The suite of commensal microbes inhabiting a host can interact with invading parasites and host physiology in ways that alter parasite growth and disease manifestation. In many systems, experimental disruptions of host microbiomes have been found to help or hinder invaders. A prior experiment in house finches (Haemorhous mexicanus) found that intact ocular microbiomes were protective against conjunctivitis caused by Mycoplasma gallisepticum (MG). Here, we conducted a dose experiment to examine whether the protective effects of the ocular microbiome vary with pathogen exposure dose. We hypothesized that protection would be strongest at low infective doses of the pathogen because protective commensal microbes would be less likely to be overwhelmed by invading pathogen. Our five dose treatments were fully factorial with an antibiotics treatment at each pathogen dose to perturb and knock down resident microbial communities. We then monitored visible pathology and collected swab samples to assess differences in disease expression and pathogen load across the study period.
Results/Conclusions
Our results were inconsistent with those from the prior experiment that found protective effects of the ocular microbiome against MG inoculation at a dose similar to our highest dose. Unexpectedly, we found a trend toward lower pathology in birds given antibiotics, regardless of dose. We also found very low infection rates at our lowest MG dose, and disruption of the ocular microbiome did not alter that. One potential reason for our results conflicting with past work is that the ocular microbiomes started with a different composition of microbes in this study, reducing its protective capacity. The antibiotics may have even knocked down community members that could facilitate the pathogen. Future data from amplicon sequencing will help us tease apart this discrepancy. Regardless of the cause, the lack of repeatability of protective effects of the ocular microbiome in house finches reveals the sometimes-limited generalizability of ecological results and the likely context dependency in many symbiotic interactions. More microbiome studies will further illustrate the extent of this variation and the contexts in which microbiomes protect hosts against invading pathogens.