Insecticidal Toxins from the Photorhabdus and Xenorhabdus Bacteria

Abstract

Insecticide resistance to the microbial insecticides Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bs) represents a serious threat to their success. Available evidence indicates that the risk for resistance to Bti is low due to the makeup of its parasporal crystal, which contains Cyt1A, Cry4A, Cry4B, and Cry11A toxic proteins. Disrupting the toxin complex in Bti enables resistance to evolve, especially in the absence of the key factor, the cytolytic toxin, Cyt1A. Cross-resistance is widespread among mosquitocidal Bacillus thuringiensis Cry toxins and the mechanisms of Cry resistance in mosquitoes are not known. Bacillus sphaericus (Bs) is at higher risk for resistance due to its singlesite action and field cases have been reported from a number of locations worldwide. Cross-resistance is reported among the various Bs isolates, although some isolates produce additional toxic proteins that can reduce cross-resistance and slow resistance evolution. Field and lab evolved resistant populations consistently show recessive and monofactorial inheritance of resistance. Resistant populations, however, have evolved a variety of molecular mechanisms causing that resistance. Traditional resistance management strategies with promise include rotations and mixtures of Bti and Bs, as well as untreated areas that provide natural refuges for susceptible alleles. Promising new strategies include genetic engineering to increase the toxin complexity targeted toward mosquito larvae, to enhance the host range of the mosquito control product, and to avoid the evolution of insecticide resistance. Regardless of the control strategy, a resistancemonitoring program alongside an integrative pest management approach is the best strategy to delay insecticide resistance.

Evaluation of responses of different ant species (Formicidae) to the scavenger deterrent factor associated with the entomopathogenic nematode-bacterium complex

“…During bacterial multiplication in the insect host, a variety of small molecules are produced that protect the cadaver from bacterial and fungal contamination (Boemare & Akhurst, 2006;Hinchliffe et al, 2010;Tobias et al, 2017) as well as invertebrates and some vertebrates (Baur et al, 1998;Foltan & Puza, 2009;Fenton et al, 2011;Gulcu et al, 2012;Jones et al, 2016;Ramalingam et al, 2017). Possibly, SDF is a small molecule (s) within the anthraquinones (AQs) which are ecologically important metabolites for Photorhabdus and appear to protect nematode-killed insects more than metabolites from Xenorhabdus that does not have AQs (Pankewitz & Hilker, 2008;Bode, 2009;Gulcu et al, 2012;Ulug et al, 2014).…”

Trophic Relationships of Entomopathogenic Nematodes in Agricultural Habitats

“…Upon infection, the physical presence of the nematodes and bacteria, along with any tissue damage, elicits the release of proteins that recognize and bind to surface sugar moieties and trigger further immune responses, e.g. C-type lectins, hemolin, peptidoglycan recognition proteins and "-1,3-glucan recognition proteins (Hinchliffe, Hares, Dowling, & ffrench-Constant, 2010). In response to these signals, the cellular immune responses include phagocytosis by hemocytes and the formation of hemocyte aggregates (nodules) (Hinchliffe et al, 2010;Lavine & Strand, 2002), whereas the humoral immune response secretes a variety of antimicrobial peptides and proteins including lysozymes, cecropins and attacin, and the prophenol-oxidase system (Hoffmann, 2003;Kanost, Jiang, & Yu, 2004;Li, Cowles, Cowles, Gaugler, & Cox-Foster, 2007).…”

“…C-type lectins, hemolin, peptidoglycan recognition proteins and "-1,3-glucan recognition proteins (Hinchliffe, Hares, Dowling, & ffrench-Constant, 2010). In response to these signals, the cellular immune responses include phagocytosis by hemocytes and the formation of hemocyte aggregates (nodules) (Hinchliffe et al, 2010;Lavine & Strand, 2002), whereas the humoral immune response secretes a variety of antimicrobial peptides and proteins including lysozymes, cecropins and attacin, and the prophenol-oxidase system (Hoffmann, 2003;Kanost, Jiang, & Yu, 2004;Li, Cowles, Cowles, Gaugler, & Cox-Foster, 2007). Nodules rapidly darken due to the synthesis of insoluble melanin related to the production of reactive oxygen species, causing the bacteria and/or nematodes to become immobile and isolated from nutrients (Cox-Foster & Stehr, 1994;Hinchliffe et al;Nappi & Vass, 2001).…”

“…The genera Xenorhabdus and Photorhabdus are motile, gram-negative bacteria in the family Enterobacteriaceae which belongs to the "-subclass of the Protobacteria (Boemare & Akhurst, 2006;Hinchliffe et. al., 2010).…”

Discovery of a Highly Virulent Strain of Photorhabdus luminescens ssp. akhurstii from Meghalaya, India

“…Sequencing and annotation of the Photorhabdus genome revealed several insecticidal toxins which vary in their modes of action; some show oral toxicity, others show injectable toxicity or both [20]. These toxins include toxin complexes (Tc) consisting of tca, tcb, tcc and tcd genes, Makes Caterpillar Toxins (MCf) which consist MCf1 and MCf2, Txp40 toxin, PirA/B, Photorhabdus insect related binary toxins, Photorhabdus virulence cassettes (PVCs), Photorhabdus insecticidal toxins (Pt toxins), Hemolysins or Hemagglutinin-Related proteins and many others [20]. However, the existence of so many different toxins in Photorhabdus is intriguing.…”