Entomopathogenic potential of bacteria associated with soil-borne nematodes and insect immune responses to their infection.

Soil-borne nematodes establish close associations with several bacterial species. Whether they confer benefits to their hosts has been investigated in only a few nematode-bacteria systems. Their ecological function, therefore, remains poorly understood. In this study, we isolated several bacterial species from rhabditid nematodes, molecularly identified them, evaluated their entomopathogenic potential on Galleria mellonella larvae, and measured immune responses of G. mellonella larvae to their infection. Bacteria were isolated from Acrobeloides sp., A. bodenheimeri, Heterorhabditis bacteriophora, Oscheius tipulae, and Pristionchus maupasi nematodes. They were identified as Acinetobacter sp., Alcaligenes sp., Bacillus cereus, Enterobacter sp., Kaistia sp., Lysinibacillus fusiformis, Morganella morganii subsp. morganii, Klebsiella quasipneumoniae subsp. quasipneumoniae, and Pseudomonas aeruginosa. All bacterial strains were found to be highly entomopathogenic as they killed at least 53.33% G. mellonella larvae within 72h post-infection, at a dose of 106 CFU/larvae. Among them, Lysinibacillus fusiformis, Enterobacter sp., Acinetobacter sp., and K. quasipneumoniae subsp. quasipneumoniae were the most entomopathogenic bacteria. Insects strongly responded to bacterial infection. However, their responses were apparently little effective to counteract bacterial infection. Our study, therefore, shows that bacteria associated with soil-borne nematodes have entomopathogenic capacities. From an applied perspective, our study motivates more research to determine the potential of these bacterial strains as biocontrol agents in environmentally friendly and sustainable agriculture.

The Silkworm as a Source of Natural Antimicrobial Preparations: Efficacy on Various Bacterial Strains.

The global spread of multi-resistant pathogens responsible for infections, which cannot be treated with existing drugs such as antibiotics, is of particular concern. Antibiotics are becoming increasingly ineffective and drug resistance is leading to more difficult-to-treat infections; therefore, new bioactive compounds with antimicrobial activity are needed and new alternative sources should be found. Antimicrobial peptides (AMPs) are synthesized by processes typical of the innate immune system and are present in almost all organisms. Insects are extremely resistant to bacterial infections as they can produce a wide range of AMPs, providing an effective first line of defense. The AMPs produced by insects therefore represent a possible source of natural antimicrobial molecules. In this paper, the possibility of using plasma preparations from silkworm (Bombyx mori) larvae as a source of antimicrobials was evaluated. After simple purification steps, insect plasma was analyzed and tested on different Gram-positive and Gram-negative bacterial strains. The results obtained are encouraging as the assays on Escherichia coli and Enterobacter cloacae showed significant decrease in the growth of these Gram-negative bacteria. Similar results were obtained on Gram-positive bacteria, such as Micrococcus luteus and Bacillus subtilis, which showed strong susceptibility to the silkworm AMPs pool. In contrast, Staphylococcus aureus displayed high resistance to Bombyx mori plasma. Finally, the tested plasma formulations were assessed for possible storage not only at 4 °C, but also above room temperature. In conclusion, partially purified plasma from silkworm could be a promising source of AMPs which could be used in formulations for topical applications, without additional and expensive purification steps.

Drosophila suzukii Susceptibility to the Oral Administration of Bacillus thuringiensis, Xenorhabdus nematophila and Its Secondary Metabolites.

Drosophila suzukii, Spotted Wing Drosophila (SWD), is a serious economic issue for thin-skinned fruit farmers. The invasion of this dipteran is mainly counteracted by chemical control methods; however, it would be desirable to replace them with biological control. All assays were performed with Bacillus thuringiensis (Bt), Xenorhabdus nematophila (Xn), and Xn secretions, administered orally in single or combination, then larval lethality was assessed at different times. Gut damage caused by Bt and the influence on Xn into the hemocoelic cavity was also evaluated. In addition, the hemolymph cell population was analyzed after treatments. The data obtained show that the combined use of Bt plus Xn secretions on larvae, compared to single administration of bacteria, significantly improved the efficacy and reduced the time of treatments. The results confirm the destructive action of Bt on the gut of SWD larvae, and that Bt-induced alteration promotes the passage of Xn to the hemocoel cavity. Furthermore, hemocytes decrease after bioinsecticides treatments. Our study demonstrates that combining bioinsecticides can improve the efficacy of biocontrol and such combinations should be tested in greenhouse and in field in the near future.