Schematic representation of the molecular markers used to study the integrons of Typhimurium from Mexico. A) Diagrammatic representation of the basic features of a class 1 integron . The positions of the primers [see Additional file 3] used to amplify the different regions are shown by arrows. A class 1 integron consist of two conserved segments (5'-CS and 3'-CS) separated by a variable region that may contain an array of one or more gene cassettes. The 5'-CS includes the gene for the integrase (intI1), the promoters for the expression of the integrase (Pint) and the gene cassettes (Pc), and an adjacent attI recombination site, where the cassettes are integrated. Gene cassettes consist of a single promoter-less gene and a recombination site known as a 59-base element (59-be or attC), which is recognized by the site-specific recombinase (intI1). The 3'-CS includes qacEΔ1 and sul1 genes, determining resistance to quaternary ammonium compounds and to sulphonamide, respectively. The structure of the integron profiles found here, IP-1, IP-2, IP-3 and IP-4, are shown with their corresponding gene cassettes. B) Diagram of the regions of the Salmonella genome island 1 (SGI1) [43, 44] that were studied. The positions of the primers [see Additional file 3] used to amplify the different regions are shown by arrows. The insertion of the island in the chromosome was detected by amplification of the right and left junctions; from the antibiotic resistance cluster the two integron-born gene cassettes (aadA2 and pse-1), floR and tetG were amplified.