Antimicrobial resistance can move through human, animal, and environmental systems, making livestock-associated bacteria important targets for One Health surveillance.
In a study published in Emerging Contaminants, researchers investigated Stutzerimonas nitrititolerans recovered from geese feces at a farm in Yangzhou, China, focusing on resistance to meropenem and reduced susceptibility to tigecycline.
Genetics of antimicrobial resistance
From 60 fecal samples, the team isolated five multidrug-resistant S. nitrititolerans strains. All five were found to be resistant to meropenem, with minimum inhibitory concentrations of 8-32 μg/mL, and showed tigecycline MICs of 1-4 μg/mL. Hybrid short- and long-read genome sequencing found chromosomes of approximately 4.17-4.36 Mb and no detectable plasmid replicons.
“The genomes contained two distinct chromosomal resistance-associated regions,” shares senior and co-corresponding author Ruichao Li. “One carried the tmexC2D2-toprJ2 efflux cluster within a mobile-element-rich region containing transposases, integron components, and phage-related integrases. A second, more conserved chromosomal locus contained blaPST-2 and showed local insertion-sequence and recombination signatures.”
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To place the finding in a wider context, the researchers compared the five isolates with 99 publicly available S. nitrititolerans genomes. Complete tmexCD-toprJ clusters occurred in only 10 of 104 genomes (9.6%).
“Co-occurrence of a complete tmexCD-toprJ cluster with blaPST was found in nine genomes (8.7%), five of which were the geese isolates,” says Li. “Apart from one closely related pair, the geese isolates were genetically diverse, indicating that the shared resistance architecture was not simply explained by expansion of a single clone.”
No transfer of resistance
Notably, transfer of meropenem- or tigecycline-non-susceptibility to Escherichia coli C600 was not detected under the filter-mating conditions used. The researchers therefore caution that the study does not demonstrate active transfer, and the direct phenotypic contribution of blaPST-2 requires further experimental confirmation.
“These data support further genomic surveillance of S. nitrititolerans in geese and other livestock-associated environments,” adds Li. “Our study provides a farm-level genomic snapshot and supports continued monitoring of resistance determinants at the animal-environment interface.”
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