A xerophile [1] is an extremophilic organism that can grow and reproduce in conditions with a low availability of water, also known as water activity.
Water activity, a thermodynamical value denoted, is defined as the partial water vapor pressure in equilibrium with the substance relative to (divided by) the (partial) vapor pressure of pure water at the same temperature: The thermodynamical water activity is thus equal to the relative humidity (RH), and the chemical activity of pure water is equal to one:
When the atmosphere above a substance, or a solution, is undersaturated in water vapor, its water activity is lower than one.
Xerophiles are "xerotolerant", meaning tolerant of dry conditions. They can often survive in environments with water activity below 0.8; above which is typical for most life on Earth. Typically xerotolerance is used with respect to matrix drying, where a substance has a low water concentration. These environments include arid desert soils. The term osmophile, or osmotolerant, is typically applied to microorganisms that can grow in solutions with high solute concentrations (salts, sugars), such as halophiles.
The common food preservation methods of reducing water activity (food drying) may not always be sufficient to prevent the growth of xerophilic organisms, often resulting in food spoilage. Some mold and yeast species are xerophilic. Mold growth on bread is an example of food spoilage by xerophilic organisms.
Complete dehydration based on the freeze-drying technique with effective protection inside a tight packaging system, strictly impervious to water and atmospheric gases (and), may be required for long-term preservation of food and pharmacochemical substances (antibiotics, vaccines…). Freeze drying can limit the microbial activity on the long term, as long as the product remains perfectly dry in a hermetically sealed and intact package, but it is not a sterilisation technique per se, because after rehydration, even if many dehydrated cells suffer irreversible and lethal damages, some resistant spores and bacterial endospores can still be revived again, and multiplied, by means of microbiological cultures if the product was not initially sterilized by applying a proven technique.
Examples of xerophiles include Trichosporonoides nigrescens,[2] Zygosaccharomyces, and cacti.