A stable atypical H₂S-negative, non-motile Salmonella Infantis variant from a human isolate associated with a barA...

Salmonella enterica serovar Infantis (S. Infantis) is an increasingly important food-borne pathogen, and detection in broiler flocks via the standardized EN/ISO 6579-1:2017 is of utmost importance. Phenotypic characteristics, such as the production of hydrogen sulfide (H₂S) and swarming behavior, are essential for detection. However, the occurrence of atypical variants lacking such phenotypic characteristics bears a risk to public health. In this study, we characterized an Austrian human-derived S. Infantis strain showing, in addition to the normal colony phenotype (NCP), also an atypical colony variant (ACV) lacking H₂S production and motility. Both phenotypes proved stability, and no differences were found in cell morphology. In comparison to NCP, ACV demonstrated altered biofilm formation on Congo Red agar, and showed lower streptomycin resistance. The genomic analysis revealed a total of 21 mutations specific to ACV, including a stop-gain mutation in barA. Using gene network analysis, a connection between barA and operons relevant for H₂S production, ttr, and phs was found. As barA is a well-known regulator with established roles in motility, the observed truncation provides a plausible regulatory basis for the stable H₂S-negative, non-motile phenotype. Furthermore, mutations in barA have been linked to persistent Salmonella infections in humans, supporting our findings. Importantly, such atypical variants might go undetected in the mandatory control programs.IMPORTANCEPhenotypic characteristics, such as the production of hydrogen sulfide and motility, are essential for culture-based detection of Salmonella. This study shows that Salmonella Infantis can lose these key diagnostic features through regulatory mutations, enabling escape from routine surveillance while maintaining infection potential. As the present mutation is known to contribute to bacterial persistence in the host, our findings highlight a dual clinical concern of diagnostic evasion and possible adaptation to host survival. This work demonstrates a critical limitation of phenotype-dependent diagnostics and underscores the need for surveillance strategies that can detect atypical but clinically relevant Salmonella variants.