A BTP1 prophage gene present in invasive non-typhoidal Salmonella determines composition and length of the O-antigen of the lipopolysaccharide.

Abstract

Salmonella Typhimurium isolate D23580 represents a recently identified ST313 lineage of invasive non-typhoidal Salmonellae (iNTS). One of the differences between this lineage and other non-iNTS S. Typhimurium isolates is the presence of prophage BTP1. This prophage encodes a gtrC gene, implicated in O-antigen modification. GtrCBTP1 is essential for maintaining O-antigen length in isolate D23580, since a gtrBTP1 mutant yields a short O-antigen. This phenotype can be complemented by gtrCBTP1 or very closely related gtrC genes. The short O-antigen of the gtrBTP1 mutant was also compensated by deletion of the BTP1 phage tailspike gene in the D23580 chromosome. This tailspike protein has a putative endorhamnosidase domain and thus may mediate O-antigen cleavage. Expression of the gtrCBTP1 gene is, in contrast to expression of many other gtr operons, not subject to phase variation and transcriptional analysis suggests that gtrC is produced under a variety of conditions. Additionally, GtrCBTP1 expression is necessary and sufficient to provide protection against BTP1 phage infection of an otherwise susceptible strain. These data are consistent with a model in which GtrCBTP1 mediates modification of the BTP1 phage O-antigen receptor in lysogenic D23580, and thereby prevents superinfection by itself and other phage that uses the same O-antigen co-receptor.