Determining the Optimal Thymidine Concentration for Growing Thy(−) Escherichia coli Strains

Changes of thymidine concentration in the growth medium affect the chromosome replication time of Thy(−) strains without at the same time causing a detectable difference in the growth rate (R. H. Pritchard and A. Zaritsky, Nature 226:126–131, 1970). Consequently, the optimal thymidine concentration cannot be determined by ascertaining which concentration produces the highest growth rate. Here we present a method for determining the optimal thymidine concentration of any Thy(−) Escherichia coli strain. Using this method, we found that the E. coli “wild-type” strain MG1655 has a partial Thy(−) phenotype.