Comparative study of antimicrobial activity of lactic acid bacteria isolated from different vegetables

Abstract

The growing demand for fresh and minimally processed vegetables has led to an increase in the number and variety of products available to consumers worldwide. Although fruits and vegetables have a natural microbiota, generally non-pathogenic, could be contaminated with microorganisms. Furthermore certain lactic acid (LAB), normally present in fruit and vegetables, can produce antimicrobial substances with able to inhibit the growth of pathogens and spoilage microorganism. The aim of this study was to evaluate the antimicrobial activity of LAB isolated from pepper, lettuce and eggplant against Escherichia coli 25922 ATCC, Listeria monocytogenes, Salmonella typhimurium, Enterococcus faecalis and Escherichia coli 700. The antimicrobial activity was determined using the drop test, a sample of LAB culture was placed on plates with MRS agar and incubated for 24 h at 30°C, then poured onto which 7 ml of broth common semi-solid inoculated with the sensitive strain (5 log cfu/ml), the plates were incubated at 37°C for 24 h. The responsible factor of the growth inhibition by LAB was investigated using cell-free culture supernatants of the LAB at the end of the exponential growth phase. Supernatants were treated as separate fraction, fraction A (neutralized pH supernatant), fraction B (neutralized pH supernatant treated with catalase 0.1mg ml−1, 37°C-1 h) and fraction C (neutralized pH supernatant treated with trypsin 1 mg ml−1, 37°C-1 h). The growth of the sensible microorganism in presence of an aliquote of each fraction was follow with the measurement of optical density at 560 in a microplate reader. Between 23 isolated LAB, only 3 (SLGR4, SLM7 and JNB23) did not possesses antimicrobial activity against tested microorganisms. In all LAB, the inhibition zone ranged from 12 to 30 mm. Salmonella typhimurium and Listeria monocytogenes were the most sensitive. The microplate assay showed that fraction ?A? of supernatants were the most effective to inhibit the growth of all bacteria, demonstrating that the production of organic acids was the mechanism of inhibition used by LAB. Only, with JP11 strain the inhibitory mechanism against Salmonella typhimurium and E. coli 25922 could be related with a bacteriocin production. Our results showed the potential application of LAB with antimicrobial activity for the elaboration of microbiological safe fermented food.