Integrons

Integrons are genetic elements that enable bacteria to capture, integrate, and express gene cassettes, often including antibiotic resistance determinants. They are not mobile on their own like transposons, but can be carried by plasmids or transposons and spread between bacteria.

A typical integron has three core features: an integrase gene (intI) that encodes a site-specific recombinase, a primary recombination site (attI) where new cassettes are integrated, and a promoter (Pc) that drives expression of the integrated cassette array. Gene cassettes usually contain a single open reading frame and an attC site; most cassettes lack their own promoters and rely on the Pc promoter for expression. The cassette array is arranged downstream of attI, and the position within the array influences expression levels, with cassettes closer to Pc generally expressed more strongly.

The integration mechanism centers on the integrase-catalyzed recombination between attI and attC sites, allowing new cassettes to be incorporated in a directional manner. Excision can occur via recombination between two attC sites, enabling cassette rearrangement. While integrons are not inherently mobile, their association with plasmids, transposons, or phages facilitates horizontal transfer between bacteria. Some integrons are part of large chromosomal structures called superintegrons, containing hundreds of gene cassettes, notably in environmental bacteria such as Vibrio species.

Integrons are major contributors to the rapid acquisition of adaptive traits, especially antibiotic resistance, by assembling diverse resistance genes into expressible arrays. They have been identified across many Gram-negative pathogens and are a focal point in antimicrobial resistance surveillance and research. The concept of integrons was described by Stokes and Hall in 1989, establishing a new paradigm for gene capture and expression in bacteria.