Emergence of ciprofloxacin- and tigecycline-resistant extended-spectrum β-lactamase (ESBL)-producing Salmonella enterica serovar Kentucky ST198 from horse, China.

The study reports the emergence of a drug-resistant bacterial strain, Salmonella enterica Kentucky ST198, found in horses in China. The strain showed resistance to various antimicrobials, highlighting a significant public health concern.

Research Objective and Methods

• The study’s main aim was to characterise the first isolated S. Kentucky ST198, a bacterial strain becoming increasingly prevalent worldwide, from a horse in China.
• To achieve this, the researchers collected faecal samples from horses at a farm in Shihezi, Xinjiang Province, China, and investigated them for the presence of Salmonella.
• The study determined the bacterial strain’s minimum inhibitory concentrations—a measure of their resistance—against 15 different antimicrobial agents using a methodology known as the broth microdilution method.
• The researchers also sequenced the genome of the isolated strain using a technique known as Illumina Hiseq and assembled them into contigs with a software called SPAdes 3.10.0.
• Phylogenetic analysis was then carried out on the basis of core-genome single-nucleotide polymorphisms to understand the genetic ancestry and similarities of this particular bacterial strain.

Findings and Conclusions

• The study was successful in isolating a strain designated Sal140, which carried 15 antimicrobial resistance genes. Its resistance spanned a host of common antimicrobial agents, including ampicillin, ceftazidime, tetracycline, ciprofloxacin, and sulfamethoxazole/trimethoprim.
• Resistance to specific antimicrobials, such as ciprofloxacin and third-generation cephalosporins, was attributed to mutations in certain genes and the production of a protein called CTX-M-55.
• Further, phylogenetic analysis revealed that the isolated strain clustered within the subclade ST198.2-2, indicating its close genetic similarity to isolates from a patient and chicken meat from other regions of China. This suggested a possible cross-species or cross-regional transmission link.
• The study concludes by highlighting the public health concern arising from the emergence of this extensively drug-resistant strain in horses and strongly recommends ongoing surveillance of this bacterial clone.