantly enhance the destructive effect on the red blood cells. This enhanced lysis creates a zone of complete clearing on the blood agar plate. The zone is much larger than the zones produced by either bacterium alone, confirming the specific biochemical interaction characteristic of L. monocytogenes.
Test Procedure and Reading the Results
The CAMP test is performed on a blood agar plate, typically using sheep blood agar, where the synergistic reaction is most clearly visible. The procedure begins by streaking a known, beta-lysin-producing Staphylococcus aureus strain in a straight line across the center of the plate. This streak acts as the source of the indicator toxin necessary for the synergistic reaction.
Next, the unknown Listeria sample is streaked perpendicular to the S. aureus line, creating a cross shape. The two streaks must come within a few millimeters of each other but must not physically touch, allowing the secreted toxins to diffuse and interact. The plate is then incubated at 35°C to 37°C for 18 to 24 hours.
A positive result for Listeria monocytogenes is indicated by a distinct, enhanced zone of hemolysis at the intersection of the two bacterial streaks. This clearing is classically described as having an “arrowhead” or “flame” shape, pointing towards the S. aureus streak. This shape forms because the LLO from the Listeria diffuses outward, causing greater breakdown where it meets the highest concentration of S. aureus beta-lysin.
A negative result shows no such enhancement; the Listeria streak shows either no hemolysis or only the faint, narrow zone it produces on its own. Although Listeria seeligeri can show some enhancement with S. aureus, the marked, clear arrowhead formation remains the standard for identifying L. monocytogenes.