Determination of in vitro postantibiotic effects in Staphylococcus aureus and Escherichia coli by [3H]thymidine incorporation

Postantibiotic effects (PAE) and control-related effective regrowth time (CERT) of dicloxacillin, vancomycin, rifampin and gentamicin in Staphylococcus aureus and imipenem, gentamicin, tobramycin, doxycycline and rifampin in Escherichia coli were measured by standard viability counting and ³H]thymidine incorporation. For PAE determination, the two methods correlated well; r ² = 0.821 for S. aureus and r ² = 0.939 for E. coli . For viable counts below the detection limits of 10⁵ to 10⁶ log₁₀ CFU/mL, the PAE was overestimated by the ³H]thymidine method. Quantitation of CERT by both methods showed a good correlation, r ² = 0.867 for S. aureus and r ² = 0.997 for E. coli . Measuring ³H]thymidine incorporation in bacteria is a novel alternative method for the determination of PAE and CERT.