Streptococcus pneumoniae and Haemophilus influenzae are commensal bacteria that inhabit the human nasopharynx. Both species can migrate from this anatomical niche and in doing so, cause various diseases. Both S. pneumoniae and H. influenzae were present in middle ear tissues of chronic OM patients1. However, the study of the interplay between these pathogens is a recent field of study, and there is controversy in the current literature about the patterns of competition or cooperation between these pathogens2,3,4,5 . This study investigates the interactions between H. influenzae and S. pneumoniae in both the planktonic and biofilm state in co-culture conditions. To date, we have determined that S. pneumoniae is able to predominate over H. influenzae in both the planktonic and biofilm state in co-culture growth studies, dramatically reducing H. influenzae viability. This effect held true for the laboratory strains and various clinical isolates of both species. These initial experiments were in batch culture and the competitive interaction was observed to occur after 24hrs in co-culture, but not after 6hrs, suggesting the competitive effect is time-specific or growth phase dependent. No extracellular component of S. pneumoniae seemed to be the damaging agent; it was not a bacteriocin or hydrogen peroxide derived from S. pneumoniae6 . The effect of pH however was shown to be important in these competitive interactions; the competitive advantage of S. pneumonaie was not observed at an alkaline pH (pH 8.0), but did occur at neutral (pH 7.4) or acidic pH (pH 6.8). An important aspect of this study is to analyse the gene expression patterns occurring between these pathogens that allow for them to co-exist. We also aim to determine the molecular pathways that vary under changes in pH and thereby alter the interaction between these bacteria. These aspects of the study will provide a systems perspective of the molecular pathways that enable these pathogens to co-exist under select conditions.