Comparison of susceptibility to opsonic killing by in vitro human immune response of Enterococcus strains isolated from dairy products, clinical samples and probiotic preparation.

The genus Enterococcus like other LAB has also been featured in food and probiotic industry for decades due to its specific biochemical traits and beneficial health claims. At the same time, some enterococcal strains present an emerging pool of opportunistic pathogens for humans and are frequently armed with potential virulence factors. Thus, there is a need to assure the safety of enterococci before their use in food and probiotic preparations. Opsonophagocytic assay is an important test for the safety assessment of enterococci. In the present study comparative safety assessment of the different enterococcal strains isolated from dairy products, faeces, clinical samples and a commercial probiotic preparation was carried out by in vitro opsonophagocytic assay. Eleven strains of Enterococcus spp. were tested for their susceptibility to killing by opsonophagocytic assay. Among them, six isolates (Enterococcus faecium strain) were from our previous study (isolated from dairy products and faecal sample), four were from clinical samples and one from a probiotic preparation. Five out of six previous isolates and the isolate from probiotic preparation showed higher susceptibility to killing in contrast to the clinical isolates. The difference in the susceptibility to opsonic killing among the clinical and non-clinical Enterococcus isolates may be attributed to the presence of a capsule in the former, which protect them against the opsonophagocytic killing. Thus, these susceptible E. faecium strains may be designated as safe. However, certain other virulence traits must be evaluated prior to their exploitation in food and probiotic preparations.