ISOLATION AND IDENTIFICATION OF HIGH POTENTIAL ANTIMICROBIAL PRODUCING LACTIC ACID BACTERIA FROM TRADITIONAL THAI FERMENTED MINCED FISH (SOM-FAK) PRODUCTS

摘要

High potential antimicrobial producing lactic acid bacteria (LAB) were isolated from traditional Thai fermented minced fish (som-fak) products sampled from 16 different small-scale industries, located in the Northeastern region of Thailand. The numbers of LAB viable cells in the products ranged from 7.59 log CFU/g to 8.68 log CFU/g with an average of 8.21 log CFU/g. The inhibitory activities of LAB isolates were determined by agar spot tests, against various Gram-positive and Gram-negative indicatorbacteria. A total of 320 LAB isolates were tested and selected based on the presences of clear zones around the spots on MRS agar containing 0.5% CaC03. It was found that only 54 LAB isolates showed antimicrobial activities against 7 strains of indicatorbacteria tested, including Lactobacillus sake TISTR 912, Enterococcus faecalis ATCC 29212, Staphylococcus aureus ATCC 25923, Listeria monocytogenes DMST 17303, Clostridium perfringens ATCC 13124, Escherichia coli 0157:H7 DMST 12743 and Salmonella Typhimurium ATCC 13311, for primary screening of antimicrobial activities. The several indicator strains including pathogenic and spoilage bacteria were used for secondary screening of antimicrobial activities of these 54 LAB isolates. It was found that only 17 selected LAB isolates exhibited excellent inhibition zone diameters on agar plates using a deferred inhibition assay, and all of them could inhibit all of tested indicator bacteria, especially Vibrio cholera DMST 2873, Listeria ivanovii DMST 9012, S. Typhimurium ATCC 13311, S. Weltevreden DMST 15677, E. coli 0157:H7 DMST 12743 and Pseudomonas fluorescens ATCC13525. The 17 selected LAB isolates were then subjected to biochemical tests using the API 50 CHL system and they were identified as Lb. plantarum (64.7%), Pediococcus pentosaceus (23.5%), Lb. pentosus (5.9%) and Lb. paracasei spp. paracasei (5.9%). Also, their identities were confirmed by 16S rRNA sequencing and they were identified as the strains of Lb. plantarum (70.6%), P. pentosaceus (23.5%)and Lb. acidophilus (5.9%). Thus, these potent antimicrobial producer strains were selected for further study aimed on the potential use as starter cultures in som-fak production to enhance product safety and shelf life extending.