Antagonistic effects of Streptococcus and Lactobacillus probiotics in pharyngeal biofilms

Direct antagonism towards pathogens including Streptococcus pyogenes is a proposed mechanism of pharyngeal probiosis but off‐target effects on the symbiotic microbiota of the throat are possible and may be beneficial, harmful or neutral. We have assessed the bacteriological effects of two candidate Lactobacillus probiotics and the established pharyngeal probiotic S. salivarius K12. Antagonism towards S. pyogenes and potential off‐target effects were determined using sessile monospecies biofilms and pharyngeal microcosms, respectively. The candidate probiotics were antagonistic towards S. pyogenes (rank order of increasing potency, L. acidophilus<L. plantarum<S. salivarius) in the absence of significant acidification or cell‐cell contact. S. salivarius and L. plantarum caused significant reductions in viable counts of streptococci in pharyngeal microbiotas, whilst S. salivarius also caused reductions in staphylococci. In contrast, changes in pharyngeal eubacterial DNA profiles were limited overall. In summary, the three candidate probiotics suppressed axenic S. pyogenes biofilms by mechanisms that did not depend on cell‐cell contact or acidification and did not markedly destabilise complex pharyngeal microbiotas derived from healthy individuals.