Isolation and screening of lactic acid bacteria from Thai traditional fermented fish (Plasom) and production of Plasom from selected strains

Lactic acid bacteria (LAB) were isolated from traditional Thai fermented fish, Plasom at various fermentation periods. It was found that 138 isolates exhibited a clear zone and growth on MRS agar supplemented with CaCO_3; however, only 133 isolates were identified as LAB. Only 14 strains showed excellent inhibition zone diameters on agar when Salmonella sp. was used as an indicator for preliminary detection of antagonistic activity. Staphylococcus aureus was used for secondary screening for antagonistic activity of these 14 strains. It was found that only 7 strains exhibited good inhibition zone diameters on agar, and all of them could inhibit E. coli as the third indicator. The strains which exhibited widest zones of inhibition against Escherichia coli, S. aureus and Salmonella sp. were LPS04, LPS17, and LD219 and LPS18 respectively. Tested pathogenic strains were inhibited by 4 selected LAB. The strain which showed the best lactic acid production and pH reduction ability was LD219. Using 16s rDNA sequence analysis, LD219 was identified as Streptococcus salivarius, LPS04, LPS17 and LPS18 were identified as Enterococcus faecalis. Plasom was produced by using a mixed culture (LD219, LPS04, LPS17 and LPS18) as a starter culture compared with spontaneous and back-slopping processes. Significant differences (p < 0.05) in pH, its titratable acidity as lactic acid and number of total viable counts (TVC), LAB and Enterobacteriaceae were found in these Plasom at 0 and 8 days. However, no significant difference (p > 0.05) was observed in terms of colour, smell, taste, sour, texture and overall acceptance of Plasom produced by non-starter cultured, back-slopping and starter cultured processes.