TABLE 2.
Frequency of recombinational replacements in cells of penicillin-susceptible S. pneumoniae strain TIGR4 transformed with chromosomal DNA of penicillin-resistant serotype 19F strain GA71 on the basis of the number of colonies selected for resistance to different concentrations of antimicrobial agents
| Antimicrobial agent used to select transformants (concn [mg/ml]) | Frequency of resistant transformants among all cells in transformation culturea | Frequency of 19F serotype cells among resistant transformants |
|---|---|---|
| Trimethoprim (25) | 3.6 × 10−3 | 2.1 × 10−5 |
| Penicillin (0.1) | 1.1 × 10−5 | 2.7 × 10−3 |
| Penicillin (0.3) | 2.5 × 10−7 | NDb |
| Trimethoprim (25) + penicillin (0.1) | 1.8 × 10−8 | ND |
| Cefotaxime (0.03) | 7.3 × 10−4 | 1.1 × 10−3 |
| Cefotaxime (0.3) | 1.3 × 10−5 | 8.5 × 10−3 |
| Trimethoprim (25) + cefotaxime (0.3) | 2.0 × 10−9 | ND |
a
The cell density in transformation cultures varied at the time of selective-medium inoculation from 1.1 × 108 to 1.5 × 108 CFU/ml.
b
ND, not detected.