Polymicrobial-Host Interactions during Infection.

Microbial pathogenesis research has, historically, focused on the study of infections as monomicrobial events. However, the advent of next generation sequencing and culture-independent identification methods has revealed that many, if not most, infections are polymicrobial either in origin or in manifestation. Polymicrobial infections are often associated with increased infection severity and poorer patient outcome. Multiple infecting microbes can interact synergistically to induce virulence traits, alter the infected niche, or modulate the host immune response, all of which can promote polymicrobial infection. Importantly, a polymicrobial environment at the time of inoculation, consisting of multiple pathogens or pathogens in combination with the native microbiota, can contribute to the pathogenic progression of a single predominant organism at the time of diagnosis. Hence, in order to completely understand and elucidate the impact of these polymicrobial interactions on infection outcomes, a thorough examination of the entire microbial community present throughout the pathogenic cascade is required: from the time of inoculation to symptomology to resolution. In this review, we highlight the themes of metabolite exploitation, immune modulation, niche optimization, and virulence induction that contribute to polymicrobial infections. We focus on recent literature about microbe-microbe and microbe-host interactions that promote polymicrobial infections with an emphasis on understanding these interactions to identify better interventions for these sometimes complex infections.