001) in MA isolates from TS (94.1%) as compared to T (76.9%) and V (56.0%) and CON (38.5%) steers (Table 4). In the MA isolates from CON,
resistance to CL was most common, and its prevalence (61.5%) was notably higher (p = 0.007) than was observed in the T (15.4%), TS (5.9%) or V (4.0%) isolates (Table 4). Table 4 Total number (n) and percentage of phenotype observed within isolates recovered from MacConkey agar amended with 50 μg/ml ampicillin after diet administration of control and three antimicrobial treatments. Treatment† Phenotype CON % ( n ) T % ( n ) TS % ( n ) V % ( n ) AMP 100 (26) 100 (13) 100 (51) 100 (25) CL 61.5a (16) 15.4b (2) 5.9b (3) 4.0b (1) STR 38.5 (10) 23.1 (3) 13.7 (7) 40.0 (10) TE 38.5c (10) 76.9b (10) 94.1a (48) 56c (14) Total ( n ) 26 13 51 25 † CON; no antibiotics added to supplement, T: chlortetracycline provided as Aureomycin 100-G fed at 11 ppm, TS: chlortetracycline + sulfamethazine, provided Torin 1 molecular weight as Aureo S-700G (Alpharma Inc.) fed at 44 ppm and V: virginiamycin provided as V-Maxed at 31 ppm. Antibiogram patterns Irrespective of the CON or antibiotic treatment administered, the majority of isolates, particularly those from MA medium, were LOXO-101 solubility dmso resistant to multiple antibiotics. Among the MT isolates, multi-resistance MLN2238 nmr whereby a single isolate displayed resistance to more than one antibiotic, was found in 69.4%, 56.8%, 76.6% and 73.9% of CON, T, TS and V isolates, respectively
(Figure 2). By comparison, in the MA isolates, multi-resistance was observed in 100, 92.3, 100, and 80.0% of isolates from CON, T, TS and V steers, respectively (Figure 3). Figure 2 Antibiogram and PFGE types of fecal E. coli isolated from feedlot cattle using MacConkey agar amended with 4 μg/ml chlortetracycline (M T ), as distributed by dietary treatment, sampling day and animal of origin. Sampling days (B to E) are depicted in Figure 1. Each box represents a single isolate from
a particular steer on a given sampling day. The first eight colors represent the most commonly observed antibiogram patterns others with grey indicating an infrequently observed antibiogram. Unfilled boxes indicate no isolate obtained on MT. Common letters indicate isolates with >90% genetic homology. Shaded boxes without a letter indicate isolates with <90% genetic homology with antibiogram data. Dietary treatments were as follows: Control: no antibiotics; Chlortetracycline (11 ppm; denoted T); Chlortetracycline + sulfamethazine (44 ppm; denoted TS); and Virginiamycin (31 ppm; V). nc: isolates not characterized. Figure 3 Antibiogram and PFGE types of fecal E. coli isolated from feedlot cattle using MacConkey agar amended with 50 μg/ml ampicillin (M A ), as distributed by dietary treatment, sampling day and animal of origin. Sampling days (B to E) are depicted in Figure 1. Each box represents a single isolate from a particular steer on a given sampling day.