Mechanisms underlying proximity between oral commensal bacteria

Complex polymicrobial biofilm communities are abundant in nature particularly in the human oral cavity where their composition and fitness can affect health. While the study of these communities during disease is essential and prevalent, little is known about interactions within the healthy plaque community. Here we describe interactions between two of the most abundant species in this healthy microbiome, Haemophilus parainfluenzae and Streptococcus mitis . We discovered that H. parainfluenzae typically exists adjacent to Mitis group streptococci in vivo with which it also positively correlated based on microbiome data. By comparing in vitro coculture data to ex vivo microscopy we revealed that this co-occurrence is density dependent and further influenced by H 2 O 2 production. We discovered that H. parainfluenzae has a more redundant, multifactorial response to H 2 O 2 than related organisms and that the integrity of this system enhances streptococcal fitness. We also show that Mitis group streptococci can act as an in vivo source of NAD for H. parainfluenzae and that streptococci in vitro evoke patterns of carbon utilization from H. parainfluenzae that are similar to those observed in vivo . Our findings describe mechanistic interactions between two of the most abundant and prevalent members of healthy supragingival plaque that contribute to their survival in vivo .