Serratia marcescens isolated from red fire ants, leaf-cutter ants, and humans: their dose effect against fire ants

Fire ants (RIFA) native from Argentina are a worldwide distributed pest inflicting great damage to the US biodiversity and economy. Chemical control has been inefficient and contaminant. Only parasitoids from Argentina have been released in the US and although established their effect on ants cannot be assessed yet. Other natural enemies should be released to act synergistically with parasitoids. We report on the effect of Serratia marcescens isolates on RIFA mortality. This enterobacterium from water effluents, previously claimed to be completely safe for humans and animals, has been patented by others and shown to kill RIFA mounds after the third week using a bacterial concentration from 109 -1013. We tested the specificity of several strains against Solenopsis invicta from Argentina. We isolated the bacterium from an Atta vollenweideri queen and RIFA workers; we also used the RM66262 clinical isolate from a patient with urinary tract infection. Strains were biotyped as Serratia with biochemical tests and Serratia marcescens by using specific 16S rDNA primers. Bacteria were cultured in YMEA and resuspended in water to count four concentration treatments: 105, 106, 107, and 108. Worker ants were sterilized and after 1-2 days inoculated and kept in Petri dishes with sugar water. All strains tested were capable of killing RIFA, with the RIFA and Atta strains being the fastest and the clinical strain the slowest. Some colonies exhibited significant differences among treatments, whereas other colonies did not. The RIFA strain showed a decrease in its virulence through time becoming less virulent reaching clinical strain values. Since the strains tested were effective in killing RIFA, like the effluent strain, infection could highly likely occur the other way around. Therefore, we recommend testing any S. marcescens strain against a wide range of organisms before releasing it into the environment for RIFA biological control.