Schematic representation of the molecular markers used to study core and plasmid accessory genes of Typhimurium from Mexico. A) The chromosomal variation was addressed by multilocus sequence typing using partial sequences of the seven housekeeping genes , denoted by boxes on the chromosome of strain LT2 [GenBank:AE006468] , and by macrorestriction analysis using the rarely cutting enzyme Xba I resolved by pulsed-field electrophoresis, represented by lines crossing the chromosome at several points. B) The presence of the Typhimurium virulence plasmid (pSTV) [GenBank:AE006471] was determined by PCR amplification of three genes involved in virulence spvC, rck and traT [19, 28], and by Southern hybridisation on plasmid profiles using spvC as probe. C) The presence of the plasmid-borne cmy-2 gene, conferring resistance to extended spectrum cephalosporins [GenBank:NC_011079] [30, 31], was determined by PCR and by Southern hybridisation on plasmid profiles. The chloramphenicol determinant floR was also assessed, since it has been reported that both resistances are often encoded by the same plasmid .