ANTIBIOTIC-RESISTANCE IN GRAM-NEGATIVE BACTERIA - THE ROLE OF GENE CASSETTES AND INTEGRONS

Resistance of gram-negative organisms to antibiotics such as beta-lact ams, aminoglycosides, trimethoprim and chloramphenicol is caused by ma ny different acquired genes, and a substantial proportion of these are part of small mobile elements known as gene cassettes. A gene cassett e consists of the gene and a downstream sequence, known as a 59-base e lement (59-be), that acts as a specific recombination site. Gene casse ttes can move into or out of a specific receptor site (attl site) in a companion element called an integron, and integration or excision of the cassettes is catalysed by a site-specific recombinase (Intl) that is encoded by the integron. At present count there are 40 different ca ssette-associated resistance genes and three distinct classes of integ ron, each encoding a distinct Intl integrase. The same cassettes are f ound in all three classes of integron, indicating that cassettes can m ove freely between different integrons. Integrons belonging to class I often contain a further antibiotic resistance gene, sull,conferring r esistance to sulphonamides. The sell gene is found in a conserved regi on (3'-CS) that is not present in all members of this class. Class I i ntegrons of the sull type are most prevalent in clinical isolates and have been found in many different organisms. Even though most of them are defective transposon derivatives, having lost at least one of the transposition genes, they are none the less translocatable and consequ ently found in many different locations. The transposon Tn7 is the bes t known representative of class 2 integrons, and Tn7 and relatives are also found in many different species.