Azithromycin-resistant Salmonella enterica Typhi with AcrB R717L/Q mutations in the United States

Azithromycin is a critical oral treatment for typhoid fever caused by Salmonella enterica serovar Typhi (Salmonella Typhi), since XDR has rendered other first-line treatment options ineffective. Azithromycin resistance conferred by amino acid changes in AcrB, an AcrAB-TolC efflux pump component, represents an emerging public health concern. Leveraging phenotypic and genotypic data from U.S. Salmonella Typhi surveillance systems, this study describes the prevalence, phenotype and genomic epidemiology of Salmonella Typhi with AcrB mutations in U.S. patients since the first detection in 2015.

AST and WGS data of >3000 Salmonella Typhi isolates were used to identify all cases with an AcrB mutation in the United States (2015-2025). We calculated annual prevalence and MIC ranges. Phylogenetic analysis was used to contextualize U.S. cases of Salmonella Typhi with an AcrB mutation within all globally reported cases.

While the prevalence of AcrB mutations in the United States is low (1.5%), it has risen significantly in recent years, from 0.2% in 2016-2022 to 2.2% in 2023-2025. This increase is predominantly driven by clonal expansion of existing strains circulating in South Asia. AcrB mutations do not reliably confer resistance to azithromycin (MIC ≥ 32 mg/L), complicating clinical interpretation.

The prevalence of AcrB mutations in Salmonella Typhi is increasing in the United States, and likely globally, given that U.S. data function as an informal proxy for regions without routine surveillance infrastructure. Clinical outcomes data are needed to inform Salmonella Typhi treatment guidelines and potentially amend clinical breakpoints for azithromycin.