A cautionary note on the mutation frequency in microbial research

Highlights: • The mutation frequency is unnormalized. • While the mean mutation frequency is infamously erratic, the median mutation frequency is not an effective remedy. • Nonmutant cell death can inflate mutation frequencies. • The mutation rate should be the only yardstick of microbial mutability. - Abstract: The mutation frequency, also known as the mutant frequency, is an unnormalized quantity, and its normalized counterpart is the mutation rate. Due to historical reasons, the mutation frequency has been a predominant yardstick of microbial mutability in the field of mutator identification. While the mean mutation frequency is infamously erratic, replacing it with the median mutation frequency is not an effective remedy. By encouraging investigators to substitute mutation rates for mutation frequencies in microbial research, this paper directs attention to substantial open problems such as false positive control and massive nonmutant cell death.