HK97 bacteriophage, illustration. Bacteriophages are viruses that infect bacteria, in this case enterobacteria such as E. coli (Escherichia coli). This bacteriophage capsid structure if formed by the protein gp5. This protein cross-links upon maturation to form a concatenated chain-mail like structure. The bacteriophage undergoes a maturation process during which it changes from spherically symmetrical to icosahedrally symmetrical. This characteristic is present in a large number of dsDNA bacteriophage capsids, as well as in the major capsid protein of eukaryotic herpesviruses.