Polymorphisms of Gene Cassette Promoters of the Class 1 Integron in Clinical Isolates.

Objective: To describe the polymorphisms of gene cassette promoters of the class 1 integron in clinical isolates and their relationship with antibiotic resistance.

Methods: Polymorphisms of the gene cassette promoter in 153 strains of were analyzed by PCR and nucleotide sequencing. Variable regions of atypical class 1 integrons were detected by inverse PCR and nucleotide sequencing. Enterobacterial repetitive intergenic consensus (ERIC)-PCR was used to analyze the phylogenetic relationships of class 1 integron-positive clinical isolates. Representative beta-lactamase genes (), including , , , , , , and , and plasmid-mediated quinolone resistance (PMQR) genes including , and were also screened using PCR and sequence analysis.

Results: Fifteen different gene cassette arrays and 20 different gene cassettes were detected in integron-positive strains. Of them, (37/96) was the most common gene cassette array. Two of these gene cassette arrays () have not previously been reported. Three different Pc-P2 variants (PcS, PcW, PcH1) were detected among the 96 strains, with PcH1 being the most common (49/96). Strains carrying the promoters PcS or PcW were more resistant to sulfamethoxazole, gentamicin and tobramycin than those carrying PcH1. Strains with weak promoter (PcH1) harbored significantly more intra- and extra-integron antibiotic resistance genes than isolates with strong promoter (PcW). Further, among 153 isolates, representative beta-lactamase genes were detected in 70 isolates ( , 54; , 40; , 12; , 12; , 5; , 2) and representative PMQR genes were detected in 87 isolates (, 6; , 3; , 5; , 46; , 5; , 7; , 13; , 32).

Conclusion: To the best of our knowledge, this study provides the first evidence for polymorphisms of the class 1 integron variable promoter in clinical isolates, which generally contain relatively strong promoters. Resistance genotypes showed a higher coincidence rate with the drug-resistant phenotype in strong-promoter-containing strains, resulting in an ability to confer strong resistance to antibiotics among host bacteria and a relatively limited ability to capture gene cassettes. Moreover, strains with relatively weak integron promoters can "afford" a heavier "extra-integron antibiotic resistance gene load". Furthermore, the gene cassettes , and the gene cassette arrays have been confirmed for the first time in clinical isolates. Beta-lactamase genes and PMQR were investigated, and and were the most common, with and also being dominant.