Exploring the Biological Control Potential Targeting the Ornithodoros lahorensis: Virulence Evaluation of Beauveria bassiana Strain XJBB-01 and Activity Characterization of Its Key Protease

Background Ornithodoros lahorensis is a soft tick species that parasitizes a variety of hosts, including humans and livestock, and can transmit pathogens such as relapsing fever spirochetes, posing a threat to animal husbandry and public health. Chemical control is the primary method for tick management, but over-reliance on acaricides can easily induce tick resistance and cause environmental pollution. Beauveria bassiana is currently the most extensively studied and applied entomopathogenic fungus worldwide which has been widely used in the biological control of agricultural and forestry pests as well as urban insects. However, research on the tick-killing efficacy of B.bassiana remains relatively scarce. Methods The hatching rate of eggs and the mortality rate of adults of O.lahorensis after infection with the B.bassiana strain XJBB-01 were determined. Then enzyme characteristics and tick-killing activity of recombinant chitinase (rBbchit1) as well as serine protease (rBbPr1) were identified after being purified through a prokaryotic expression system. Results After infection with B.bassiana XJBB-01 strain at t spore concentrations of 1.0×10 ⁷ , 1.0×10 ⁸ , and 1.0×10 ⁹ cfu/mL, the hatching rate of eggs was 20.83%, 8.33%, and 0%, and the mortality rate of adult ticks was 79.17%, 87.5%, and 100% at 14 d respectively. The enzyme characteristics results showed that the relative molecular mass of rBbchit1 and rBbPr1were 39 kDa and 42 kDa, respectively, after expression and purification. rBbchit1 exhibits an enzyme activity of 103 U/mg at its optimal temperature of 40 °C and optimal pH 5.0. In terms of rBbPr1, the enzyme activity was 172 U/mg at its optimal temperature of 30 °C and optimal pH 8.0. After submerged for 5 s in the recombinase solution, there was no significant difference between the rBbchit1 treatment group and the control group under the scanning electron microscope, and the hatching rate was 91.67 % by day 10. In the rBbPr1 treatment group, the contents of the eggs were dissolved, forming cavities, and the hatching rate dropped to 16.67% by day 10, which revealed a significant difference compared to the control group. After treating the adults of O.lahorensis with rBbchit1, a large number of hemorrhagic spots appeared on the cuticle, and some molting ticks died before the molting progress was completed, with white crystallization occurring on the abdomen. The highest mortality rate of adult ticks was 66.77% by day 10. In the rBbPr1 treatment groups, a few hemorrhagic spots appeared on the cuticle of ticks, and the exoskeleton of dead ticks became wrinkled and brittle. The maximum mortality rate of rBbPr1against adult ticks reached to 58.33% by day 10. Conclusion This study revealed that B.bassiana XJBB-01 strain and its key enzymes had obvious pathogenic effects on O.lahorensis . Among both enzymes, rBbchit1 showed excellent insecticidal activity against adult ticks, and rBbPr1 showed significant ovicidal activity. These findings provided a theoretical basis for the development of biocontrol agents for the prevention and treatment of ticks especially in O.lahorensis.