88D-18 |
Growth of lactic acid bacteria and pathogenic bacteria on MRS and Kang-Fung medium |
S. KIM, Animal Sciences and Industry, Kansas State University, 202 Call Hall, Manhattan, KS 66506-1600 and D. Y. C. Fung. Lactic acid bacteria (LAB) are important in food fermentation, food spoilage, and food preservation. Lactobacillus deMan Rogosa Sharpe (MRS) medium has been routinely used for enumeration and cultivation of Lactobacillus species, however, various kinds of microorganism other than Lactobacillus spp. can grow well in MRS due to its nutritive components. Kang-Fung (KF) medium developed for selection of Pediococcus spp. in mixed culture was reported to inhibit the growth of some pathogens by the effect of phenylethanol in the formulation. In this study, MRS and KF media were used as basal media and 0.2 % v/v of lactic acid (L) was supplemented to produce MRSL (pH 5.5) and KFL (pH 5.5), respectively. These media were developed to selectively cultivate lactic acid bacteria only. Pure cultures of pathogenic bacteria and LAB were diluted with 0.1 % peptone water to give 106-107 CFU/mL and appropriate portions of diluted cultures were plated using a Spiral Plater (Spiral Biotech, model SFB-410, Bethesda, MD). After incubation at 35-37 °C for 24-36 hrs, the population of colonies on each medium was determined. Both MRSL and KFL medium completely inhibited the growth of the 18 pathogenic bacteria tested such as E. coli O157:H7, Salmonella typhimurium, Listeria monocytogenes, Yersinia enterocolitica, and Staphylococcus aureus, while Lactobacillus plantarum, Pediococcus pentosaceus, and Leuconostoc mesenteroides grew well on MRSL and KFL medium under both aerobic and anaerobic condition. Lactococcus lactis did not grow well on KFL. By lowering the pH to 5.5 in MRS and KF medium, pathogenic bacteria can be eliminated in the media while allowing LAB to grow. These media can be used to selectively enumerate and monitor growth of LAB in mixed culture studies such as effects of starter cultures on food-borne pathogen in sausage fermentation.
Session 88D, Food Microbiology: General
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