Detection and genomic characterization of a multidrug-resistant Salmonella Newport co-harbouring blaCMY-2, qnrB19 and mcr-9 from the diarrheic faeces of a foal.

The research involves the discovery and genetic study of a multidrug-resistant strain of Salmonella Newport obtained from a sick foal in Brazil. This specific strain co-harbours the antibiotic resistance genes mcr-9, bla, and qnrB19.

Methods

• Researchers found a bacterial isolate positive for the mobile colistin resistance gene, known as mcr-9.
• This isolate underwent antimicrobial susceptibility testing to determine its resistance profile. This testing was performed through disk diffusion and broth microdilution for both colistin and polymyxin B, types of antibiotics.
• The researchers then performed a whole genome sequencing of the isolate using both Illumina technology and Nanopore Sequencing.
• Additional investigations were performed to determine plasmid sizes and content. The methods included conjugation assays, S1-PFGE, and hybrid assembly after MinIon long-read sequencing.

Results

• Based on sequence typing, the researchers identified the isolate (termed 195_20) as ST45.
• Isolate 195_20 showed resistance to a wide range of antibiotics, including penicillin, cephalosporins, aminoglycosides, phenicol, quinolones, fluoroquinolones, folate pathway antagonists, and tetracycline.
• The researchers identified a transferable plasmid (a piece of DNA that can move from one bacteria to another) of approximately 262kb. This plasmid carried the mcr-9 gene as part of a module containing other resistance elements.
• Two additional plasmids, one of 174kb and the other of 48kb, were also identified. These carried the bla and qnrB19 resistance genes, respectively.

Conclusions

• Despite its resistance to other antibiotics, isolate 195_20 was found to be susceptible to polymyxins, likely due to the absence of the qseBC regulatory operon.
• The presence of mobile colistin resistance (mcr-9), third-generation cephalosporins (bla), and quinolone (qnrB19) resistance genes in zoonotic pathogens from animals in close contact with humans suggests a potential transmission route between these different reservoirs.

The study offers important insights into the genetic makeup of a multidrug-resistant strain of Salmonella. Such data can help guide future research and intervention strategies in the control of antibiotic resistance.