A model experimental gel surface for the growth of bacteria on foods

A method is presented for the growth of bacteria on the surfaces of gels as a model of the changes that occur on food surfaces, and is demonstrated for Sal monella typhimurium. The method is based on a gel cassette, which consists of a frame holding a layer of gel, in this case gelatin, formed between two PVC windows. After formation of a sterile gel, one of the PVC windows is removed to expose the gel surface for inoculation. Using this technique, the effect of sucrose and sodium chloride on the growth rate of S. typhimurium growing as surface colonies was compared with that of immersed colonies and planktonic cells in broth and was found to follow the order: broth>immersed colonies>surface colonies. The maximum numbers of cells decreased with increasing sucrose concentration and this decrease was more marked for surface colonies than for immersed colonies or planktonic cells. These results indicate that predictions of the growth rate of bacteria on food surfaces may be erroneous where these predictions are based on data collected in broth. (C) 1997 Academic Press Limited.