Diagnosis of Human Illness

In the diagnosis of staphylococcal foodborne illness, proper interviews with the victims and the gathering and analysis of epidemiological data are essential. Incriminated foods should be collected and examined for staphylococci. The presence of relatively large numbers of enterotoxigenic staphy-lococci is good circumstantial evidence that the food contains toxin. The most conclusive test is the linking of an illness with a specific food or, in cases where multiple vehicles exist, the detection of the toxin in the food sample(s). In cases where the food may have been treated to kill the staphylo-cocci, as in pasteurization or heating, direct microscopic observation of the food may be an aid in the diagnosis. A number of serological methods for determining the enterotoxigenicity of S. aureus isolated from foods as well as methods for the separation and detection of toxins in foods have been developed and used successfully to aid in the diagnosis of the illness. Phage typing may also

FIGURE 1 Comparative numbers of reported outbreaks and cases caused by staphylococcal species in the United States, 1993-1997.

be useful when viable staphylococci can be isolated from the incriminated food, from victims or from suspected carriers, such as food handlers. However, this approach can be limiting because there are strains of S. aureus that are not typable by this system. More recently, genetic fingerprinting techniques are being applied to characterize strains of staphylococci. Two of these approaches are pulse-field gel electrophoresis (PFGE) of DNA restriction fragment profiles (7) and DNA restriction fragment polymorphism of rRNA genes (ribotyping) (15).

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