Staphylococcus aureus Testing

Staphylococcus aureus (S. aureus) is gram-positive, round-shaped bacteria. It has been identified as the causative agent in many foodborne outbreaks.

Figure 1. Electron micrograph of Staphylococcus aureus and related contaminated foods

S. aureus is widely present in human throat, nose, and skin. It usually does not cause illness in healthy people but can produce several enterotoxins to cause food poisoning. Its prevalence makes it important to food manufactures. The most common way of foods to be contaminated by S. aureus is through the contact with food handlers with skin infection or nasal carriers. Foods that can be associated with S. aureus contamination are foods involving manual handling and no reheating afterwards, such as cakes, sandwiches, and pastries. The toxins can be produced by S. aureus at the temperature from 10 to 45℃. Therefore, the refrigeration conditions can inhibit the formation of the toxin. S. aureus is salt-tolerant and can grow at a water activity as low as 0.85. So it can be found in salt-containing food products with low water activity. The bacteria is highly sensitive to heat treatment and all sanitizing agents. It can be destroyed by normal cooking procedure and pasteurization. The presence of S. aureus or its enterotoxins on food processing equipment or in processed foods can indicate the poor sanitation. The toxins produced by S. aureus are more resistant to heat and may survive during food processing.

The poisoning of S. aureus is a gastrointestinal illness caused by the consumption of foods contaminated with toxins formed by S. aureus. The main symptoms of S. aureus poisoning include vomiting, nausea, abdominal cramping, and diarrhea as well as sometimes fever. People usually recover within two days and some severe cases may require hospitalization.

The presence of S. aureus in processed foods with high numbers (>10⁴ CFU/g) indicates that an unacceptable level of contamination. The level of S. aureus less than 20 CFU/g is generally regarded as satisfactory. The Microbiological Criteria for non-bottled drinks requires the level of S. aureus to be less than 100 CFU/mL. The infective dose of S. aureus is approximately greater than 10⁵ CFU/g to produce toxins to cause people sick. In most cases the monitoring of S. aureus is required by regulations.

Staphylococcus aureus Testing of Food at Alfa Chemistry

The standard methods (EN/ISO, FDA BAM and AOAC) that have been widely used to enumerate S. aureus are culture-based methods using general microbiological testing supplies. Selective mediums are used to incubate S. aureus for appropriate time. Then it is followed by the plate counting technique or the most probable number (MPN) to determine the level of S. aureus in samples. Plating method for S. aureus uses solid medium of Baird-Parker agar. MPN method is a multi-step assay including presumptive, confirmed, and completed phases based on statistical analysis. The confirmation test (such as coagulase test) is then performed using biochemical identification technique.

Alfa Chemistry offer accurate and validated testing platforms for the enumeration and confirmation of S. aureus in various food samples to meet customers' needs. We also have experience working on different standards and regulations.

Platform: Plate counting or MPN technique

Limit of detection: <10 CFU/g

Sample type: Solid and liquid food samples

Sample size: 25 g