Opsonophagocytic assay as a potentially useful tool for assessing safety of enterococcal preparations.

Enterococci are commonly used in foods and food additives. A number of putative virulence factors are usually evaluated to assure that the strains used are not harmful. We propose an additional test to assess the safety of these bacteria by testing the susceptibility to opsonophagocytic killing. One probiotic Enterococcus faecalis strain was compared to a collection of 27 clinical isolates and our results indicate that 89% of the clinical strains were less susceptible to killing mediated by normal rabbit sera. Opsonophagocytic killing is the best in vitro surrogate for a protective immune response against bacteria, and the susceptibility of bacteria against normal rabbit sera indicates that these strains may not be able to survive in the bloodstream of the host. Further studies comparing a larger collection of pathogenic strains with commensal isolates are necessary to confirm these findings.