The O-polysaccharide of Escherichia coli F5, which is structurally related to that of E. coli O28ab, provides cells only weak protection against bacteriophage attack.

Several types of Escherichia coli O-antigens form highly effective shields protecting the bacterial cell surface and preventing bacteriophages from interacting directly with their secondary (terminal) receptors. However, it is not clear if O-antigens of various types (O-serotypes) differ in their anti-phage protection efficacy. Here, we describe a new E. coli strain, F5, which has an E. coli O28ab-related O-antigen. Although the amount of O-antigen produced by this strain is comparable to that produced by other E. coli strains we tested, it appears to give the cells significantly lower protection against phage attack than other O-antigen types, such as the O-polysaccharide of E. coli F17, which we studied earlier.