Integrons: an antibiotic resistance gene capture and expression system

Bacteria can transfer genetic information to provide themselves with protec tion against most antibiotics. The acquisition of resistance gene arrays in volves genetic mobile elements like plasmids and transposons. Another class of genetic structures, termed integrons, have been described and contain o ne or more gene cassettes located at a specific site. Integrons are defined by an intl gene encoding an integrase, a recombination site attl and a str ong promoter. At least six classes of integrons have been determined accord ing to their intl gene. Classes 1, 2 and 3 are the most studied and are lar gely implicated in the dissemination of antibiotic resistance. A gene casse tte includes an open reading frame and, at the 3'-end, a recombination site attC. Integration or excision of cassettes occur by a site-specific recomb ination mechanism catalyzed by the integrase. However, insertion can occur, albeit rarely, at non-specific sites leading to a stable situation for the cassette. Cassettes are transcribed from the common promoter located in th e 5'-conserved segment and expression of distal genes is reduced by the pre sence of upstream cassettes. Most gene cassettes encode antibiotic resistan t determinants but antiseptic resistant genes have also been described. Int egrons seem to have a major role in the spread of multidrug resistance in G ram-negative bacteria but integrons in Gram-positive bacteria were describe d recently. Moreover, the finding of super-integrons with gene-cassettes co ding for other determinants (biochemical functions, virulence factors) in V ibrio isolates dating from 1888 suggests the likely implication of: this mu lticomponent cassette-integron system in bacterial genome evolution before the antibiotic era and to a greater extent than initially believed.