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ARS Home » Southeast Area » Raleigh, North Carolina » Food Science Research » Research » Publications at this Location » Publication #89884


item Fleming, Henry

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: The presence of disease-causing microorganisms on some fresh vegetables and vegetable products has been documented by researchers. The potential for these organisms to grow and cause disease may be enhanced by new packaging methods that make vegetable products look fresher and last longer on store shelves. One approach to preventing the growth of disease-causing bacteria in foods is to add harmless bacteria to a food product such as the bacteria normally used in cheese or vegetable fermentations which produce acid. Then, if the product spoils, the bacteria that grow will be the "safe" added culture which will produce acid and, thereby, prevent growth by organisms that may cause disease. Very little research has been carried out, however, to study the competitive growth of bacteria in foods. To understand how this competition works, we developed a mathematical model to help explain our experimental observations of the growth of bacteria grown singly or in combination with other bacteria. We developed a computer program to predict the outcome of the competitive growth of bacteria. Predictions from the model also indicate which characteristics of the bacterial strains are most important in controlling the outcome of competitive growth. This research will allow us to choose bacterial cultures that may best ensure the safety of selected vegetable products, and will benefit both the consumers and producers of these products.

Technical Abstract: Current mathematical models used by food microbiologists do not address the issue of competitive growth among mixed cultures of bacteria. We have developed a mathematical model which consists of a system of non-linear differential equations describing the growth of competing bacterial cell cultures. In this model, bacterial cell growth is limited by the accumulation of protonated lactic acid and decreasing pH. In our experimental system, growth of pure and mixed cultures of Lactococcus lactis and Listeria monocytogenes was carried out in a vegetable broth medium. Predictions of the model indicate that pH is the primary factor limiting the growth of L. monocytogenes in competition with a strain of L. lactis which does not produce the bacteriocin nisin. The model also predicts the values of parameters affecting the growth and death of the competing populations. Further development of this model will incorporate the effects of additional inhibitors, such as bacteriocins, and may aid in the selection of lactic acid bacteria (LAB) cultures for use in the competitive inhibition of pathogens in minimally processed foods.