Structure, assembly and regulation of expression of capsules in Escherichia coli

Many Escherichia coli strains are covered in a layer of surface-associated polysaccharide called the capsule, Capsular polysaccharides represent a maj or surface antigen, the K antigen, and more than 80 distinct K serotypes re sult from structural diversity in these polymers, However, not all capsules consist of K antigen. Some are due to production of an extensive layer of a polymer structurally identical to a lipopolysaccharide O antigen, but dis tinguished from lipopolysaccharide by the absence of terminal lipid A-core. Recent research has provided insight into the manner in which capsules are organized on the Gram-negative cell surface, the pathways used for their a ssembly, and the regulatory processes used to control their expression. A l imited repertoire of capsule expression systems are available, despite the fact that the producing bacteria occupy a variety of ecological niches and possess diverse physiologies. All of the known capsule assembly systems see n in Gram-negative bacteria are represented in E. coli, as are the majority of the regulatory strategies. Escherichia coli therefore provides a variet y of working models on which studies in other bacteria are (or can be) base d, In this review, we present an overview of the current molecular and bioc hemical models for capsule expression in E, coli, By taking into account th e organization of capsule gene clusters, details of the assembly pathway, a nd regulatory features that dictate capsule expression, we provide a new cl assification system that separates the known capsules of E. coli into four distinct groups.