Inhibition of pneumococcal growth and biofilm formation by human isolates of <i>Streptococcus mitis</i> and <i>Streptococcus oralis</i>

<i>Streptococcus pneumoniae</i> (pneumococcus) infections remain a major public health issue despite the use of vaccines and antibiotics. Pneumococci asymptomatically colonize the human upper respiratory tract, a niche shared with several commensal <i>Streptococcus</i> species. Competition for space and nutrients among species sharing the same niche is well documented and tends to be more intense among closely related species. Based on this rationale, a screening of several commensal streptococci isolated from the human upper respiratory tract led to the identification of strains of <i>Streptococcus mitis</i> and <i>Streptococcus oralis</i> capable of inhibiting most pneumococcal strains, across diverse serotypes and genotypes. This inhibition was partially or wholly linked to the expression of novel bacteriocins. The selected <i>S. mitis</i> and <i>S. oralis</i> strains significantly disrupted pneumococcal biofilms, indicating a potential for using commensals as biotherapeutics to control pneumococcal colonization, a key step in preventing disease and transmission.