TABLE 3.
Susceptibilities to ampicillin, ceftazidime, cefpodoxime, gentamicin, and tetracycline observed in E. coli isolates recovered from feedlot cattle, pooled across subtherapeutic antimicrobial treatment groups
| Antibiotic and isolation medium | No. of isolates (%) at MICa:
|
Total no. of isolates | ||||||
|---|---|---|---|---|---|---|---|---|
| 0.5 μg ml−1 | 1 μg ml−1 | 2 μg ml−1 | 4 μg ml−1 | 8 μg ml−1 | 16 μg ml−1 | 32 μg ml−1 | ||
| Ampicillin | ||||||||
| MAC | 0 (0) | 3,014 (85.8) | 411 (11.7) | 8 (0.2) | 79* (2.2) | 3,512 | ||
| MAC+TET | 0 (0) | 1,391 (63.1) | 555 (25.2) | 8 (0.4) | 251* (11.4) | 2,205 | ||
| MAC+AMP | 2 (0.3) | - | - | 0 (0) | 635* (99.7) | 637 | ||
| MAC+GEN | 159 (19.6) | 538 (66.3) | 71 (8.7) | 0 (0) | 44* (5.4) | 812 | ||
| Subtotal | 161 (2.2) | 4,943 (69.0) | 1,037 (14.5) | 16 (0.2) | 1,009* (14.1) | 7,166 | ||
| Ceftazidime | ||||||||
| MAC | 3,486 (99.3) | 3 (0.1) | 23* (0.7) | 3,512 | ||||
| MAC+TET | 2,203 (99.9) | 1 (0.1) | 1* (0.1) | 2,205 | ||||
| MAC+AMP | 545 (85.6) | 1 (0.2) | 91* (14.3) | 637 | ||||
| MAC+GEN | 792 (97.5) | 4 (0.5) | 16* (2.0) | 812 | ||||
| Subtotal | 7,026 (98.0) | 9 (0.2) | 131* (1.8) | 7,166 | ||||
| Cefpodoxime | ||||||||
| MAC | 3,441 (98.0) | 49 (1.4) | 22* (0.6) | 3,512 | ||||
| MAC+TET | 2,096 (95.1) | 106 (4.8) | 3* (0.1) | 2,205 | ||||
| MAC+AMP | 432 (67.8) | 102 (16.0) | 103* (16.2) | 637 | ||||
| MAC+GEN | 763 (94.0) | 34 (4.2) | 15* (1.8) | 812 | ||||
| Subtotal | 6,732 (94.0) | 291 (4.1) | 143* (2.0) | 7,166 | ||||
| Gentamicin | ||||||||
| MAC | 3,480 (99.1) | 30 (0.8) | 2 (0.1) | - | - | 3,512 | ||
| MAC+TET | 2,107 (95.6) | 95 (4.3) | 3 (0.1) | - | - | 2,205 | ||
| MAC+AMP | 246 (38.6) | 363 (57.0) | 28 (4.4) | 0 (0) | 0 (0) | - | 637 | |
| MAC+GEN | 209 (25.7) | 233 (28.7) | 97 (11.9) | 200 (24.6) | 66 (8.1) | 7* (0.9) | 812 | |
| Subtotal | 455 (6.3) | 6,183 (86.3) | 250 (3.5) | 205 (2.9) | 66 (0.9) | 7* (0.1) | 7,166 | |
| Tetracycline | ||||||||
| MAC | 2,981 (84.9) | 0 (0) | 531* (15.1) | 3,512 | ||||
| MAC+TET | 19 (0.9) | 3 (0.1) | 2,183* (99.0) | 2,205 | ||||
| MAC+AMP | 0 (0) | 59 (9.3) | 83 (13.0) | 0 (0) | 495* (77.7) | 637 | ||
| MAC+GEN | 75 (9.2) | 490 (60.3) | 35 (4.3) | 8 (1.0) | 204* (25.1) | 812 | ||
| Subtotal | 75 (1.0) | 549 (7.7) | 3,118 (43.5) | 11 (0.2) | 3,413* (47.6) | 7,166 | ||
For each antibiotic, the MIC columns in which isolate numbers are marked with asterisks are the breakpoint values (≥) distinguishing sensitive and resistant isolates of E. coli, with the exception of ceftazidime and cefpodoxime, for which the indicated concentration was used to screen for candidate E. coli potentially encoding ESBLs. Data were analyzed across all of the antibiotic treatments (including control, no antibiotics) described in Materials and Methods. Isolates were recovered from MacConkey agar containing no antibiotics (MAC) or onto MAC amended with ampicillin (MAC+AMP), tetracycline hydrochloride (MAC+TET), or gentamicin (MAC+GEN) as defined in the text. Dashes indicate that isolates were not tested at the specified antibiotic concentration.