<p>Carbohydrates are used for a wide variety of functions in animals, plants and microbes. As well as providing the main energy source for most organisms,carbohydrates are ideally suited for molecular recognition [<cite idref="PUB00010401"/>]. For example, the stereochemistry of each hydroxyl substituent in a simple six-carbon six-oxygen pyranose ring can be varied to give up to 10 different molecules.An organism's ability to change and control the stereochemistry of moietieslike glucose and galactose is, therefore, important.</p><p>Epimerases or "dehydratases", enzymes that catalyse these changes in carbohydrates, exist in both prokaryotes and eukaryotes. The processes they arbitrate are known as "epimerisation" and are involved in many metabolic pathways [<cite idref="PUB00010334"/>]. For example, the UDP-galactose epimerase in <taxon tax_id="3702">Arabidopsis thaliana</taxon> (Mouse-ear cress) mediates the reversible epimerisation of UDP-galactose, and is considered essential for this reason, providing a secondary growth metabolite to glucose [<cite idref="PUB00010334"/>]. </p><p>Bacterial homologues of the mouse-ear cress epimerase are involved in the synthesis of extracellular polysaccharide capsule components, as well as metabolic pathways [<cite idref="PUB00010328"/>]. For this reason, they are considered virulence factors if present in a pathogenic strain. <taxon tax_id="672">Vibrio vulnificus</taxon>, the causativeagent of septicemia and infectious disease from contaminated seafood, relies on a thick polysaccharide capsule to evade host immune cells [<cite idref="PUB00010328"/>]. Epimerasedeletion mutants were unable to infect a mouse model, suggesting that thegene is essential for bacterial virulence. Deletion studies with othercapsular pathogens, like <taxon tax_id="1280">Staphylococcus aureus</taxon>, have shown that theepimerase enzyme is needed for capsule polysaccharide formation and,ultimately, virulence.</p>