An indigenous gut bacterium, enterococcus faecalis (Lactobacillales: Enterococcaceae), increases seed consumption by harpalus pensylvanicus (Coleoptera: Carabidae)

Microbial symbioses likely drive the evolution of diet within animals, yet these symbiotic relationships remain poorly understood for many organisms. The bacterial endosymbiont Enterococcus faecalis is found in the intestinal tract of the beetle Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) and is thought to contribute to the digestion of the insect's seed diet. We tested whether E. faecalis increases seed consumption by H. pensylvanicus. The feeding assay consisted of 4 dietary treatments fed: 1) antibiotics and E. faecalis; 2) antibiotics and no E. faecalis; 3) no antibiotics and E. faecalis; and 4) no antibiotics and no E. faecalis, in which seed consumption of the beetles was measured. Beetles administered antibiotics and then E. faecalis consumed greater weights of seeds and had both decreased efficiency of conversion of ingested material to biomass (E.C.I.) per beetle and decreased efficiency of conversion of digested material (E.C.D.) to biomass per beetle. These data provide further evidence that a gut microbiota dominated by E. faecalis facilitate seed consumption by H. pensylvanicus, possibly by contributing digestive enzymes to their host. Further research is needed on the evolutionary relationship between E. faecalis and granivorous insects, and on how these facultative symbioses could influence the trophic placement of animals within complex food webs.