Morphological responses to high sugar concentrations differ from adaptation to high salt concentrations in the xerophilic fungi Wallemia spp.

Fungi from the food-borne basidiomycetous genus Wallemia, which comprises Wallemia ichthyophaga, Wallemia muriae and Wallemia sebi, are among the most xerophilic organisms described. Their morphological adaptations to life at high NaCl concentrations are reflected in increased cell-wall thickness and size of cellular aggregates. The objectives of this study were to examine their growth and to define cell morphology and any ultrastructural cell-wall changes when these fungi are grown in low and high glucose and honey concentrations, as environmental osmolytes. We analysed their growth parameters and morphological characteristics by light microscopy and transmission and scanning electron microscopy. Wallemia ichthyophaga grew slowly in all of the sugar-based media, while W. muriae and W. sebi demonstrated better growth. Wallemia ichthyophaga adapted to the high glucose and honey concentrations with formation of larger cellular aggregates, while cell-wall thickness was increased only at the high glucose concentration. Wallemia muriae and W. sebi demonstrated particularly smaller sizes of hyphal aggregates at the high glucose concentration, and different and less explicit changes in cell-wall thickness. Adaptive responses show that the phylogenetically more distant W. ichthyophaga is better adapted to high salt conditions, whereas W. muriae and W. sebi cope better with a high sugar environment.