Class 1 Integrons, Gene Cassettes, Mobility, and Epidemiology

Abstract

Integrons are genetic elements that, although unable to move themselves, contain gene cassettes that can be mobilized to other integrons or to secondary sites in the bacterial genome. The majority of approximately 60 known gene cassettes encode resistance to antibiotics. Recently, a number of gene cassettes encoding extended-spectrum beta-lactamases or carbapenemases have been described. Up to at least five cassettes may be present in an integron, which leads to multiresistance. Frequently, more than one integron is observed within the same bacterial cell. Integrons are widespread in their species distribution. Although integrons are normally reported from Enterobacteriaceae and other gram-negative bacteria, an integron has been described in Corynebacterium glutamicum, a gram-positive species. The gene cassette in this integron showed even higher expression when compared to the expression in Escherichia coli. Integrons have been reported from all continents and are found frequently. The widespread occurrence of integrons is thought to be due to their association with transposon plasmids, conjugative plasmids, or both. Integrons form an important source for the spread of antibiotic resistance, at least in gram-negative bacteria but also potentially in gram-positive bacteria. The aim of this review is to describe the versatility of integrons, especially their mobility and their ability to collect resistance genes.

Emergence of Salmonella epidemics: The problems related to Salmonella enterica serotyp Enteritidis and multiple antibiotic resistance in other major serotypes

“…The antibiotic resistance gene cluster is located near the 3' end of SGI1 and constitutes a complex class 1 integron belonging to the In4 group. Class 1 integrons contain a 5'-conserved segment (5'-CS) which consists of the intI1 gene encoding the site-specific integrase and the associated attI1 site, the primary site of recombination, and a 3'-conserved segment (3'-CS) of variable length but generally consisting of qacE1 encoding low level resistance to some antiseptics, the sul1 gene encoding sulfonamide resistance, and orf5, a gene of unknown function [50]. One or more gene cassettes consisting of the coding region(s) and the downstream 59-base element (59-be), which is responsible for recognition and mobilisation of cassettes, are found between the 5'-CS and 3'-CS [50].…”

“…Class 1 integrons contain a 5'-conserved segment (5'-CS) which consists of the intI1 gene encoding the site-specific integrase and the associated attI1 site, the primary site of recombination, and a 3'-conserved segment (3'-CS) of variable length but generally consisting of qacE1 encoding low level resistance to some antiseptics, the sul1 gene encoding sulfonamide resistance, and orf5, a gene of unknown function [50]. One or more gene cassettes consisting of the coding region(s) and the downstream 59-base element (59-be), which is responsible for recognition and mobilisation of cassettes, are found between the 5'-CS and 3'-CS [50]. Transposon Tn402 is a mobile class 1 integron that contains the 5'-CS and a transposition module (tni region) consisting of four genes required for transposition [25].…”

The evolutionary dynamics of integrons in changing environments

“…Class 1 integrons have been extensively studied mechanistically with respect to gene-cassette acquisition. They are widely dispersed in Proteobacteria (including most clinical Gram-negatives) and occasionally found in Gram-positives (Fluit and Schmitz, 1999;Partridge et al, 2009).…”

Rapid Emergence of Ciprofloxacin-Resistant Enterobacteriaceae Containing Multiple Gentamicin Resistance-Associated Integrons, the Netherlands

“…The integron encodes an integrase enzyme that mediates the recombination events (18,19). Integrons are not independently mobile but may be found as part of transposons or plasmids, and the genes that they contain may not always be expressed with equal effectiveness (33).…”

“…Most bacterial strains derived from bloodstream infections acquired in northern European countries were still highly susceptible to ciprofloxacin (16,17). An additional, complicating factor is that antibiotic-resistance genes are frequently trapped in cassettes, the so-called integrons (18)(19)(20), which provide an efficient means for capturing and exchanging resistance genes. This implies that both bacterial strain characteristics and the exchange and dissemination of resis-tance genes need to be considered during ongoing outbreaks of infection (21).…”