Fungal Xerophiles (Osmophiles)

Abstract

Xerophilic fungi are yeasts and moulds that are capable of growth at or below a water activity (aw) of 0.85. These microorganisms have developed physiological mechanisms that enable their biochemical pathways to function in environments where little water is available. External conditions of low aw are sensed by membrane osmosensors, and xerophiles then accumulate glycerol as a compatible solute to balance the internal and external osmotic pressure. They also modify their membranes to retain this glycerol within the cell. As a group, xerophiles are extremely important in the spoilage of many processed foods and stored commodities, and in indoor environments. Moderate xerophiles include species within Aspergillus, Penicillium and Eurotium. Extreme xerophiles compete poorly at high aw, because they require decreased aw for growth. Some xerophiles have a preference for salt or sugar substrates, whereas other species can be isolated from both jam and salterns. Xerophiles are widely spread on the fungal tree of life.

Key Concepts:

  • Xerophiles have requirement for reduced water activity; fungi are the most successful xerophilic organisms, and the most extreme xerophiles are found among fungi.

  • Compatible solutes are accumulated intracellularly to maintain the osmotic balance in response to decreased water availability.

  • Glycerol is the primary compatible solute among xerophilic fungi.

  • Membrane composition plays an important role in retaining glycerol within the cells of xerophiles.

  • Moderate xerophiles of the genera Aspergillus, Eurotium and Penicillium are economically important spoilage organisms.

  • Some xerophiles are isolated from both high‐salt and high‐sugar environments; others have a preference for either salt or sugar.

  • Xerophiles can cause spoilage of food and feed: stored commodities, sugary products and salted foods.

  • Xerophilic species are widely spread on the fungal tree of life.

Keywords: xerophilic fungi; water activity; osmophiles; yeasts; food spoilage; extremophiles; glycerol; membrane permeability; compatible solutes

Figure 1.

Eurotium chevalieri, a xerophilic species occurring commonly in stored products. (a) Aspergillus head. Bar, 20 μm. (b) Ascospores showing equatorial crests typical of this species. Bar, 10 μm.

Figure 2.

Some less common extreme xerophiles found in foods. (a) Aleurioconidia of Chrysosporium farinicola, the anamorphic form of Bettsia alvei. Bar, 20 μm. (b) Cleistothecia and ascospores of Be. alvei. Bar, 50 μm. (c) Aleurioconidia and arthroconidia of Chrysosporium xerophilum. Bar, 20 μm. (d) Asci and ascospores of Eremascus albus, a rare species which is occasionally isolated from mustard powder. Bar, 20 μm.

Figure 3.

Two highly specialised xerophilic fungi. (a) Polyphialides of Polypaecilum pisce, a halophilic xerophile associated with dried salted fish. Bar, 20 μm. (b) Cleistothecium and ascospores of Xeromyces bisporus, the most xerophilic fungus known. Bar, 20 μm.

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Further Reading

Williams JP and Hallsworth JE (2009) Limits of life in hostile environments: no barriers to biosphere function? Environmental Microbiology 11: 3292–3308.

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Vinnere Pettersson, Olga, and Leong, Su‐lin L(Aug 2011) Fungal Xerophiles (Osmophiles). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000376.pub2]