Mobilized integrons: Team players in the spread of antibiotic resistance genes

Abstract

Integrons possess a site-specific recombination system and comprise a family of elements that are broadly distributed amongst the Proteobacteria. The units of capture into these elements are gene cassettes, which normally comprise of only a single gene along with an attachment site recognized by the recombination system. The class 1 integron has at least two features that distinguishes it from most other members of the integron family of integrase elements. The first of these is that they are located on mobile elements as opposed to being fixed in the chromosome and the second is that most of the associated gene cassettes include genes that encode antibiotic resistance. The linkage of the class 1 integron to mobile elements was an important step since it has meant that diverse molecular processes act cooperatively to disseminate resistance genes in Gram-negative bacteria. The selection for resistance in the antibiotic era has now led to an enormous diversity of elements that in many cases has resulted in conjugation, transposition, and site-specific recombination processes combining to spread large clusters of resistance genes. All these processes existed in nature prior to the antibiotic era but the level and extent of cooperation did not. Here we discuss how some of these complex class 1-associated mobile resistance regions evolved and their ramifications for the management of the antibiotic resistance problem.