Antibiotic production is a wide-spread tactic adopted by microbial organisms to harm microbial competitors. Whether antimicrobial compounds are capable of impacting animal competitors is much less resolved. Janzen was one of the first to propose that antibiotic compounds may be capable of deterring animal competitors, but few tests of this theory exist. Because saprotrophic invertebrates compete with microbial taxa over habitat and food resources, we hypothesize that invertebrates may be targets for antibiotic compounds. Here we test whether antibacterial and antifungal compounds impact the survival and fitness of three detritus-feeding invertebrates: isopods, millipedes, and termites. We then assess whether the mechanisms of antibiotic effects is the disruption of invertebrate microbiomes. Finally, we test whether invertebrate organisms detect and avoid areas of antibiotic activity.
Results/Conclusions
The effect of antibiotics on growth and survival for our three focal taxa--isopods, millipedes, and termites—varied but was generally negative. Isopods and millipedes had lower survival rates when raised in all three antibiotic environments, though the magnitude of these effects was much larger for isopods. Termite survival was decreased by synthetic antifungals and antibacterials, but not by the natural antibacterial, streptomycin. Contrary to predictions, the composition of invertebrate microbiomes was not altered by exposure to antibiotic treatments. Isopods avoided all areas of antibiotic activity, while termites only avoid antifungal environments. By contrast, millipedes did not avoid antibiotic compounds. Our results support the hypothesis that antibiotic production by microbes, long considered as a potent mechanism of competition between microbes, can also be effective between phylogenetic domains. At the same time, we document intriguing variation among three common invertebrate taxa in patterns of mortality and avoidance among three antibiotic compounds. Combined, our results suggests that, as predicted by Janzen, antibiotic production may be one of the underlying chemical mechanisms microbes use to deter animal competitors and contribute to the diversity of the detrital food webs.