Adhesion properties and cell surface characteristics of the entomopathogenic fungus Beauveria bassiana: A link between morphology and virulence

The entomopathogen Beauveria bassiana produces three distinct in vitro, single cell propagules: aerial conidia, blastospores and submerged conidia. Atomic force microscopy (AFM) was used to visualize the surface of aerial conidia and confirmed the presence of a rodlet layer that was absent from the surface of both blastospores, which were smooth, and submerged conidia, which appeared coarse. Interfacial free energies of interaction and hydrophobicity indicies, derived from contact angle data, and hydrocarbon partitioning revealed differential properties of the three propagules regarding cell surface hydrophobicity ranging from strongly hydrophobic (aerial conidia) to hydrophilic (blastospores). Adhesion studies with fluorescently labeled cells suggested that (1) aerial conidia bound better to hydrophobic surfaces, (2) blastospores bound better to hydrophilic surfaces, and (3) submerged conidia bound equally well to both types of surfaces. The effective surface charge (zeta potential) of the three single cell propagules was predominantly positive at low pH (pH 3-4) decreasing to negative values at higher pH values (pH 6-8). Conidial surface charge varied the most with respect to pH (+22 mV to -47 mV), while submerged conidia varied moderately (+7 mV to -13.4 mV) and the blastospores showed minor variation (+ 3.2 mV to -4.65 mV). The gene for a beauverial hydrophobin (bhd1) was identified from a suppression-subtracted library generated from cells grown in the presence of insect cuticle as opposed to glucose. Real-time, reverse transcriptase PCR of two Beauveria bassiana specific hydrophobins and a hydrophobin like protein (bhd1, bhd2 and bsn ) showed that bhd1 mRNA levels were relatively high in most cell types analyzed, with the highest abundance in submerged conidia. Transcript for bhd2 was detected primarily in aerial conidia and submerged conidia, whereas transcript for bsn was not detected under the conditions tested. A SDS insoluble/TFA soluble constituent of the cell wall of B. bassiana conidia was identified as bhd2 by peptide mass fingerprinting.