Assessing the inhibitory activity of culture supernatants against foodborne pathogens of two psychrotrophic bacteria isolated from river trout.

There is a need for new natural products with antimicrobial activity to treat multidrug resistant bacteria that can cause human illness. Some of them are foodborne pathogens. Two different Gram-negative psychrotrophic strains were isolated from healthy trout river samples (Salmo trutta). Based on phenotypic characterization, proteomics, genotyping and phylogenetic analyses of 16 rRNA gene, strains TCPS12 and TCPS13 were identified as Shewanella baltica and Pseudomonas fragi, respectively. Both of them produced an exopolysaccharide that showed antimicrobial activity against four foodborne pathogens. P. fragi supernatant (AS13) showed higher antimicrobial activity than S. baltica supernatant (AS12) against all tested pathogens. The stability of the antimicrobial activity of AS13 was assessed against Enterococcus faecalis ATCC 29212 under different conditions. This solution was stable when exposed for 30 min to temperatures ranging from 40 to 100 °C. In addition, it retained its activity within a pH range of 2-8 during 2 h of incubation, showing higher activity at pH 6. Serine proteases and α-amylase inactivated significantly the antimicrobial activity of AS13, suggesting that the active molecule could most likely be a glycoprotein. These products are interesting for their possible application as biopreservatives in the food industry.