Table 3.

Antimicrobial susceptibility of all isolates and PCR-confirmed E. coliO157:H7 isolatesa

Antibiotic (concn [μg/ml])No. of isolates with susceptibility
All (n = 57)PCR confirmed (n = 26)
SIRSIR
Not approved for feedlot animals
E. coli targeting
  Imipenem (4, 8)57002600
  Ciprofloxacin (1, 2)57002600
  Norfloxacin (4, 8)57002600
  Nitrofurantoin (32)57002600
  Gentamicin (4, 8)56102600
  Cefazolin (8, 16)48092402
  Cephalothin (8, 16)3314101853
 Non-E. coli targeting
  Oxacillin (2)00570026
  Vancomycin (4, 16)00570026
Approved for feedlot animals
E. coli targeting
  Trimethoprim-sulfamethoxazole (2/38)53042600
  Ceftiofur (0.015–32)b50072204
  Ampicillin (8) (0.015–32)b47 (48)010 (9)24 (21)02 (5)
  Tetracycline (4, 8) (0.015–32)b31 (39)3 (0)23 (18)15 (18)1 (0)10 (8)
  Amoxicillin/clavulanic acid (4/2)3002716010
  Florfenicol (0.015–32)b05700260
  Spectinomycin (0.25–32)b05700260
  Rifampin (1, 2)02550125
 Non-E. coli targeting
  Tylosin (0.312–20)b05700260
  Penicillin (0.03, 0.12, 8)00570026
  Clindamycin (0.5, 2)00570026
  Erythromycin (0.5, 4) (0.015–32)b00570026
  Tilmicosin (0.015–32)b00570026
  • a Analysis includes multiple isolates from a single sample. S, susceptible; I, intermediate susceptibility; R, resistant. Concentrations of antibiotics are in parentheses after the antibiotic name and are expressed in micrograms per milliliter when reconstituted in 50 μl of broth. If there are two sets of antibiotic concentrations, the second set is for the broth microdilution method. When both methods were used and the number of isolates differed between the methods, the number of isolates in parentheses was determined by the broth microdilution method.

  • b Susceptibility determined by broth microdilution instead of the breakpoint methods.