A single nucleotide polymorphism collapses phase variation and drives short-sighted evolution of O-antigen in...

The O-antigen of Salmonella is a key virulence determinant and serotype-defining structure, yet the regulatory mechanisms underlying its dynamic modification and evolutionary trade-offs remain incompletely understood. Salmonella Pullorum is a pathogen that displays both O-antigen unstable phase variation and stable antigenic conversion between standard (O12₃) and variant (O12₂) antigenic types. In this study, we identified the glycosyltransferase encoded by gtrC ^III as responsible for the variant-specific glucosylation of the O-antigen. The expression of gtrC ^III is co-regulated by the transcriptional repressor OxyR and methyltransferase Dam, generating stochastic ON‒OFF switching of the O-antigen modification, which is a classic phase variation and promotes immune evasion. Remarkably, a single C-to-A point substitution at position -34 of the gtrABC ^III promoter disrupts OxyR binding and simultaneously enhances intrinsic promoter activity, resulting in constitutive expression of the variant antigenic type. This monophasic variant strain exhibits enhanced resistance to standard-type antibody-mediated killing, conferring a short-term immune evasion advantage. However, in long-term colonization, it is outcompeted by the phase-variable standard type strain, exemplifying "short-sighted evolution", where an adaptive mutation trades off persistence for transient immune escape. Our findings reveal how a single nucleotide change can subvert a complex epigenetic switch to drive sub-serotype conversion, and provide insights into the evolutionary constraints shaping O-antigen diversity.