The ability to adapt a particular trait in order to overcome a change in circumstances is important for all biological systems. Adapting to changes in the environment allows microorganisms to colonize a wide range of ecological niches and to survive sudden alterations in conditions. For example, on page 330 Krulwich, Sachs and Padan describe how bacteria have evolved diverse mechanisms for pH sensing and homeostasis that have allowed them to tolerate and grow at external pH values outside the cytoplasmic pH that they must maintain for growth.
For pathogens, adapting to better exploit a host will provide a selective advantage over competing organisms and can allow the colonization of a new niche. Bacteria of the genus Xanthomonas can cause disease in hundreds of plant hosts, including many important crop species, as described by Dow and colleagues on page 344. They discuss the host specificity and pathogenicity factors that have allowed this group of bacteria to adapt in order to exploit an extraordinary diversity of plant hosts.
