Prevalence of class 1 integron and its gene cassettes carrying antibiotic resistance genes in drinking water treatment and distribution systems

Class 1 integrons are vital mobile genetic elements involved in the environmental transmission of antibiotic resistance genes (ARGs). However, knowledge about the diversity and abundance of class 1 integrons and gene cassettes during drinking water treatment and distribution is still limited. In this study, we aimed to uncover the prevalence of class 1 integrons in the drinking water treatment and distribution systems with the combination of culture-dependent and culture-independent methods. Further, we applied the nanopore sequencing method to characterize the diversity and arrangement of ARGs carried by class 1 integron-associated gene cassettes. A total of 42 isolates were intI1-positive among the 208 strains isolated from drinking water, which tended to confer multi-drug resistance compared with intI1-negative isolates. The absolute abundance of the intI1 average 1.15 × 10⁹ copies/L in the source water and underwent the most significant reduction of over 99.9% after liquid chlorine disinfection. Furthermore, nanopore sequencing revealed that the class 1 integron-associated gene cassettes carried 51 subtypes of ARGs in drinking water, mainly conferring resistance to aminoglycosides and trimethoprim. The treatment processes, especially liquid chlorine disinfection, reduced most of the ARGs carried by gene cassettes, though some of the ARG subtypes persisted along the treatment and distribution like aac(6′)-II, aadA, and dfrB2. The antibiotic resistance gene cassette array ∣aac(6′)-II∣arr∣ was most frequently detected, especially in the chlorinated water. This study underlined that drinking water was potential reservoir for integron-mediated ARGs transfer, indicating that the health risks of resistance gene cassettes in class 1 integrons deserved urgent attention.