Streptococcus mitis bacteriocins drive contact-dependent lysis of S. pneumoniae facilitating transformation in multispecies environments

Many streptococci are naturally competent, acquiring environmental DNA through transformation. This includes pathogens like Streptococcus pneumoniae and commensals like Streptococcus mitis, which can exchange genetic material through horizontal gene transfer (HGT). For example, S. mitis can acquire pneumococcal capsules, leading to its misidentification in polymicrobial samples such as those obtained from the upper respiratory tract. Understanding the drivers of HGT between these species is therefore critical. Here, we characterize a competence-induced bacteriocin cluster in S. mitis. These bacteriocins lyse pneumococci, promoting DNA release and enhancing gene transfer in dual-species biofilms. Our findings uncover a mechanism by which competence-associated predation promotes interspecies HGT, shaping the evolution and epidemiology of streptococcal populations.