Reproductive failure of Heterorhabditis marelatus in the Colorado potato beetle: Evidence of stress on the nematode symbiont Photorhabdus temperata and potential interference from the enteric bacteria of the beetle

The nematode Heterorhabditis marelatus and its symbiont, Photorhabdus temperata, have been documented to successfully attack and kill, but cannot reproduce within Leptinotarsa decemlineata. In vitro growth of P. temperata in L. decemlineata hemolymph did not cause the bacterium to switch to a secondary phenotype based on dye absorption from NBTA medium, and its growth in hemolymph was similar to that found in nutrient broth. Microscopic examination of body fluid from P. temperata-killed L. decemlineata revealed a high prevalence of P. temperata exhibiting abnormal growth characteristics, such as filamentous forms that regularly exceeded 50 μm in length. We also noted that many P. temperata infections were accompanied by substantial growth of competing bacteria that could interfere with growth of either P. temperata or H. marelatus. We isolated six species of enteric bacteria from our laboratory colony of L. decemlineata and identified them as species of Serratia, Pseudomonas, Acinetobacter, Pantoea, Klebsiella, and Lactococcus. When P. temperata was grown alongside these enteric isolates on plates, all but the Lactococcus isolate inhibited the growth of P. temperata to a greater extent than P. temperata was able to inhibit their growth. Surprisingly, the Serratia isolate appeared to be slightly stimulated by the presence of P. temperata.