Staphylococcal growth and enterotoxin production in ready to eat food at different storage time and temperatures.

Abstract

Abstract : Foodborne intoxication caused by Staphylococcus aureus is one of the commonly reported illnesses that is caused by the ingestion of staphylococcal enterotoxin preformed in contaminated foods. Although Staphylococcus aureus and other coagulase positive Staphylococcus species are the main responsible for foodborne intoxication, investigators emphasise that staphylococcal enterotoxins are able to be produced by some coagulase negative staphylococci strains that could be a potential cause of foodborne intoxication. In the present study, the effect of different storage time and temperatures on the growth and subsequent enterotoxin production by coagulase positive and negative Staphylococci representative strains (Staphylococcus aureus (NCTC 10657), Staphylococcus aureus (NCTC 8532), and Staphylococcus epidermidis (NCTC 12100) was observed in a laboratory system. The growth and enterotoxins production by these commercially available cultures was examined at 3 different temperatures (5°C, 30°C and 44°C) considering the food product which is likely to encounter under the anticipated duration and storage (5°C), the storage temperature to which the product may be subjected by retailer to consumer (30°C) and the temperature (44°C the average summer temperature in Bahrain) at which the product may subjected by retailer or consumer during transferring and also supporting the growth and enterotoxin production (Maximum enterotoxin production temperature) by enterotoxigenic Staphylococci. Rapid analytical methods are essential for routine monitoring purpose and public health protection. Different enterotoxins detection methods have been developed recently and in this project reversed passive latex agglutination (RPLA) assay has been used for staphylococcal enterotoxins A, B, C and D detection and semi quantitative estimation in ready to served custard samples filtrates which have been spiked with the enterotoxigrnic Staphylococcus aureus NCTC 10657 and Staphylococcus epidermidis NCTC 12100. The obtained results suggest that SET-RPLA toxin detection kit can be applied directly to food with good correspondence of culture results. Neither significant changes in the original cell's count nor toxin production were observed among the three examined strains at 5°C during the 3 storage days. Lowering the incubation (storage) temperature of the contaminated food samples inhibit the growth and enterotoxins production by the coagulase positive and coagulase negative staphylococci (CPS & CNS) strains. The optimum growth temperature for the examined staphylococci species was at 30°C, therefore, the long storage of foodstuff at this Staphylococcal critical growth temperature encourages the growth and the subsequent enterotoxins production. However, the optimum enterotoxins production temperature was at 44°C where the growth rate of Staphylococcus species is fast due to high metabolic activity. The growth rate of Staphylococcus epidermidis was observed to be slower than the other examined Staphylococcus aureus species therefore no enterotoxins was detected in the first and second storage days at both 30°C and 44°C though the bacterial count was more than 107CFU/ml. However, SEC was detected in the third day at 30°C and 44°C with an agglutination degree of (+2) and (+3) respectively. There was no direct effect observed of changing the pH value on the staphylococcal enterotoxin production during the 3 incubation day. The drop in pH value due to microbial growth decreases the rate of growth and the subsequent enterotoxin production. Staphylococcal growth and enterotoxin production can not be controlled by single factor and the effectiveness of the storage temperature on the growth and the subsequent and enterotoxin production by staphylococci influenced by the effectiveness of other growth factors such as time, pH, water activity (section 1.7). Coagulase activity and mannitol fermentation test have been suggested by several studies as good indicators of enterotoxigenicity. However, neither mannitol fermentation nor coagulase activities appeared to be specific for enterotoxigenicity of all Staphylococcus species (e.g. Staphylococcus epidermidis NCTC 12100). Both tests can be useful to differentiate between these species and to detect the potentially enterotoxigenic CPS and CNS strains. The most reliable test for enterotoxigenicity is the direct detection of these enterotoxins directly in food. In this study, SET RPLA toxin detection kit represent a simple, sensitive, quick and versatile method for routine monitoring purposes, however, its regarded as presumptive and must be confirmed by other standard methods (Feng, 1996).