Staphylococci and Salmonellae in Commercial Wrapped Sandwiches

Abstract

A total of 820 commercially prepared, wrapped sandwiches of various kinds were evaluated bactiologically when fresh and after 48 hr. exposure to conditions of ambient temperature encountered routinely. The pH of samples was also determined at these intervals. Coliform counts tended to decrease in salad-type sandwiches during holding. No salmonellae were found. Staphylococci were sometimes very numerous, but few were coagulase-positive. No significant increase in these could be demonstrated with aging of sandwiches. Coagulase-positive staphylococci inoculated into egg salad and chicken salad sandwiches failed to increase appreciably when held at room temperature for 48 hr., but large saprophytic populations developed under these conditions. Competition from these saprophytic organisms was thought to have been partially responsible for suppressing the growth of inoculated staphylococci. Acidity also appeared to be an important factor. Spiced ham and cheese sandwiches seem to be more hazardous. However, growth of coagulase-positive staphylococci was affected by placing the inoculum on the side of the spiced ham in contact with the mayonnaise. The customary use of commercial mayonnaise and other acid ingredients produes a low pH which with time tends to become more acid, often creating an unfavorable environment for the growth or even survival of the limited numbers of coagulase-positive staphylococci which occur. Results of this study offer some insight into the epidemiologic observation that these products, although seemingly mishandled, are rarely incriminated in food-poisoning outbreaks.