Regulatory role of ISEcp1 and spacer sequences in blaCTX-M-mediated cephalosporin resistance in Shigella.

BACKGROUND

Extended-spectrum β-lactamases, especially CTX-M enzymes, play a critical role in cephalosporin resistance in several bacterial species. The ISEcp1 insertion sequence, a mobile genetic element, regulates the high expression and horizontal transfer of the blaCTX-M gene. Diverse types of spacer regions exist, the length of which varies across the different types of blaCTX-M genes and their upstream ISEcp1 elements. In this study, we aimed to investigate the distribution and role of ISEcp1 and different length spacers in antibiotic-resistant Shigella.

METHODS

We selected 1393 cephalosporin-resistant Shigella strains isolated from China between 2004 and 2016 to analyse the distribution of ISEcp1 and spacers using polymerase chain reaction and sequencing. To functionally evaluate the role of these elements, we designed different combinations of ISEcp1, spacer sequences, and blaCTX-M gene fragments to generate and express recombinant plasmids containing these fragments in Escherichia coli. We then assessed the cephalosporin resistance phenotype of recombinant clones using the E-TEST method.

RESULTS

The carriage rate of ISEcp1 in cephalosporin-resistant Shigella was 90.09%, with an ISEcp1-CTX-M element carriage rate of 67.19%, and was significantly higher in the blaCTX-M-9 group than that in the blaCTX-M-1 group. Moreover, E. coli strains containing different combinations exhibited varying bacterial resistance to β-lactam antibiotics, indicating the regulatory role of ISEcp1 and spacers.

CONCLUSIONS

Our results demonstrate that the configuration of ISEcp1, spacer sequences, and blaCTX-M in plasmids fosters a robust resistance phenotype against various cephalosporins, providing new insights into antimicrobial resistance mechanisms and an important basis for developing novel anti-resistance drugs.