Integron involvement in environmental spread of antibiotic resistance

Abstract

The spread of antibiotic-resistant bacteria is a growing problem and a public health issue. In recent decades, various genetic mechanisms involved in the spread of resistance genes among bacteria have been identified. Integrons - genetic elements that acquire, exchange, and express genes embedded within gene cassettes (GC) - are one of these mechanisms. Integrons are widely distributed, especially in Gram-negative bacteria; they are carried by mobile genetic elements, plasmids, and transposons, which promote their spread within bacterial communities. Initially studied mainly in the clinical setting for their involvement in antibiotic resistance, their role in the environment is now an increasing focus of attention. The aim of this review is to provide an in-depth analysis of recent studies of antibiotic-resistance integrons in the environment, highlighting their potential involvement in antibiotic-resistance outside the clinical context. We will focus particularly on the impact of human activities (agriculture, industries, wastewater treatment, etc.).

Keywords: agriculture; antibiotic resistance; aquatic ecosystems; integron; soil; wastewater; water.

Figure 1

General organization of an integron and gene cassette (GC) recombination mechanism. The IntI1 protein catalyzes the insertion (A) and excision (B) of the GC in the integron, with GC integration occurring at the attI recombination site. In example (A), the circularized GC3 is integrated in linear form inside the integron platform via a specific recombination mechanism between the attI site and the attC3 site of the GC3. GC excision preferentially occurs between two attC sites. In example (B), the GC1 is excised following the recombination between the two attC1 and attC3 sites. Pc: gene cassette promoter; attI: integron recombination site; attC1, attC2, and attC3: attC GC recombination sites; intI: the integrase gene; GC1, GC2, GC3 are the gene cassettes, and arrows indicate the direction of coding sequences (Barraud and Ploy, 2011).

Figure 2

Main route of MIs dissemination from anthropogenic sources to the environment (pictures derived from PILLS project: http://www.pills-project.eu).