Microbial survival on food contact surfaces in the context of food hygiene regulation

Bacterial food poisoning causes substantial suffering and financial loss worldwide. One way organisms enter foods is via cross contamination directly or indirectly from structural and food contact surfaces. An 'in situ' method was developed for the detection of surviving bacteria on surfaces. Samples of test surfaces were overlaid with agar and after incubation, colonies were visualised by reaction with nitroblue tetrazolium, which was reduced to a purple insoluble dye. It was shown that the death of bacteria applied as liquid films to surfaces, occurred largely at the point of drying. For impervious surfaces (ceramic, stainless steel, glass and polystyrene), surface type had little effect on survival. In contrast, survival was markedly affected by the nature of the suspension fluid in which cells were dried. In deionised water, survival was low and for Gram negative organisms was strongly influenced by cell density. Where cells were dried in simulated food films (containing brain heart infusion, NaCI, serum or sucrose), survival values increased with increasing concentrations and approached 100% for Staphylococcus aureus cells suspended in 10% w/v sucrose. The survival of Gram positive organisms on impervious surfaces was generally greater than for Gram negative organisms and consistent with this observation, scanning electron microscopy indicated that Gram negative cells collapsed during drying. On wood surfaces, survival was generally similar to or higher than on impervious surfaces. However, neither of the Gram positive organisms tested (Staphylococcus aureus and Listeria monocytogenes) could be recovered following inoculation onto the surface of the African hard-wood, iroko, although Gram negative organisms survived well. Scanning electron microscopy confirmed that cells had not been adsorbed below the wood surface and an ethanol-soluble toxic factor was extracted from iroko, which killed Staphylococcus aureus cells, but had no effect on the viability of Salmonella enteritidis PT4. In metabolic studies, it was also shown that this extract caused greater inhibition of ethanol and glucose oxidation in Gram positive than in Gram negative bacteria. Following the washing of iroko in ethanol for 36 days, the survival of Staphylococcus aureus was similar to or higher than on beech wood or ceramic, showing that the toxic factor had been effectively removed. The results are discussed in relation to current legislation and associated guidance concerning the use of surface materials in food businesses and domestic settings. (author)Available from British Library Document Supply Centre- DSC:DXN057411 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo