Effects of beta-lactam antibotics on thymidine incorporation of bacteria

Five strains each ofEscherichia coli, Proteus mirabilis, andStaphylococcus aureus were grown in Trypticase soy broth (TSB) with or without penicillin, oxacillin, ampicillin, amoxicillin, or cefamandole. ³H]Thymidine incorporation and the optical density of the cultures were determined at an hourly interval for the duration of incubation. All strains ofP. mirabilis showed after 2 and 3 h of incubation of 5-to 16-fold increase in the specific activity of ³H]thymidine incorporated in the presence of MIC of the antibiotics as compared to controls grown without the drugs.E. coli andS. aureus showed smaller increases in thymidine incorporation than didP. mirabilis. After 4–5 h the antibiotics produced an inhibition of ³H]thymidine incorporation. At the MAC, the responses were of a smaller magnitude. Regardless of whether these changes are the result of specific interference or just lack of ³H]thymidine incorporation, they are directly related to the antibiotic activity of agents known not to affect DNA synthesis. The monitoring of ³H]thymidine incorporation detects early antibiotic activity probably earlier than other current systems which are used for this purpose.