CHARACTERIZATION OF ANTI-LISTERIAL LACTIC-ACID BACTERIA ISOLATED FROMTHAI FERMENTED FISH PRODUCTS

Thai fermented fish products were screened for lactic acid bacteria ca pable of inhibiting Listeria sp. (Listeria innocua). Of 4150 assumed l actic acid bacteria colonies from MRS agar plates that were screened b y an agar-overlay method 58 (1.4%) were positive. Forty four of these strains were further characterized and 43 strains were inhibitory agai nst Listeria monocytogenes. The strains were inhibitory to other Gram- positive (lactic acid) bacteria probably because of production of bact eriocins. All 44 strains inhibited both Vibrio cholerae and Vibrio par ahaemolyticus and 37 were inhibitory to a mesophilic fish spoilage bac terium tan Aeromonas sp.). Inhibition of Gram-negative bacteria was at tributed to production of lactic acid. Most strains were identified as Lactobacillus spp., and all grew well at ambient temperatures (25-37 degrees C) and tolerated up to 6.5% NaCl. Glucose was fermented rapidl y in laboratory media whereas pH decreased only very slowly in fish ju ice supplemented with 4% glucose and 3.5% NaCl or in a rice-fish mixtu re. Only four of 44 isolates could degrade and ferment complex carbohy drates such as rice, potatoes and maize starch. This indicates that ot her types of bacteria may be responsible for the rapid spontaneous fer mentation of the products or that other yet-unknown factors ensure rap id fermentation. Overall anti-listerial lactic acid bacteria do occur in fermented fish products and the antibacterial activity against path ogenic bacteria indicates that they may be important in product safety . (C) 1998 Academic Press Limited.