Assessment of Pathogenic Bacteria Transfer From Pristionchus Entomophagus (Nematoda: Diplogasteridae) to the Invasive Ant Myrmica Rubra and Its Potential Role in Colony Mortality in Coastal Maine

Abstract Background:Necromenic nematode Pristionchus entomophagus has been frequently found in nests of the invasive European ant Myrmica rubra in coastal Maine, United States. The nematodes may contribute to ant mortality and collapse of colonies by transferring environmental bacteria. M. rubra ants naturally hosting nematodes were collected from collapsed wild nests in Maine and used for bacteria identification. Virulence assays were carried out to validate acquisition and vectoring of environmental bacteria to the ants.Results:Multiple bacteria species, including Paenibacillus spp., were found in the nematodes’ digestive tract. Serratia marcescens, Serratia nematodiphila, and Pseudomonas fluorescens were collected from the hemolymph of nematode-infected Galleria mellonella larvae. Variability was observed in insect virulence in relation to the site origin of the nematodes. In vitro assays confirmed uptake of RFP-labeled Pseudomonas aeruginosa strain PA14 by nematodes. Bacteria were highly concentrated in the digestive tract of adult nematodes, a small amount of bacteria were observed in the digestive tract of juveniles with a more significant amount on their cuticle, and none on the cuticle of adults. RFP-labeled P. aeruginosa were not observed in hemolymph of G. mellonella larvae, indicating an apparent lack of bacterial transfer from juvenile nematodes to the insects despite larval mortality.Host species was the primary factor affecting bacterial community profiles. Spiroplasma sp. and Serratia marcescens sequences were shared across ants, nematodes, and nematode-exposed G. mellonella larvae. Alternative to the idea of transferring bacteria from environment to host, we considered whether nematode-exposure might disorder or depauperate the endobiotic community of an insect host. While total bacterial diversity was not statistically lower in nematode-exposed G. mellonella larvae when compared to controls, 16 bacterial sequence variants were less abundant in nematode-exposed larvae, while three were increased, including Serratia, Pseudomonas, and Proteus.Conclusions: This study suggests that transfer of bacteria from nematodes to ants is feasible, although largely serendipitous, and may possibly contributed to ant death and/or collapse of wild colonies in Maine. Hypothetically, the use of an engineered biological control, such as nematodes carrying specifically-seeded bacterial species, may be effective, especially if the pathogenic bacteria are normally found in soil ecosystems and represents a low risk for biosafety control.