Confirmatory Testing for Staphylococcus Aureus in Chicken

Staphylococcus aureus is a bacterium that can infect the skin, contaminate food, produce toxins and cause gastrointestinal illness. When testing for contamination in chicken, the first step is to isolate and multiply the bacteria so there is enough of a sample to analyze. For example, growing S.aureus on Baird-Parker agar produces gray or black circular colonies.

Biochemical Tests

• Biochemical tests are performed on the isolated colonies of bacteria obtained from the original chicken sample.

  S. aureus produces an enzyme called catalase, which breaks down toxic oxygen compounds like hydrogen peroxide. When trying to figure out if bacteria are S.aureus, analysts mix a scoop of the bacteria from the agar plate with hydrogen peroxide. If bubbles form, this indicates the presence of S. aureus.

  An ability to break down glucose in the absence of oxygen is also a characteristic of S. aureus. Presence of both these traits indicates the colonies are S. aureus.

Coagulase

• S. aureus produces the coagulase enzyme. The presence of the enzyme strongly indicates the bacteria isolated from the first special growth step are S.aureus. Coagulase can be tested for by mixing bacteria with a commercial preparation of fibrin, a protein used to form blood clots. A positive result occurs when the mixture sticks together in a lump.

Deoxyribonucleic Acid Analysis

• S. aureus is commonly found on human skin and in food products, and can be tested using methods such as polymerase chain reaction (PCR), where the bacterial DNA is extracted and copied to readable levels. While such a test can be prohibitively expensive in normal circumstances, DNA testing is useful in outbreaks when the origin and spread of the contamination needs to be accurately identified.