TABLE 3.
β-Lactam susceptibility tests with S. Saintpaul isolates (n = 55)
| Susceptibility testa | Antimicrobial agents |
Concn tested | No. (%) of isolates |
|||||
|---|---|---|---|---|---|---|---|---|
| Subclass | Antibiotic | Resistant | Intermediate | Susceptibleb | Decreased susceptibility | Fully susceptible | ||
| Agar dilution (MIC) | First-generation cephalosporins | Cephalothin | 1-32 μg/ml | 41 (74.5) | 4 (7.3) | 10 (18.2) | ||
| Second-generation cephalosporins | Cefuroxime | 1-32 μg/ml | 35 (63.6) | 10 (18.2) | 10 (18.2) | |||
| Third-generation cephalosporins | Cefotaxime | 1-64 μg/ml | 55 (100) | |||||
| Ceftazidime | 1-32 μg/ml | 5 (9.1) | 50 (90.1) | |||||
| Cefoxitin | 1-32 μg/ml | 8 (14.5) | 40 (72.7) | 7 (12.7) | ||||
| Ceftiofur | 0.5-8 μg/ml | 19 (34.5) | 31 (56.4) | 5 (9.1) | ||||
| Disk diffusion (zone diam) | Penicillins | Ampicillin | 10 μg | 45 (81.8) | 10 (18.2) | |||
| Carboxypenicillins | Ticarcillin | 75 μg | 45 (81.8) | 10 (18.2) | ||||
| Acylaminopenicillins | Piperacillin | 100 μg | 45 (81.8) | 10 (18.2) | ||||
| β-Lactamase inhibitors | Amoxicillin-clavulanic acid | 30 μg | 13 (23.6) | 27 (49.1) | 15 (27.3) | |||
| Third-generation cephalosporins | Cefpodoxime | 10 μg | 6 (10.9) | 49 (89.1) | ||||
| Ceftriaxone | 30 μg | 55 (100) | ||||||
| Fourth-generation cephalosporins | Cefepime | 30 μg | 55 (100) | |||||
| Carbapenems | Imipenem | 10 μg | 55 (100) | |||||
| Monobactams | Aztreonam | 30 μg | 55 (100) | |||||
The MIC breakpoints and zone diameter standards were defined as described by the CLSI (16). The MIC breakpoints for cephalothin, cefuroxime, ceftazidime, and cefoxitin were as follows: resistant, ≥32 μg/ml; intermediate, 16 μg/ml; and susceptible, ≤8 μg/ml (decreased susceptibility, 8 to 2 μg/ml; fully susceptible, <2 μg/ml). The MIC breakpoints for cefotaxime were as follows: resistant, ≥64 μg/ml; intermediate, 32 to 16 μg/ml; and susceptible, ≤8 μg/ml (decreased susceptibility, 8 to 2 μg/ml; fully susceptible, <2 μg/ml). The MIC breakpoints for ceftiofur were as follows: resistant, ≥8 μg/ml; intermediate, 4 μg/ml; and susceptible, <4 μg/ml (decreased susceptibility, 2 to 1 μg/ml; fully susceptible, <1 μg/ml). The zone diameter standards for cefpodoxime and piperacillin were as follows: resistant, ≤17 mm; intermediate, 18 to 20 mm; and susceptible, ≥21 mm. The zone diameter standards for ampicillin were as follows: resistant, ≤13 mm; intermediate, 14 to 16 mm; and susceptible, ≥17 mm. The zone diameter standards for ceftriaxone were as follows: resistant, ≤13 mm; intermediate, 14 to 20 mm; and susceptible, ≥21 mm. The zone diameter standards for amoxicillin-clavulanic acid were as follows: resistant, ≤13 mm; intermediate, 14 to 17 mm; and susceptible, ≥18 mm. The zone diameter standards for cefepime were as follows: resistant, ≤14 mm; intermediate, 15 to 17 mm; and susceptible, ≥18 mm. The zone diameter standards for ticarcillin were as follows: resistant, ≤14 mm; intermediate, 15 to 19 mm; and susceptible, ≥20 mm. The zone diameter standards for imipenem were as follows: resistant, ≤13 mm; intermediate, 14 to 15 mm; and susceptible, ≥16 mm. And the zone diameter standards for aztreonam were as follows: resistant, ≤15 mm; intermediate, 16 to 21 mm; and susceptible, ≥22 mm.
Includes isolates with decreased and full susceptibility.