The role of integrons in dissemination of antibiotic resistance

Annales de Biologie Clinique. Volume 58, Number 4, 439-44, Juillet - Août 2000, Revues générales

Author(s) : M.-C. Ploy, T. Lambert, A. Gassama, F. Denis, Laboratoire de bactériologie-virologie-hygiène, CHU Dupuytren, 2, avenue Martin-Luther-King, 87042 Limoges cedex.

Summary : Bacteria can transfer genetic information to get protection against most antibiotics. The acquisition of resistance genes involves genetic mobile elements such as plasmids and transposons. Another genetic structures, named integrons, have been described and contain one or more gene cassettes located at a specific site. Integrons contain an intI gene encoding a site-specific recombinase belonging to the integrase family and a recombination site attI. A gene cassette includes an open reading frame and, at the 3’-end, a recombination site attC. Integration or excision of cassettes occurs by a site-specific recombination mechanism catalyzed by the integrase. However, insertion can rarely occur, at non-specific sites leading to a stable situation for the cassette. Cassettes are transcribed from a common promoter located in the 5’-conserved segment and expression of distal genes is reduced by the presence of upstream cassettes. Most gene cassettes encode antibiotic resistant determinants but antiseptic resistant genes have also been described. Integrons seem to have a major role in the spread of multidrug resistance in Gram-negative bacteria but integrons in Gram-positive bacteria have been recently described. Moreover, the finding of super-integrons with gene cassettes coding for other determinants (biochemical functions, virulence factors) in different Gram negative bacteria suggests that integrons are probably implied in bacterial genome evolution.

Keywords : Integron – Resistance – Dissemination.

Pictures


		Figure 1. Structure d'un intégron et mouvement des cassettes. La région 5' conservée contient un gène intl codant pour une intégrase, un site d'attachement attI et un promoteur P. Le mouvement des cassettes se fait par insertion-excision sous forme circulaire par recombinaison entre le site attI et un site attC ou entre deux sites attC.


		Figure 2. Structure d'une cassette. Une cassette est constituée d'un gène et d'un site attC excepté les 6 dernières paires de bases. Le site attC contient aux deux extrémités les séquences core et core inverse.