MODELING OF THE COMPETITIVE GROWTH OF LISTERIA MANOCYTOGENES AND LACTOCOCCUS-LACTIS IN VEGETABLE BROTH

Current mathematical models used by food microbiologists do not addres s the issue of competitive growth in mixed cultures of bacteria. We de veloped a mathematical model which consists of a system of nonlinear d ifferential equations describing the growth of competing bacterial cel l cultures. In this model, bacterial cell growth is limited by the acc umulation of protonated lactic acid and decreasing pH. In our experime ntal system, pure and mixed cultures of Lactococcus lactis and Listeri a monocytogenes were grown in a vegetable broth medium. Predictions of the model indicate that pH is the primary factor that limits the grow th of L. monocytogenes in competition with a strain of L. lactis which does not produce the bacteriocin nisin. The model also predicts the v alues of parameters that affect the growth and death of the competing populations. Further development of this model will incorporate the ef fects of additional inhibitors, such as bacteriocins, and may aid in t he selection of lactic acid bacterium cultures for use in competitive inhibition of pathogens in minimally processed foods.