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.