Culture filtrate of Bacillus atrophaeus induced abnormalities in Ascosphaera apis

The antifungal activity of broth filtrate of Bacillus atrophaeus isolate #35 was studied on Ascosphaera apis (Ascosphaeraceae-Ascomycota), a pathogen of honeybee larvae. Inhibition of hyphal growth was evident but varied with batch of fermentation broth, dilution, and duration of treatment. Significant growth inhibition and hyphal lysis were observed when the strength of the culture filtrate was at maximum. At the scanning electron microscope level, the treatment induced morphological abnormalities including swelling, stunting, constriction, and rupture of hyphae. We also found aberrations in the plasma membrane ranging from a serrated appearance in profile to total degradation in lysed hyphae. In some hyphae, almost all membrane systems including mitochondria, endoplasmic reticulum, and Golgi bodies, had changed drastically and in many cases formed membranous inclusions. Hyphae also contained electron-dense, bizarrely shaped structures that extended throughout. In the most inhibitory treatment, the hyphal wall was uneven in thickness, yet had N-acetyl-D-glucosamine label equal to that of the control, suggesting the irregular thickness of the wall was due to deposition of other polysaccharides.