Microbial diversity of raw bovine milk as a source of microorganisms endowed with antimicrobial activity against pathogens

Cheesemaking procedures applied for the production of traditional dairy products, including Protected Designation Origin (PDO) cheeses, usually include wild bacteria from raw milk and natural starters. These sources of microbial diversity have been largely exploited for the improvement of dairy products quality and safety. However, recent studies demonstrated these traditional foods are also a good source of probiotic bacteria. The application of a validated novel digestion/enrichment protocol to 15 raw milk samples allowed the isolation of 11 lactic acid bacteria strains belonging to Enterococcus faecium, Lactobacillus brevis, L. gasseri, , L. johnsonii, L. reuteri L. vaginalis and Lactococcus lactis species. This work reports the isolation of potentially probiotic lactobacilli from raw milk samples paying particular attention to antimicrobial activities played by the natural isolate Lactobacillus reuteri LB1-7 against shiga toxin-producing enterohaemorrhagic Escherichia coli (EHEC) strains. L. reuteriLB1-7 was assayed for the production of hydroxypropionaldehyde (HPA) verifying its antimicrobial activity against several EHEC strains. Antimicrobial assays were carried out in bovine rumen fluid as well as in samples collected from the bovine rectum. Under both conditions the EHEC was totally suppressed when incubated together with L. reuteri LB1-7 and supplemented with 80 mM glycerol. The addition of LB1-7 or glycerol alone did not modify EHEC survival in any condition. This work underlines that microbial diversity still occurring in raw milk could be an useful source of strains to be employed in the improving of food quality and safety. In addition, results related to the suppression of EHEC strains open the possibility to counteract the EHEC shedding by cattle thanks to a direct-fed microbial supplementation.