COMPARATIVE STUDY OF ANTIMICROBIAL ACTIVITY OF LACTIC ACID BACTERIA ISOLATED FROM DIFFERENT VEGETABLES

L.V. RIVERO, M.J. RODRÍGUEZ VAQUERO, M.A. TORRES SOPORSKY , F.M. SAGUIR

Córdoba

Congreso; Congreso Argentino de Microbiología General.SAMIGE; 2015

SAMIE

Consumer demand today is for fresh and minimally processed (MP) vegetables with high level of safety. Lactic acid bacteria (LAB) can inhibit the pathogen microorganisms growth in contaminated vegetables by the formation of antimicrobial substances like organic acids and/or bacteriocins. The aim of this study was evaluate comparatively the antimicrobial activity of LAB isolated from different vegetables cultivated in our region (pepper, lettuce and eggplant) against pathogens mainly found in contaminated vegetables such as Escherichia coli ATCC 25922, Listeria monocytogenes, Salmonella typhimurium, Enterococcus faecalis and Escherichia coli 700 and their action mode. Antagonistic activities of LAB were screened using the drop test for which an active culture was spotted onto the surface of MRS agar and incubated for 48 h at 30°C to allow colonies to develop. Then, cells of the strains to be tested for sensitivity (indicator) were inoculated (5 log10 cfu/ml) into 7 ml of soft BHI and poured over the plate on which LAB were grown. Plates were incubated at 37°C for 48 h. The effect ofthe antibacterial activity on sensitive cells was investigated in cell-free culture supernatants (SN) obtained at the end of exponential growth of each LAB strain. SN were treated as separate fractions (F), FA (neutralized pH SN), FB (neutralized pH SN treated with catalase 0.1mg/ml, 37°C,1 h) and FC (neutralized pH SN treated with trypsin 1 mg/ml, 37°C, 1 h). The growth of the sensible microorganisms in control medium and added with each SN fraction was measured by optical density at 560nm in a microplate reader. Only 3 strains of the total of 16 LAB tested (SLGR4, SLM7 from lettuce) and (SB1 from eggplant) showed no antimicrobial activity. The remaining strains showed moderate (halo diameters £ 20) and good (halo diameters > 20 mm) inhibitory activity profiles when screened by the drop test depending on LAB and indicator strain tested. In general S. typhimurium, L. monocytogenes and E. coli ATCC 25922 were the most sensitive to the inhibitory action with inhibition halos up to 20 mm for 100% of the positive LAB strains, while only one strain from lettuce showed a good inhibitory level against E. faecalis. When the FA was added to control medium there was no inhibition of sensible bacteria growths except for the JP11 strain from pepper. In this case E. coli and S. typhimurim grew lower than 27% and similar to control in the FA or FB and FC presence respectively. Thus, the LAB isolated from lettuce (4), eggplant (7) and pepper (2) showed, in general good antimicrobial activity which would be associated to the organic acids production and in the JP11 strain also to a bacteriocin like substance, supporting their potential as biocontrol agents of MP