Table 2. Effect of ΔrecA mutation on ciprofloxacin-induced hypermutation in STEC EDL933.
| Experiment Number | Strain | Rifampin Resistance Frequency per million | Fold-Change in Rifampin- Resistance Frequency with cipro, Mean ± SD | Fold-Decrease in Rifampin Resistance with Zinc compared to cipro alone Mean ± SD | |||
|---|---|---|---|---|---|---|---|
| Control | Cipro Conc Used * | Cipro-Induced Rif Resistance | + Cipro + 0.2 mM zinc | ||||
| 1 | EDL933 (wt) | 0.05 ± 0.01 | 6 ng/ml | 1.48 ± 0.36 | not done | 29.5-fold increase | not done |
| 2 | EDL933 | 0.141 ± 0.05 | 6 ng/ml | 0.625 ± 0.17 | not done | 4.3-fold increase | not done |
| 3 | EDL933 | 0.063 ± 0.01 | 7 ng/ml | 0.544 ±0.33 | 0.102 ± 0.1 | 8.6-fold increase | 5.3 -fold |
| 4 | EDL933 | 0.116 ± 0.05 | 7 ng/ml | 4.29 | 0.06 ± 0.05 | 37-fold increase | 71-fold |
| 5 | EDL933 | 0.256 ± .038 | 8 ng/ml | 5.4 ± 1.5 | not done | 21.1-fold increase | not done |
| 20.1 ± 13.7-fold increase with cipro | 27.2 ± 37-fold decrease due to zinc | ||||||
| Cipro Conc Used ** | |||||||
| 1 | EDL933R ΔrecA | 0.54 ± 0.11 | 0.7 ng/mL | 0.25 ± 0.19 | not done | 2.1-fold decrease | N/A |
| 2 | EDL933R | 2.00 ± 0.27 | 1 ng/ml | 1.53 ± 0.43 | not done | 1.3- decrease | N/A |
| 3 | EDL933R | 11.2 ± 3.2 | 1.2 ng/ml | 4.17 ± 1.62 | not done | 2.7-fold decrease | N/A |
| Mean 2.0 ± 0.7 fold decrease with cipro | N/A | ||||||
*, As shown in Table 1, the ciprofloxacin MIC of the wild-type EDL933 is 23 ng/mL
**, Ciprofloxacin MIC for EDL933R mutant is 3 ng/mL.
For both strains we used concentrations of ciprofloxacin bracketing the value of one-third of the MIC. The concentration of rifampin in the LB plates used for detection of rifampin resistance was 10 μg/mL for the wild-type and 4 μg/mL for the EDL933R mutant.