A modified Luria-Delbrück fluctuation assay for estimating and comparing mutation rates.

We have investigated the accuracy with which mutation rates may be estimated using a modification of the Luria and Delbrück fluctuation experiment protocol. The modification involves growing a larger-than-usual culture, and plating out a small aliquot of it. Monte Carlo simulations of the experiments confirm that the modification leads to a decrease in the coefficient of variation of the estimated mutation rate where this is based on the median number of mutants detected in a number of cultures grown in parallel. If sets of experimental and control cultures are compared using the Mann-Whitney U-test, then fractional increases in mutation rate can be reliably detected using relatively small numbers of cultures. The modified protocols promise better estimates of mutation rates, offer a powerful test of differences in mutation rates, and are easier to implement in practice.