Halophily reloaded: new insights into the extremophilic life-style of Wallemia with the description of Wallemia hederae sp. nov

Wallemia comprises air- and food-borne, mycotoxigenic contaminants including the halophilic W. ichthyophaga, xerotolerant W. sebi and xerophilic W. muriae. Wallemia isolates are easily overlooked and only a comparably small number of strains have been deposited in culture collections so far. In order to better understand the natural distribution of Wallemia spp. and to encounter their natural habitats, we tested more than 300 low-water-activity substrates and 30 air samples from a wide geographical coverage. We isolated more than 150 new Wallemia strains. Wallemia sebi and W. muriae were isolated mostly from hypersaline water, low-water-activity foods, plant materials and indoor. Wallemia muriae is the dominant Wallemia species in the air of natural and human influenced environments in Europe. New isolates of W. ichthyophaga were obtained from hypersaline environments such as brine, salt crystals, salty foods and MgCl₂-rich bitterns, and from the air of hay barns in Denmark. Five halotolerant strains were recognised as a hitherto un-described species Wallemia hederae, the phylogenetic sister of the halophilic W. ichthyophaga. Wallemia spp. show in-vitro growth on media that contain the chaotropic salt MgCl₂. Wallemia ichthyophaga can grow in liquid medium enriched with 2 M MgCl₂. Never before has a microorganism been grown on comparably high MgCl₂ concentrations. Tests of the activity of a wide range of extracellular enzymes in the presence of NaCl also suggested that Wallemia is well-adapted to substrates with a reduced water activity.