Escherichia coli is a Gram-negative bacterium which colonizes the intestinaltract of man and other animals. In addition to being a part of the normal bacterial flora of the humanintestine, there are a number of enteropathogenic strains of E. coli which cause infections ranging inconsequence from diarrhoea to colitis. Antigen 43 (Ag43) is the major phase-variable protein in the outermembrane of E. coli. One benefit for bacteria resulting from phase variation of surface antigens is usuallyascribed to evasion of host defences. However, results of recent studies indicate that infection of E.coli by different bacteriophages is inhibited in the presence of certain bile salts and carbohydrates(components present in the human intestine but absent in standard bacteriological media) when cells arein the 'OFF' state for production of Ag43. The inhibition of bacteriophage development was found to bedue to a significant impairment in the process of phage adsorption and evidence was presented for thebinding of phage to Ag43. Here we present a hypothesis that in the case of Ag43, phase-variation mightbenefit the host bacterium by modulating the susceptibility to phage infection in the gut. If this hypothesisis true, it may have important implications not only for basic research but also for development of bacteriophagetherapy, a re-discovered method of treatment of patients with infectious diseases.

Keywords: Antigens, Bacterial, Bacterial Outer Membrane Proteins, Coliphages, Escherichia coli, Escherichia coli Proteins, Research Support, Non-U.S. Gov