Assessment of Insecticidal Virulence of Pir-Like Toxins From Vibrio Parahaemolyticus (Gamma Proteobacteria, Vibrionaceae)

Abstract

Vibrio parahaemolyticus Pir-like toxins are the causative agent of Acute Hepatopancreatic Necrosis Disease (AHPND) in several shrimp species. These toxins are similar to the Pir toxins produced by Photorhabdus bacteria, which are known to be insecticidal to a wide range of insects. Both the Vibrio and Photorhabdus Pir toxins have a binary structure with two components: Pra and Prb. Together, the two components are required to cause mortality in insects. However, it is unknown if V. parahaemolyticus PirAB-like toxins have insecticidal activity. Thus, the current study focused on assessing the activity of these toxins considering last instar larvae of the greater wax moth, Galleria mellonella, as the model organism. The effects of temperature, portal of entry, toxin concentration, and toxin source were the variables tested to assess the toxins’ virulence. Infected larvae were processed for histology to observe the effect of the toxins on tissue pathology. When comparing the two portals of entry, the toxins showed a higher insect mortality when delivered through the digestive route. Interestingly, a higher mortality was achieved at the lower concentration tested, and toxins seemed to be more virulent at 22°C. The spiracles portal of entry presented low virulence at the tested higher dose. When comparing recombinant and purified proteins from overnight cultures, a higher insect mortality was achieved with the recombinant proteins. Preliminary histology observations showed a lack or shortening of microvilli in the gut of treated larvae compared to the healthy control. Overall, V. parahaemolyticus PirAB-like toxins had very low insecticidal activity in G. mellonella larvae. Future studies should focus on expanding the host range to other insect orders, conducting dose response assays, and assessing the effect of in vitro association of Pra and Prb.