Amplification of class 1 integrons from STEC O118:[H16] ET-A strains with primers 5′CS/3′CS (Table 1). Nineteen representative integron-positive and -negative strains from humans and cattle are shown. Lanes 1 and 23, 100-bp ladder; lane 2, CB7503 (negative); lane 3, CB7512 (dfrA1-aadA1a, 1.6 kb); lane 4, CB7882 (dfrA1-aadA1a, 1.6 kb); lane 5, CB8153 (negative with primers 5′CS/3′CS, positive for integrase and sulI); lane 6, CB8220 (negative); lane 7, CB8255 (negative); lane 8, CB8271 (negative); lane 9, E-D143 (aadA1a, 1.0 kb); lane 10, VTH28 (dfrA1-aadA1a, 1.6 kb); lane 11, VTH62 (aadA1a, 1.0 kb); lane 12, T17968 (aadA1a, 1.0 kb); lane 13, EC970130 (dfrA1-aadA1a, 1.6 kb); lane 14, 666/89 (aadA1a, 1.0 kb); lane 15, CB6069 (O118: H12, ET-B, negative); lane 16, 1874-99 (negative); lane 17, EH78 (negative); lane 18, RW2017 (aadA1a, 1.0 kb); lane 19, RW2030 (negative); lane 20, RW2173 (dfrA1-aadA1a, 1.6 kb); lane 21, RW2266 (dfrA1-aadA1a, 1.6 kb); lane 22, negative PCR control.