Abstract
Entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae) are a group of organisms capable of infecting larvae of insects living in soil, including representatives of the family Scarabaeidae. Their insecticidal activity is related to the presence of symbiotic bacteria Xenorhabdus spp. or Photorhabdus spp. in the alimentary tract, which are released into the insect body, leading to its death caused by bacterial toxins and septicemia. Although the antibacterial activities of symbionts of entomopathogenic nematodes have been well described, there is insufficient knowledge of the interactions between these bacteria and microorganisms that naturally inhabit the alimentary tract of insects infested by nematodes. In this study, 900 bacterial strains isolated from midgut samples of Amphimallon solstitiale larvae were tested for their antagonistic activity against the selected five Xenorhabdus and Photorhabdus species. Cross-streak tests showed significant antibacterial activity of 20 isolates. These bacteria were identified as Bacillus [Brevibacterium] frigoritolerans, Bacillus toyonensis, Bacillus wiedmannii, Chryseobacterium lathyri, Chryseobacterium sp., Citrobacter murliniae, Enterococcus malodoratus, Paenibacillus sp., Serratia marcescens and Serratia sp. Since some representatives of the intestinal microbiota of A. solstitiale are able to inhibit the growth of Xenorhabdus and Photorhrhabdus bacteria in vitro, it can be assumed that this type of bacterial interaction may occur at certain stages of insect infection by Steinernema or Heterorhabditis nematodes.
Highlights
Representatives of the genera Photorhabdus and Xenorhabdus (γ-Proteobacteria, Morganellaceae) occur naturally in two types of environments
At the beginning of the study, 150 bacterial strains were randomly isolated from midguts sampled from 10 A. solstitiale individuals (L2 and L3 larvae freshly collected from the natural environment)
Potential antibacterial properties of these isolates were investigated using cross-streak tests against the following entomopathogenic nematodes (EPNs) bacteria species: At the beginning of the study, 150 bacterial strains were randomly isolated from midguts sampled from 10 A. solstitiale individuals (L2 and L3 larvae freshly collected from the natural environment)
Summary
Representatives of the genera Photorhabdus and Xenorhabdus (γ-Proteobacteria, Morganellaceae) occur naturally in two types of environments. Photorhabdus species form mutualistic relationships with nematodes of the genus Heterorhabditis, while Xenorhabdus are symbionts of nematodes from the genus Steinernema [1]. Nematodes release bacteria through regurgitation (Heterorhabditis) or defecation (Steinernema) into the hemocoel [3]. This cavity and the entire larval cadaver become another habitat of nematode-associated bacteria. Photorhabdus and Xenorhabdus bacteria produce numerous enzymes that decompose insect tissues, e.g., proteases, lipases, hemolysins, and chitinases, which enriches the environment with nutrients thereby supporting rapid growth of bacteria and nematodes [4]. EPN bacteria can dominate this habitat, as they are able to produce numerous bacteriocins and other substances with antibacterial properties [5,6,7]
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