**3.5.** *Staphylococcus aureus*

The foods most associated with staphylococcal poisoning are meat products, dairy products and cream filled bakery products. In processed foods in which *S. aureus* is destroyed by processing, its presence usually indicates post-processing contamination from human skin, mouth, nose or food handlers (Lancette & Bennett, 2003). Due to the high salt tolerance of *S. aureus*, it can grow in table olives even though the low pH and the olive phenols may represent natural inhibitors (Tassou & Nychas, 1994). However, it may be isolated and enumerated in table olives for the same above mentioned reason (as contamination index).

A variety of coagulase-positive and coagulase-negative staphylococci are able to produce enterotoxins, but *S. aureus* still plays a predominant role in staphylococcal food poisoning.

Baird-Parker agar and Rabbit Plasma Fibrinogen agar are the media recommended by the International Organisation for Standardisation (ISO). Moreover, Baird-Parker agar is also used in the official AOAC method in the United States (Zangerl &Asperger, 2003). Tellurite reduction, egg yolk reaction and a high level of sodium chloride are the most applied selective chemicals added in media for *S. aureus* isolation and enumeration.

**Figure 3.** *S. aureus* colonies on Mannitol Salt Agar.

*S. aureus* colonies in Baird-Parker agar are black, with an opaque zone around the colony because of the egg yolk reaction. Another medium frequently used is the Mannitol Salt agar, which contains mannitol. Coagulase-positive staphylococci grow and produce acid from mannitol showing a yellow colony and halo in a red medium. For all these media, the chemical inhibitors usually used are not completely selective, so additional diagnostic tests may be necessary to identify *S. aureus* colonies. Microscopic examination, catalase test and coagulase test may be rapidly executed to identify *S. aureus* from isolates. Common MPN procedures may be used also for enumeration of *S. aureus*. In most cases, the methodologies need a liquid enrichment procedure to detect low numbers of staphylococci (<100 UFC/g).

Culture conditions are usually 37°C for 48 hours aerobically.

**Figure 4.** *S. aureus* strain detected by fluorescence microscopy.
