Antimicrobial resistance and molecular characterization of gene cassettes from class 1 integrons in Salmonella strains

Abstract

Introduction. The emergence of antibiotic-resistant Salmonella isolates is a global concern and has been attributed to the indiscriminate use of antibiotics in humans and animals. Integrons are mobile gene elements closely related to bacterial drug resistance. Among them, class 1 integrons containing various resistance gene cassettes could play an important role in disseminating and maintaining antibiotic resistance in Salmonella isolates. Hypothesis. Salmonella class 1 integrons have a relationship with Salmonella drug resistance. Aim. This study aims to investigate the distribution of class 1 integrons and their variable regional molecular characteristics, as well as the diversity of the promoters and drug sensitivity among Salmonella strains. Methodology. A total of 111 Salmonella strains, collected between 2018 and 2020, underwent fully automated bacterial identification using the VITEK 2 Compact system and an antibiotic sensitivity test. PCR was employed to screen class 1 integrase genes (IntI1) and integron variable regions, while promoter type and variable region gene cassette characteristics were determined using sequencing analysis. Results. A total of 24 IntI1-positive strains were detected in 111 Salmonella strains. Moreover, IntI1-positive strains exhibited statistically significant resistance to ceftazidime, ciprofloxacin, levofloxacin, ceftriaxone, trimethoprim/sulfamethoxazole and azithromycin compared to integron-negative strains (P<0.05). The multidrug resistance rate of IntI1-positive strains was significantly higher than that of negative strains. Variable regions were observed in 6 of the 24 IntI1-positive strains. Four gene cassettes were detected, namely dfrA17-aadA5, dfrA12-aadA2, aadA22 and aar-3-dfrA27. Finally, 3 types of class 1 integron variable region promoters were identified in 24 strains, including PcW, PcH1 and PcWTGN-10; they are all relatively weak promoters. Conclusion. The integron and the drug resistance genes carried by integron have a certain relationship with drug resistance.