2008) was learn more used to perform a log-linear analysis separately for each medium to evaluate differences among recovered isolates for antimicrobial resistance phenotypes, PD0332991 treatments and their interaction. P values ≤ 0.05 were interpreted as indicative of a significant difference. PFGE patterns were either classified as unique or grouped into clusters based on ≥ 90% homology using Dice similarity coefficients using unweighted pair group methods with arithmetic average algorithms built into Bionumerics. The position tolerance and optimization were set at 1% and 0.5% respectively. Results

Antimicrobial susceptibility Resistance to AMI, FOX, AXO, GEN, or NAL was not observed in any of the 531 E. coli isolates examined (isolated on MC, MT or MA). Populations selected on Mc plates Forty-five of 55 isolates (81.8%) from non-selective medium MC were susceptible to all antimicrobials tested. Phenotypes observed in the MC isolates expressing AMR included resistance to SMX (7/10 isolates), STR (5/10), CHL (2/80), TE (2/10) and CL (1/10). Six of the 10 isolates obtained exhibited multi-drug resistance. Populations selected on MT plates Resistance to TE at the breakpoint level was nearly ubiquitous (>98.8%) among the isolates from the MT plates (Table 3). Isolates from MT plates exhibiting AMP, STR, SMX and TE were recovered from animals across all three treatments. A

treatment × phenotype interaction (p = 0.003) was observed with an increased number of isolates (p = 0.014) exhibiting resistance to SMX in TS group (55.1%) Tariquidar chemical structure as compared to other groups (Table 3). Resistance to STR was higher (p = 0.018) among CON (52.3%) and V (50.7%) groups as compared to T (35.1%) and TS (32.7%)

treatments (Table 3). Resistance of MT isolates to AMP was highest (p = 0.017) in isolates recovered from TS (18.7%) and was less common among isolates from groups V (13.0%), CON (6.3%) and T (2.7%). Table 3 Total number (n) and percentage of phenotype observed within isolates recovered from MacConkey agar amended with 4 μg/ml tetracycline hydrochloride after diet administration of control and three antimicrobial treatments.   Treatment† Phenotype CON % ( n ) T % ( n ) TS % ( n ) V % ( n ) AMP 6.3b (7) 2.7c (2) 18.7a (20) 13.0b (9) STR 52.3a (58) 35.1b,c (26) 32.7b (35) 50.7a (35) SMX 42.3c (47) 47.3b,c (35) 55.1a (59) 42.0b (29) TE 99.1ba (110) 100a (74) 100a (107) 98.6b (68) Total ( n ) 111 74 Isotretinoin 107 69 †CON; no antibiotics added to supplement, T: chlortetracycline provided as Aureomycin 100-G fed at 11 ppm, TS: chlortetracycline + sulfamethazine, provided as Aureo S-700G (Alpharma Inc.) fed at 44 ppm and V: virginiamycin provided as V-Maxed at 31 ppm. Population selected on MA plates As expected, given that the concentration of ampicillin in the selection medium was above the breakpoint level, resistance to AMP was confirmed in all of MA isolates (Table 4). Isolates exhibiting resistance to TE, CL and STR were obtained from cattle fed all diets.