Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2002
Publication Date: 2/1/2003
Citation: OSCAR, T.P. COMPARISON OF PREDICTIVE MODELS FOR GROWTH OF PARENT AND GREEN FLUORESCENT PROTEIN PRODUCING STRAINS OF SALMONELLA. JOURNAL OF FOOD PROTECTION. 2003. V. 66(2). pg. 200-207. Interpretive Summary: Computer models, which predict when, how fast and to what extent pathogens, such as Salmonella, will grow in food that has been improperly stored, are a valuable tool for helping processors make the proper decision about the safety of food. However, current computer models were developed in laboratory broths and may not accurately predict the growth of pathogens in food. Models have not been routinely developed in food because it is difficult to detect and count pathogens in food that contains larger numbers of other bacteria. To make it easier to detect pathogens in food, scientists have used recombinant DNA methods to develop new strains of pathogens that produce a green fluorescent protein from jellyfish that makes them visible in the presence of large numbers of other bacteria. However, in order for these modified pathogen strains to be useful for developing food safety models, they must first be demonstrated to grow the same as the parent strains from which they were made. In the current study, three green fluorescent strains of Salmonella were studied for the ability to grow on sterile chicken and it was found that they did not grow the same as the parents. Thus, it was concluded that they would not be good strains for developing food safety models. However, based on the results of the study, a new strategy was envisioned for producing green fluorescent strains of Salmonella that will grow the same as the parents.
Technical Abstract: The green fluorescent protein (GFP) from the jellyfish Aequorea victoria can be expressed in and used to follow the fate of Salmonella in microbiologically complex ecosystems, such as food. As a first step in evaluating GFP as a tool for the development of predictive models for naturally contaminated food, the current study was undertaken to compare the growth kinetics of parent and GFP producing strains of Salmonella. Growth kinetics of stationary phase cells of parent and GFP strains of Salmonella Enteritidis, S. Typhimurium and S. Dublin were compared using a previously established sterile chicken burger model system. Growth curves, which were conducted at constant temperatures from 10 to 48 C, were fit to a three-phase linear model to determine lag time, specific growth rate, and maximum population density. Secondary models for the growth parameters as a function of temperature were generated and compared between the parent and GFP strain pairs. Effects of GFP on the three growth parameters were significant and were affected by serotype and incubation temperature. Expression of GFP reduced specific growth rate and maximum population density while having only a small effect on lag time of the three serotypes. Results indicated that the GFP strains had different growth kinetics than the parent strains and thus, would not be good marker strains for developing predictive models for naturally contaminated food.