How do the interactions within the microbiota and between the microbiota and its host influence host fitness? An organism’s microbiota is a complex set of microorganisms, interacting to benefit themselves and sometimes the host organism. Mutualist bacteria have been shown to increase host fitness in multiple ways, including diet supplementation and pathogen defense. However, many microbiota studies examine effects of single species through total microbiota removal and recolonization. We used three targeted antibiotics to isolate the effects of individual bacterial groups on the fitness of the zooplankton, Daphnia magna. We also used two environmental stressors (food quality and temperature) to investigate microbiota compositional shifts in conjunction with antibiotic treatment, as environment has previously been shown to affect the composition of the microbiota. Hypotheses: For host fitness, we expected (1) host fitness to decrease as antibiotic treatments targeted more dominant groups of the microbiota, and we expected (2) host fitness to decrease when exposed to harsher environments in combination with antibiotic treatments. We also expected (3) changes in microbiota structure, composition, and diversity with antibiotic and environmental treatments.
Results/Conclusions
Hosts dosed with antibiotics that targeted larger groups within the microbiota showed decreases in total reproduction. There were no significant effects of antibiotic treatment on host survival; however, low food quality decreased Daphnia survival substantially. Microbiota composition changed with antibiotic treatment. Daphnia treated with antibiotics to knock out majority groups of the microbiota showed between an 80-95% reduction in total microbes, while those treated with all antibiotics showed a 99% reduction. Our results suggest that it is possible to selectively remove parts of the microbiota to examine host fitness effects, and that this method can be used to explore interactions among bacterial groups within the microbiota.