Surface characteristics of the entomopathogenic fungus Beauveria (Cordyceps) bassiana

Abstract

Marked differences in surface characteristics were observed among three types of single-cell propagules produced by the entomopathogenic fungus Beauveria bassiana. Atomic force microscopy (AFM) revealed the presence of bundles or fascicles in aerial conidia absent from in vitro blastospores and submerged conidia. Contact angle measurements using polar and apolar test liquids placed on cell layers were used to calculate surface tension values and the free energies of interaction of the cell types with surfaces. These analyses indicated that the cell surfaces of aerial conidia were hydrophobic, whereas those of blastospores and submerged conidia were hydrophilic. Zeta potential determinations of the electrostatic charge distribution across the surface of the cells varied from +22 to "30 mV for 16-day aerial conidia at pH values ranging from 3 to 9, while the net surface charge ranged from +10 to "13 mV for submerged conidia, with much less variation observed for blastospores, +4 to "4 mV, over the same pH range. Measurements of hydrophobicity using microbial adhesion to hydrocarbons (MATH) indicated that the surfaces of aerial conidia were hydrophobic, and those of blastospores hydrophilic, whereas submerged conidia displayed cell surface characteristics on the borderline between hydrophobic and hydrophilic. Insect pathology assays using tobacco budworm (Heliothis virescens) larvae revealed some variation in virulence among aerial conidia, in vitro blastospores and submerged conidia, using both topical application and haemocoel injection of the fungal cells.

Lectin mapping reveals stage-specific display of surface carbohydrates in in vitro and haemolymph-derived cells of the entomopathogenic fungus Beauveria bassiana

“…Studies on the adhesion of submerged conidia and blastospores have revealed that the former cells are able to bind a diverse range of substrata, including hydrophobic, weakly polar and hydrophilic surfaces, whereas the latter cells display some binding to weakly polar substrata but strong binding to hydrophilic surfaces (Holder & Keyhani, 2005). The relationship between the loss of fucose residues and the observed surface properties of submerged conidia is difficult to evaluate; however, the loss of multiple carbohydrate epitopes that might mask charged residues on the surface of in vitro blastospores as compared with aerial conidia is consistent with the idea that electrostatic charge plays a significant role in adhesion of these cells (Holder et al, 2007).…”

“…However, it is unclear whether the carbohydrates detected would contribute to the net negative surface charge of aerial conidia at neutral pH (Holder et al, 2007), particularly since little to no sialic acid, which would contribute to the surface charge, was detected. Studies on the adhesion of submerged conidia and blastospores have revealed that the former cells are able to bind a diverse range of substrata, including hydrophobic, weakly polar and hydrophilic surfaces, whereas the latter cells display some binding to weakly polar substrata but strong binding to hydrophilic surfaces (Holder & Keyhani, 2005).…”

“…Ultimately, the hyphal bodies form elongating hyphae which emerge from the insect cadaver, producing conidiophores for dispersal. Throughout this process, the interaction between the pathogen and the host is mediated by the fungal cell wall, which is able to respond to diverse surface cues and nutrient availability (Holder & Keyhani, 2005;Holder et al, 2007;Lewis et al, 2009). Little is known concerning the arrangement of carbohydrate molecules on the surfaces of the different developmental stages of the fungus, although such changes may have important consequences regarding adhesion, host detection, cuticle penetration and non-self recognition by host defence mechanisms.…”

Tenebrionid secretions and a fungal benzoquinone oxidoreductase form competing components of an arms race between a host and pathogen

“…Fungal spores (conidia) of the insect pathogenic fungus, B. bassiana, will attempt to attach and initiate infection essentially anywhere on host cuticle, although preferential infection sites on some hosts have been observed (21,49,50). The infection program is initiated by surface cues, and the fungus is capable of assimilating long-chain alkanes and other lipids that are often found as major constituents of the insect epicuticle (41).…”

Could insect phagocytic avoidance by entomogenous fungi have evolved via selection against soil amoeboid predators?

“…Infection by M. anisopliae and B. bassiana begins when the fungal conidium contacts the insect cuticle, initiating the expression of genes involved in cuticle adhesion and degradation (Holder & Keyhani, 2005;Cho et al, 2006a, b;Holder et al, 2007). The conidium germinates and the fungal hyphae transgress the insect cuticle, gaining access to the insect haemolymph.…”