Characterisation of a class 1 integron associated with the formation of quadruple bla cassettes from an IncP-1β group plasmid in Pseudomonas aeruginosa.

Integrons are genetic platforms responsible for the dissemination of antimicrobial resistance genes among Gram-negative bacteria, primarily due to their association with transposable elements and conjugative plasmids. In this study, a cassette array containing four identical bla genes embedded in a class 1 integron located on an IncP-1β group plasmid from a clinical Pseudomonas aeruginosa strain was identified. Comparative genome analysis and conjugation assay showed that the plasmid pICP-4GES lacked the trbN, trbO and trbP genes but was conjugable. Antimicrobial susceptibility testing revealed that compared with single-copy bla complementary strains, both the cloned and chromosome-targeted expression of four copies of bla increased the minimum inhibitory concentration (MIC) by one to two dilutions for most of the selected antimicrobials. Quantitative real-time reverse transcription PCR (RT-qPCR) showed that the four consecutive cassettes increased bla expression by approximately two-fold compared with the single-copy bla strain, suggesting that the level of gene expression was not directly proportional to copy number. In addition, the gene cassette capture assay showed that the global bla transfer frequency reached 5.38 × 10.