Escherichia coli K1's capsule is a barrier to bacteriophage T7

Abstract

Escherichia coli strains that produce the K1 polysaccharide capsule have long been associated with pathogenesis. This capsule is believed to increase the cell's invasiveness, allowing the bacteria to avoid phagocytosis and inactivation by complement. It is also recognized as a receptor by some phages, such as K1F and K1-5, which have virion-associated enzymes that degrade the polysaccharide. In this report we show that expression of the K1 capsule in E. coli physically blocks infection by T7, a phage that recognizes lipopolysaccharide as the primary receptor. Enzymatic removal of the K1 antigen from the cell allows T7 to adsorb and replicate. This observation suggests that the capsule plays an important role as a defense against some phages that recognize structures beneath it and that the K1-specific phages evolved to counter this physical barrier.

FIG. 1.

Infection of EV36 by T7 is blocked by capsule. A lawn of EV36 was spotted with 0.3 mg/ml of endosialidase, which had little effect on growth (a); 0.3 mg/ml of endosialidase plus 10⁴ T7 particles (growth is indicated by clearing); T7 alone, which could not adsorb (c); and 10⁴ phage K1-5 particles (K1 specific) (d).