Xerophilic Fungi - Life in the Dry Zone

Currently, about 5–10% of all food produced is spoiled by fungi. This wastage is set to increase, as climate change makes spoilage more difficult to manage – unpredictable weather conditions hamper efficient drying of grain before storage, and moulds grow faster in warm, moist conditions. Likewise, safe and effective storage of animal feed and substrates for biofuel production is critical to meet future challenges of supply and demand.

Efforts to minimise fungal spoilage of dry commodities require both fundamental and applied research.

Fundamental aspects

At the molecular level, we seek to understand how xerophilic (dry-loving) moulds grow in commodities with little available water – which cellular processes are active, and how do these differ in organisms that cannot grow in these conditions?

When the surroundings of a cell are drier or have a lower osmotic potential than the cytoplasm, cells must retain water against a concentration gradient. Bacterial, fungal and plant cells maintain turgor by accumulating or synthesising ‘compatible solutes’ internally to high concentrations.  These are often small organic molecules (polyols, sugars, amino acids) that protect enzymes from unfolding in dehydrated conditions and permit normal cellular processes. The mould, Xeromyces bisporus, is an expert at accumulating the compatible solute glycerol, and is able to grow at the lowest water availability of any organism on earth to date. For example, at its driest limits for growth, it is estimated that X. bisporus cells must withstand an internal osmotic pressure of –70 MPa (for comparison, the permanent wilting point of plants is –1.5 MPa.)

As a first step in discovering the secrets of its dry-tolerance, we have sequenced and assembled (de novo) the genome of X. bisporus, and we are currently analysing its transcriptome. We also compare patterns of glycerol accumulation and modification of membrane fatty acids among X. bisporus and other relevant spoilage moulds such as Aspergillus niger.

Applied aspects

The majority of molecular investigations are based on cultures grown in laboratory media, conditions which bear little semblance to stored cereals, bakery goods, and other dry food matrices spoiled by fungi. We are interested in whether fungi display different responses to conditions of water stress, preservatives, etc when growing on food matrices than when in agar (or broth) systems. And if so, how do these responses differ? We are currently constructing different model food matrices on which fungal growth can be examined.

More broadly, we have interests in the ecology of moulds in food and feed systems, with expertise in identification of relevant species by combined molecular and morphological techniques.