The Wallemiomycetes are a classoffungi in the division Basidiomycota. It consists of the single orderWallemiales, containing the single familyWallemiaceae, which in turn contains the single genusWallemia. The phylogenetic origin of the lineage was placed to various parts of Basidiomycota, but according to the analysis of a larger dataset it is a sister groupofAgaricomycotina.[3][4] The genus contains species of xerophilic molds that are found worldwide. The seven described species (W. sebi, W. ichthyophaga, W. muriae, W. mellicola, W. canadensis, W. tropicalis, and W. hederae) are distinguished by conidial size, xerotolerance, halotolerance, chaotolerance, growth temperature regimes, extracellular enzyme activity profiles, and secondary metabolite patterns.[1][5] They are typically isolated from low-moisture foods (such as cakes, bread, sugar, peanuts, dried fish), indoor air dust, salterns and soil.[1]W. sebi is thought to be one of the causes of the hypersensitivity pneumonitis known as the farmer's lung disease,[6] but since the other species were recognised and separated from W. sebi only recently, their role in the disease cannot be excluded.[1]
Tolerance to low water activity is generally much more frequent among ascomycetous than basidomycetous fungi, and xerotolerant fungi are also able to grow in regular growth media with normal water activity (unlike, for example, halophilic Archaea).[7] However, species from the genus Wallemia are an exception to both of these rules: all species can tolerate high concentrations of sugars and salts (W. ichthyophaga grows even in media saturated with sodium chloride), while W. muriae and W. ichthyophaga cannot be cultivated unless the water activity of the medium is lowered.[1]
Studies on Wallemia sebi showed that it produces numerous secondary metabolic compounds, including walleminol, walleminone, wallemia A and C, and azasteroid UCA1064-B.[8] A comprehensive research on other species of the class discovered that secondary metabolites are consistently produced by Wallemiomycetes and their production is – contrary to common presumptions – increased as a response to increasing NaCl concentration. In particular an increase in NaCl concentration from 5% to 15% in the growth media increased the production of the toxic metabolites wallimidione, walleminol and walleminone.[9]
Cell wall and morphological changes of Wallemia species are thought to play a major role in adaptation to low water activity.[10]
The whole genome sequences of W. sebi[3] and W. ichthyophaga[4] are available.
^ abcdeZalar P, Sybren de Hoog G, Schroers HJ, Frank JM, Gunde-Cimerman N (May 2005). "Taxonomy and phylogeny of the xerophilic genus Wallemia (Wallemiomycetes and Wallemiales, cl. et ord. nov.)". Antonie van Leeuwenhoek. 87 (4): 311–28. doi:10.1007/s10482-004-6783-x. PMID15928984. S2CID4821447.
^Sneh B, Jabaji-Hare S, Neate S, Dijst G (1996). Rhizoctonia species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control. Springer. p. 20. ISBN978-0-7923-3644-0.
^Reboux G, Piarroux R, Mauny F, Madroszyk A, Millon L, Bardonnet K, Dalphin JC (June 2001). "Role of molds in farmer's lung disease in Eastern France". American Journal of Respiratory and Critical Care Medicine. 163 (7): 1534–9. doi:10.1164/ajrccm.163.7.2006077. PMID11401869.
