Distribution and Molecular Characterization of Functional Class 2 Integrons in Clinical Proteus mirabilis Isolates

Integrons are the main mode of horizontal transmission of drug-resistance genes and are closely related to drug resistance in clinical bacteria. In this study, the distributions of class 1, 2, and 3 integron gene cassettes were investigated in 150 Proteus mirabilis (P. mirabilis) isolates from patients, and molecular characterization of functional class 2 integrons was further analyzed.

Class 1, 2, and 3 integrons were screened by polymerase chain reaction (PCR) in 150 clinical P. mirabilis isolates. The variable regions of the integrons were determined by restriction analysis and sequencing. Internal stop codons mutations in class 2 integrons and their common promoters were also determined by sequencing. Enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) was used to analyze the phylogenetic relations of class 2 integron-positive isolates.

Class 1 integrons were detected in 69 (46%) of 150 P. mirabilis isolates, and six different gene cassette arrays were detected, with the most prevalent being dfrA32-aadA2. Class 2 integrons were detected in 61 (40.7%) of 150 P. mirabilis isolates, and three different gene cassette arrays were detected, including sat2-aadA1, which was detected for the first time in a class 2 integron. Nearly similar ERIC-PCR fingerprinting patterns were detected in 45 (73.8%) of 61 class 2 integron-positive isolates. The functional class 2 integron was detected in three P. mirabilis isolates having the same gene cassette, dfrA1-sat2-aadA1, in the variable region and four novel open reading frames with unknown functions. Same PintI2 and Pc promoters were detected in these three functional class 2 integron isolates, as was found in other class 2 integron isolates. However, these three strains did not totally show identical homology and drug sensitivity.

Although functional class 2 integrons have low distribution and relatively conserved molecular characteristics, they can still form clinical dissemination and drug resistance expression.