Antimicrobial resistance patterns in bacterial isolates from mice and rats in Australian laboratory rodent facilities between 2015 and 2024

Antimicrobial resistance (AMR) is a critical global challenge threatening human and animal health, food security and economic stability. Despite advances in understanding AMR trends of bacteria from humans, domestic animals, wildlife and environmental sources, there is limited understanding regarding AMR profiles of bacteria isolated from laboratory animals, particularly rodents. To address this, a 10-year dataset from an Australian research animal health testing laboratory was analysed, investigating antimicrobial susceptibility patterns in bacterial isolates from laboratory mice and rats. Data from 205 isolates were reviewed, comparing AMR profiles across microbial species. Organisms undergoing antimicrobial sensitivity testing were most commonly associated with disease investigations, rather than routine health screening. In descending order, the most frequently isolated bacteria were Rodentibacter pneumotropicus, Staphylococcus xylosus, Enterococcus spp., E. faecalis, Enterobacter cloacae, Escherichia coli, Staphylococcus aureus, Klebsiella oxytoca and Proteus mirabilis. All isolates exhibited some AMR. Significant multidrug resistance (MDR) - with resistance to at least three of the following: tetracycline, trimethoprim-sulphamethoxazole, amoxicillin-clavulanic acid or ampicillin, or enrofloxacin - was observed in 22.0% of isolates. AMR was greatest in isolates from rats. Some MDR isolates included zoonotic bacteria, highlighting potential occupational health risks for humans and a risk of treatment failure in humans and laboratory rodents. Resistance patterns for R. pneumotropicus, E. faecalis and S. aureus were compared with those reported in similar studies from other regions, with greater rates of AMR observed in Australian isolates. Quantitative analysis was limited by small sample sizes; however, the data suggest further investigation and antimicrobial stewardship initiatives are urgently required.