TABLE 4.
Resistance and coresistance of fecal E. coli isolates from steers fed subtherapeutic levels of antibiotics as determined according to the NCCLS standard MIC of antibiotics for human use
| Isolation mediumb | Dietary groupc | No. of isolates | No. of isolates (%)a exhibiting breakpoint resistance to:
|
||||||
|---|---|---|---|---|---|---|---|---|---|
| AMP | TET | GEN | AMP+TET | AMP+GENT | TET+GENT | AMP+TET+GENT | |||
| MAC | Control | 695 | 6 (0.9) | 36 (5.2) | - | 2 (0.3) | - | - | - |
| TET-SUL | 683 | 53 (7.8) | 284 (41.6) | - | 50 (7.3) | - | - | - | |
| TET | 670 | 4 (0.6) | 99 (14.8) | - | 1 (0.1) | - | - | - | |
| VIR | 455 | 6 (1.3) | 21 (4.6) | - | - | - | - | ||
| MON | 457 | 38 (8.3) | - | - | - | - | |||
| TYL | 552 | 7 (1.3) | 53 (9.6) | - | 5 (0.9) | - | - | - | |
| Subtotal | 3,512 | 76 (2.2) | 531 (15.1) | - | 58 (1.7) | - | - | - | |
| MAC+TET | Control | 332 | 33 (9.9) | 328 (98.8) | - | 33 (9.9) | - | - | - |
| TET-SUL | 503 | 101 (20.1) | 499 (99.2) | - | 101 (20.1) | - | - | - | |
| TET | 376 | 15 (4.0) | 373 (99.2) | - | 15 (4.0) | - | - | - | |
| VIR | 354 | 35 (9.9) | 352 (99.4) | - | 35 (9.9) | - | - | - | |
| MON | 330 | 20 (6.1) | 329 (99.7) | - | 20 (6.1) | - | - | - | |
| TYL | 310 | 47 (15.2) | 302 (97.4) | - | 47 (15.2) | - | - | - | |
| Subtotal | 2,205 | 249 (11.3) | 2,188 (99.2) | - | 249 (11.3) | - | - | - | |
| MAC+AMP | Control | 63 | 62 (98.4) | 45 (71.4) | - | 45 (71.4) | - | - | - |
| TET-SUL | 252 | 252 (100) | 240 (95.2) | - | 240 (95.2) | - | - | - | |
| TET | 86 | 85 (98.8) | 56 (65.1) | - | 56 (65.1) | - | - | - | |
| VIR | 78 | 78 (100) | 47 (60.3) | - | 47 (60.3) | - | - | - | |
| MON | 63 | 63 (100) | 36 (57.1) | - | 36 (57.1) | - | - | - | |
| TYL | 95 | 95 (100) | 71 (74.7) | - | 71 (74.7) | - | - | - | |
| Subtotal | 637 | 635 (99.7) | 495 (77.7) | - | 495 (77.7) | - | - | - | |
| MAC+GEN | Control | 160 | 12 (7.5) | 23 (14.4) | 1 (0.6) | 8 (5.0) | - | 1 (0.6) | - |
| TET-SUL | 125 | 18 (14.4) | 87 (69.6) | 2 (1.6) | 15 (12.0) | - | - | ||
| TET | 134 | 4 (3.0) | 29 (21.6) | 3 (2.2) | 3 (2.2) | 2 (1.5) | 2 (1.5) | 2 (1.5) | |
| VIR | 140 | 5 (3.6) | 19 (13.6) | 1 (0.7) | 3 (2.1) | 1 (0.7) | 1 (0.7) | 1 (0.7) | |
| MON | 133 | 3 (2.3) | 23 (17.3) | - | 1 (0.8) | - | - | - | |
| TYL | 120 | 2 (1.7) | 22 (18.3) | - | 2 (1.7) | - | - | - | |
| Subtotal | 812 | 44 (5.4) | 203 (25.0) | 7 (0.9) | 32 (3.9) | 3 (0.4) | 4 (0.5) | 3 (0.4) | |
The values shown are the numbers of isolates exhibiting breakpoint resistance to ampicillin (AMP), tetracycline (TET), and gentamicin (GEN) at 32, 16, and 16 μg ml−1, respectively. The numbers of isolates showing resistance to antibiotics are not cumulative. Dashes indicate that no isolates were resistant.
Isolates were recovered from MacConkey agar containing no antibiotics (MAC) or MAC amended with ampicillin (MAC+AMP), tetracycline hydrochloride (MAC+TET), or gentamicin (MAC+GEN).
Control, no antimicrobial agents added to the diet; the TET-SUL,TET, VIR, MON, and TYL antimicrobial feeding groups are defined in Materials and Methods.