Adhesion of the Entomopathogenic Fungus Beauveria ( Cordyceps ) bassiana to Substrata

Abstract

The entomopathogenic fungus Beauveria bassiana produces at least three distinct single-cell propagules, aerial conidia, vegetative cells termed blastospores, and submerged conidia, which can be isolated from agar plates, from rich broth liquid cultures, and under nutrient limitation conditions in submerged cultures, respectively. Fluorescently labeled fungal cells were used to quantify the kinetics of adhesion of these cell types to surfaces having various hydrophobic or hydrophilic properties. Aerial conidia adhered poorly to weakly polar surfaces and rapidly to both hydrophobic and hydrophilic surfaces but could be readily washed off the latter surfaces. In contrast, blastospores bound poorly to hydrophobic surfaces, forming small aggregates, bound rapidly to hydrophilic surfaces, and required a longer incubation time to bind to weakly polar surfaces than to hydrophilic surfaces. Submerged conidia displayed the broadest binding specificity, adhering to hydrophobic, weakly polar, and hydrophilic surfaces. The adhesion of the B. bassiana cell types also differed in sensitivity to glycosidase and protease treatments, pH, and addition of various carbohydrate competitors and detergents. The outer cell wall layer of aerial conidia contained sodium dodecyl sulfate-insoluble, trifluoroacetic acid-soluble proteins (presumably hydrophobins) that were not present on either blastospores or submerged conidia. The variations in the cell surface properties leading to the different adhesion qualities of B. bassiana aerial conidia, blastospores, and submerged conidia could lead to rational design decisions for improving the efficacy and possibly the specificity of entomopathogenic fungi for host targets.Under intensive study for use as a biopesticide, the entomopathogenic fungus Beauveria bassiana displays a broad host range and is able to target a number of diverse arthropod species (7,15,16,23,26,27,31). Strains of B. bassiana have been selected for control of insects and other arthropods that act as disease vectors, including mosquitoes and ticks (6, 22); crop pests, such as whiteflies, caterpillars, grasshoppers, and borers (5, 9, 21, 25, 36); and even ecologically hazardous, invading pests, such as fire ants and termites (4,8). The varied cuticles of these organisms represent the first barrier to the pathogen, and attachment of fungal propagules to the cuticle is the initial event in establishing mycosis. Air currents, dispersion via water droplets, and saprophytic growth over substrata inhabited by insects are considered the major routes for contact of fungal spores with host cuticles (2). Upon contact, fungal cells bind to the cuticle and initiate a developmental program that includes the production of specialized infection structures, such as germ tubes and penetrant hyphae (2,17). If the infection is successful, the fungus grows across the cuticle surface and penetrates the host cuticle to invade and proliferate within the hemolymph, which ultimately results in the death of the host.Fungal cell attachment to the cutic...

Highly specific host-pathogen interactions influence Metarhizium brunneum blastospore virulence against Culex quinquefasciatus larvae

“…Conidia of EPF such as M. brunneum and B. bassian a, possess hydrophobins in the outer layer of the cell wall whereas these are absent in blastospores [22]. These differences could influence adhesion and consequently impact on EPF efficacy and probably host specificity [22]. The current study shows for the first time that M. brunneum blastospores have different degrees of adhesion to the cuticles of different insect host species.…”

“…Spore attachment is influenced by hydrophilic and hydrophobic characteristics of the host surface and the fungal cell wall [41]. Holder and Keyhani [22] demonstrated that hydrophilic blastospores of Beauveria bassiana bind rapidly to hydrophilic surfaces and bind poorly to hydrophobic surfaces. However, blastospores require a longer incubation time to bind to weakly polar surfaces than to hydrophilic surfaces.…”

“…However, blastospores require a longer incubation time to bind to weakly polar surfaces than to hydrophilic surfaces. In contrast, aerial conidia of B. bassiana adhere poorly to weakly polar surfaces and rapidly to both hydrophobic and hydrophilic surfaces but could be readily washed off the latter [22]. Conidia of EPF such as M. brunneum and B. bassian a, possess hydrophobins in the outer layer of the cell wall whereas these are absent in blastospores [22].…”

“…In contrast, aerial conidia of B. bassiana adhere poorly to weakly polar surfaces and rapidly to both hydrophobic and hydrophilic surfaces but could be readily washed off the latter [22]. Conidia of EPF such as M. brunneum and B. bassian a, possess hydrophobins in the outer layer of the cell wall whereas these are absent in blastospores [22]. These differences could influence adhesion and consequently impact on EPF efficacy and probably host specificity [22].…”

“…The first step is mediated by preformed physio-chemical properties of the spores themselves e.g. hydrophobic and electrostatic forces [21,22], and the second step involves secretion of enzymes and mucilage. Hydrolytic enzymes degrade the cuticle, release nutrients and facilitate penetration [23–25], while mucilage is often secreted to enhance binding to the host cuticle [26].…”