Antimicrobial resistance and genomic relationships of Salmonella enterica from Australian cattle

The aim of this study was to evaluate phenotypic and genotypic AMR characteristics of Salmonella enterica isolates from Australian cattle collected through a structured national survey utilizing 1001 faecal samples collected from healthy cattle at slaughter. A total of 184 Salmonella isolates were subsequently derived and subjected to microbroth dilution to 16 drugs from 11 classes with interpretation of minimum inhibitory concentrations (MICs) using epidemiological cut off (ECOFF) values to distinguish between wild-type and non-wild type populations. Most isolates were susceptible (wild type) to all antimicrobials tested, with no resistance (non wild type) detected for colistin, nalidixic acid, meropenem, gentamicin, florfenicol or chloramphenicol. Low rates of resistance were detected for ampicillin (2.2%), cefoxitin (2.2%), ceftiofur (2.2%), ceftriaxone (2.2%), ciprofloxacin (0.5%), streptomycin (3.3%) and tetracycline (0.5%). Isolates resistant to ceftriaxone (a critically important antimicrobial, CIA) carried the extended spectrum cephalosporin gene blaCMY-2 while no known mutation in the QRDR region or qnrS genes were detected for the CIA ciprofloxacin-resistant isolate. Thirty-six serovars were detected among the 184 Salmonella isolates using whole genome sequencing, dominated by Typhimurium (n = 36), Saintpaul (n = 22) and Anatum (n = 16). Genomic analysis clustered the cattle isolates based on serovar, with the majority of serovars containing a single sequence type (ST). Further analysis of the bovine Typhimurium isolates (ST19) and comparison with publicly available data from human Typhimurium isolates from Australia, revealed the majority of cattle isolates were unrelated to human isolates. In conclusion, this study demonstrates the continued low prevalence of AMR among Salmonella within the beef, dairy and veal industries in Australia. Salmonella Typhimurium from cattle were genetically distinct from isolates sourced from human infections. Further investigations are warranted to expand on the potential clinical and public health relevance of these findings to inform risk-management of this key pathogen.