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Bacteriophage K1F specifically infects Escherichia coli strains that produce the K1 polysaccharide capsule. Like several other K1 capsule-specific phages, K1F encodes an endo-neuraminidase (endosialidase) that is part of the tail structure which allows the phage to recognize and degrade the polysaccharide capsule. The complete nucleotide sequence of the K1F genome reveals that it is closely related to bacteriophage T7 in both genome organization and sequence similarity. The most striking difference between the two phages is that K1F encodes the endosialidase in the analogous position to the T7 tail fiber gene. This is in contrast with bacteriophage K1-5, another Kl-specific phage, which encodes a very similar endosialidase which is part of a tail gene 'module' at the end of the phage genome. It appears that diverse phages have acquired endosialidase genes by horizontal gene transfer and that these genes or gene products have adapted to different genome and virion architectures.

The achievement of redirection of the bacterial activity of R-type pyocins to different target bacteria is discussed.

The expression of the K1 capsule in Escherichia coli physically blocks infection by T7, a phage that recognizes lipopolysaccharide as the primary receptor. It suggests that the capsule plays an important role as a defense against some phages that recognize structures beneath it and the K1-specific phages evolved to counter this physical barrier.