Sxt1, Isolated from a Therapeutic Phage Cocktail, Is a Broader Host Range Relative of the Phage T3

Using <i>Escherichia coli</i> BW25113 as a host, we isolated a novel lytic phage from the commercial poly-specific therapeutic phage cocktail Sextaphage^® (Microgen, Russia). We provide genetic and phenotypic characterization of the phage and describe its host range on the ECOR collection of reference <i>E. coli</i> strains. The phage, hereafter named Sxt1, is a close relative of classical coliphage T3 and belongs to the <i>Teetrevirus</i> genus, yet its internal virion proteins, forming an ejectosome, differ from those of T3. In addition, the Sxt1 lateral tail fiber (LTF) protein clusters with those of the phages from the <i>Berlinvirus</i> genus. A comparison of T7, T3, and Sxt1 LTFs reveals the presence of insertions leading to the elongation of Sxt1 tail fibers, which, together with the difference in the HRDRs (host range-determining regions), might explain the expanded host specificity for the Sxt1.