Dendrogram showing genetic relationships among 136 S. Infantis isolated from 58 commercial farms of 5 integrated broiler operations characterized by PFGE profiles. S. Infantis isolates showing similarities of < 90% in PFGE were considered to be unrelated. Abbreviations: AM, ampicillin; AMC, amoxicillin-clavulanate; C, chloramphenicol; CIP, ciprofloxacin; CTX, cefotaxime; CZ, cefazolin; FEP, cefepime; FOX, cefoxitin; GM, gentamicin; IPM, imipenem; NA, nalidixic acid, SXT, trimethoprim/sulfamethoxazole; TE, tetracycline.