TABLE 1.
Antimicrobials and concentrations used to test susceptibility of E. coli and Salmonella isolates
| Antimicrobial (abbreviation) | Breakpoint concn (μg/ml)a
|
|
|---|---|---|
| Susceptibility | Resistance | |
| Amikacin (AMK) | 16 | ND |
| Ampicillin (AMP) | ND | 32 |
| Amoxicillin-clavulanate (AMC) | ND | 64/16b |
| Apramycin (APR)c | ND | 32d |
| Carbadox (CRB)c | ND | 30e |
| Cephalothin (CEF) | ND | 32 |
| Ceftriaxone (CRO) | 8 | ND |
| Ceftiofur (CTF) | ND | 8 |
| Cefoxitin (FOX) | ND | 32 |
| Chloramphenicol (CHL) | ND | 32 |
| Ciprofloxacin (CIP) | 0.125f | ND |
| Florfenicol (FLO)c | ND | 16g |
| Gentamicin (GEN) | ND | 16 |
| Kanamycin (KAN) | ND | 64 |
| Nalidixic acid (NAL) | ND | 32 |
| Neomycin (NEO)c | ND | 16d |
| Nitrofurantoin (NIT) | ND | 64h |
| Spectinomycin (SPT)c | ND | 64d |
| Streptomycin (STR)c | ND | 32d |
| Sulfisoxazole (SUL) | ND | 512 |
| Sulfamethoxazole-trimethoprim (SXT) | ND | 76/4 |
| Tetracycline (TET) | ND | 16 |
| Tobramycin (TOB) | ND | 8 |
| Trimethoprim (TMP) | ND | 16 |
The breakpoint concentrations to determine susceptibility and resistance are those specified by NCCLS standards M31-A (26) and M100-S12 (27). ND, not determined.
The strains were considered resistant when growing on agar plates with amoxicillin-clavulanic acid at 64 and 16 μg/ml; respectively.
There are no interpretive criteria specified by NCCLS standards M31-A (26) or M100-S12 (27) for these drugs.
Strains were considered resistant to apramycin, neomycin, spectinomycin, and streptomycin at 32, 16, 64, and 32 μg/ml; respectively.
Strains were considered resistant to carbadox, a veterinary growth promoter for pigs, at 30 μg/ml (12).
A 0.125-μg/ml concentration of ciprofloxacin determines reduced sensitivity to ciprofloxacin (34).
Strains were considered resistant to florfenicol at the level of 16 μg/ml (36).
Strains were considered resistant to nitrofurantoin at 64 μg/ml; human urinary tract isolates are considered resistant at 128 μg/ml (27).