The house finch ocular microbiome is altered by infection, but does not predict disease transmission

Recent studies have highlighted the importance of host-associated microbial communities in inhibiting growth or colonization of pathogenic species and mitigating disease severity. Much research has been conducted on how the microbiome of individuals may impact their disease susceptibility; however, less attention has been given to the role of microbiomes in population-level disease processes. In this study, we examined the effects of the ocular microbiome on pathogen load, disease severity, and transmission success of the conjunctival bacterial pathogen Mycoplasma gallisepticum (MG) in house finches (Haemorhous mexicanus). Fifteen male house finches were exposed to MG and housed pairwise with uninfected females for 32 days. Swabs for ocular microbiome characterization via 16 S rRNA gene amplicon sequencing were collected from male and female finches before and nine days after MG inoculation. MG load and disease severity were quantified throughout the experiment for all individuals. We examined whether ocular microbiome composition prior to MG infection was associated with pathogen load, disease severity, or transmission efficacy. Although the presence of certain taxa in the ocular microbiome is associated with disease protection in this species, we did not find any evidence that ocular microbiome composition was associated with any metric used to quantify disease transmission. MG infection did alter the resident bacterial communities, which aligns with results from previous studies. However, this was only true for the indicator birds that were directly inoculated with MG. Our results suggest the resident ocular microbiome of house finches shifts in responses to direct contact with pathogens, but may be more resilient, at least at the timescales examined, when host exposure is via an infected conspecific. This suggests that outcomes of interactions between pathogens and resident microbial communities may depend on the mode or dose of pathogen exposure.