Comparison of infection dynamics using two strains of Beauveria bassiana which have different virulence in Anopheles stephensi

Abstract: Entomopathogenic fungi kill insects mainly by “invasion to host tissue/organ and multiplication” and “production of secondary metabolites in hemocoel”. In our previous study, we detected a high virulent strain Beauveria bassiana 60-2 and low virulent strain B. bassiana 2112. B. bassiana 2112 showed low virulence on standard contact integument infection, but it expressed high virulence on conidia micro-injection to hemocoel. It was possible to suppress pathogenicity of B. bassiana 2112, despite this strain intrinsically having virulence. In this study, infection dynamics of these two different strains were compared by observing paraffin sections of fungus-infected Anopheline to discern factors comprising differences of virulence. As a result, intense hyphal growth in haemocoel and other host tissues were observed in B. bassiana 60-2 infected mosquitoes, but only agglomerated fungal propagules were detected in B. bassiana 2112 infected individuals. B. bassiana 2112 formed mass-structures of propagules, and there was no hyphal growth at haemocoel. It is known that after invasion to host haemocoel, conidia of B. bassiana are quickly encapsulated by host immune system, then they will thwart this immunity by hyphal extension. In the case of B. bassiana 60-2, this strain can express pathogenicity by overcoming the host immune system. On the other hand, B. bassiana 2112 cannot break through the host defense systems with inhibit fungal growth and/or multiplication. These results show that virulence of B. bassiana is related to the factors which can overcome host immune system.