The ability for a virus to expand its host range is dependent upon a successful mode of viral entry. As such, the host range of the well-studied ΦX174 bacteriophage is dictated by the presence of a particular lipopolysaccharide (LPS) on the bacterial surface. The mutant ΦX174 strain JACS-K, unlike its ancestor, is capable of infecting both its native host Escherichia coli C and E. coli K-12, which does not have the necessary LPS. The conversion of an alanine to a very reactive threonine on its virion surface was found to be responsible for the strain's expanded host range.