Staphylococcus aureus

Staphylococcus aureus is a gram-positive bacterium that grows in grape-like clusters, which is what its name records: staphyle, a bunch of grapes, and kokkos, a berry, with aureus, golden, for the colour of its colonies. It forms no spores and grows with or without air. The property that makes it a concern in curing is its tolerance of salt and dryness. It grows in up to twenty per cent salt and down to a water activity of 0.86, the lowest of any foodborne pathogen. It is a common resident of skin and the nose, which is one route onto the meat.

The organism was named by Alexander Ogston, an Aberdeen surgeon, who described the clustered cocci in wound pus around 1880. Friedrich Rosenbach isolated it in pure culture in 1884 and named the golden species aureus, separating it from the white Staphylococcus albus, now Staphylococcus epidermidis. Its role in food poisoning was established by Barber in 1914.

The hazard is a toxin rather than an infection, and that distinction governs how it is controlled. When the organism grows to high numbers it produces staphylococcal enterotoxins, which are heat-stable. They survive boiling, so once they are in a food, cooking, drying and fermentation will not remove them. Growth and toxin production have different limits, and this is the practical point: the organism grows from 7 to 48 degrees C, across pH 4 to 10, and down to a water activity of 0.86, but it produces toxin only over a narrower range, needing a water activity above about 0.90 and a pH above about 4.5. Toxin production falls further without air and when other organisms compete strongly.

**How it endangers cured meat.** Because it tolerates salt, the salt of a cure does not control it. The point of risk is the warm early stage of a fermented sausage, after the meat is salted and stuffed but before the starter culture has lowered the pH. If that stage is held too warm for too long, Staphylococcus aureus from the raw meat or from handling can grow and produce heat-stable toxin, which then stays in the product through all the drying and acidifying that follow. This is why a reliable starter culture and enough sugar to feed it matter: they lower the pH quickly, and the lactic acid bacteria and the useful Staphylococcus xylosus compete the organism down before it produces toxin. It is also why the warm time before the pH falls is limited, measured as degree-hours. Within the Temple it sits at the Water Activity corner, because it shows that a low water activity alone is not a sufficient control.

The toxin produces no change that the senses can detect. A sausage that carries it looks, smells and tastes normal, so control has to be preventive rather than by inspection: clean handling to keep starting numbers low, a prompt and reliable fermentation to close the warm window, and fermentation temperatures and times kept within the degree-hours limits.