Soluble O16-antigen LPS complex delays T4 bacteriophage adsorption to Escherichia coli K-12 MG1655

MG1655 is an Escherichia coli K-12 strain that does not express O-antigen, a distal sugar component of lipopolysaccharide (LPS) on Gram-negative bacteria. DFB1655 L9 is an isogenic strain of MG1655 with O16-antigen expression restored. Studies have shown that DFB1655 L9 is resistant to T4 bacteriophage infection, whereas MG1655 is susceptible. How O16-antigen confers resistance to T4 is unknown. Elucidating this resistance mechanism will provide insight into the functional role of O-antigen. Here, we investigated the effects of soluble O16-antigen on T4 adsorption to E. coli K-12 by performing a time series adsorption assay of T4 to MG1655 using T4 pre-treated with LPS with or without O16-antigen. The free phage concentration in the supernatant, representing the unadsorbed phage, was quantified via double agar overlay plaque assays. We observed a similar trend between the control group and the treatment group of T4 incubated with LPS without O16-antigen, in which free phage concentration decreased rapidly to 3.10% and 2.99% at t = 5 minutes, respectively. When T4 was incubated with LPS with O16-antigen, a delayed decrease of free phage concentration was observed, with 17% of the initial free phage concentration remaining at t = 5 minutes. However, eventually phage in the supernatant of all groups depleted. Therefore, we conclude that soluble O16-antigen LPS complex was able to delay T4 adsorption to MG1655 but did not confer long-term resistance. This provides evidence that soluble O16-antigen likely affects T4 bacteriophage infectivity through reversible interactions and delays its rate of adsorption to the cell surface.