Table 1.
Distribution of Minimum Inhibitory Concentrations (MICs) as determined by broth microdilution of 16 antibiotics to 41 Streptococcus thermophilus strains isolated from raw milk.
| Antibiotic | EFSA's cut-offs (μg mL−1) | Number of strains with the following MICs (μg mL−1) | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ≤0.06 | 0.12 | 0.25 | 0.5 | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 | 256 | ||
| Gentamicin | 32 | 10 | 21 | 8 | 2 | |||||||||
| Kanamycin | 64 | 2 | 12 | 18 | 9 | |||||||||
| Streptomycin | 64 | 3 | 20 | 13 | 4 | 1a | ||||||||
| Neomycin | – | 6 | 18 | 9 | 4 | 3 | 1a | |||||||
| Tetracycline | 4 | 3 | 8 | 17 | 5 | 1 | 1 | 3 | 3 | |||||
| Erythromycin | 2 | 23 | 10 | 6 | 2b,c | |||||||||
| Clindamycin | 2 | 35 | 3 | 1 | 2c,d | |||||||||
| Chloramphenicol | 4 | 1 | 29 | 11 | ||||||||||
| Ampicillin | 2 | 10 | 17 | 9 | 3 | 2 | ||||||||
| Penicillin G | – | 13 | 15 | 5 | 4 | 2 | 1 | 1 | ||||||
| Vancomycin | 4 | 3 | 34 | 2 | 1 | 1 | ||||||||
| Virginiamycin | – | 10 | 23 | 8 | ||||||||||
| Linezolid | – | 2 | 34 | 5 | ||||||||||
| Trimethoprim | – | 1 | 12 | 13 | 15d | |||||||||
| Ciprofloxacin | – | 1 | 5 | 17 | 14 | 4 | ||||||||
| Rifampicin | – | 7 | 5 | 18 | 9 | 2 | ||||||||
Gray-shaded boxes show strains considered resistant in this work.
The strain with the highest MIC to both streptomycin and neomycin was the same.
These two strains were able to grow at the highest erythromycin concentration assayed in the microdilution assay using the VetMIC plates (8 μg mL−1); the current MIC value was obtained in a subsequent E-test assay, testing a larger range of antibiotic concentrations.
The two strains resistant to erythromycin and clindamycin were the same.
The highest concentration assayed for clindamycin and trimethoprim in the microdilution assay was 8 and 64 μg/ml, respectively; MICs of these two antibiotics should be read as ≥16 and ≥128 μg/ml, respectively.