Duplication of blaCTX-M-1 and a class 1 integron on the chromosome enhances antimicrobial resistance in Escherichia coli isolated from racehorses in Japan.

The research investigates how certain gene duplications in bacteria can enhance their resistance to antimicrobial treatments. Two strains of Extended-spectrum β-lactamase (ESBL)-producing Escherichia coli were examined, and researchers found that these strains underwent specific genetic changes that bolstered their ability to withstand treatment.

Genetic Analysis of E. coli Strains

• The study analyzed two strands of ESBL-producing E.coli bacteria that were isolated from the heart and liver of racehorses in Japan who suffered from endocarditis and sepsis. These strains were given the designations E148 and E189.
• The complete genomic sequences of these strains were obtained using a PacBio RSII sequencer, a machine which is designed to sequence DNA for these kinds of analyses.

Antimicrobial Susceptibility Testing

• The researchers performed antimicrobial susceptibility testing on the bacteria. This was done via the agar dilution method, which involves exposing the bacteria to various dilutions of a drug to determine the concentration at which microbial growth is inhibited.
• The bacterial strains were found to have a chromosomal antimicrobial resistance (AMR) gene cluster containing a particular gene (bla), similar to a type found in an E.coli strain from a racehorse in the Czech Republic.

Findings and Implications

• One of the strains had a duplication of the 16-kb region containing the antibiotic resistance gene, and this duplication was found to increase the bacteria’s minimum inhibitory concentrations (MICs) associated with the AMR genes. Increased MICs indicate that it takes more of the drug to inhibit bacterial growth – thus the bacteria are more drug-resistant.
• The researchers concluded that the bla gene possibly derived from a plasmid (a standalone piece of DNA that can replicate independently from the chromosome) found in the Czech strain was present in the Japanese strains. The presence and accumulation of these AMR genes, including bla and the class 1 integron, in one region on the chromosome significantly enhanced the antimicrobial resistance of the bacteria.
• These findings show an instance of bacteria evolving to make themselves more resistant to treatment, highlighting the need for ongoing research in this area to develop new treatments that can bypass or disarm these evolved defenses.