Spontaneous mutation rates come into focus in Escherichia coli.

Although the long-term outcome of mutagenesis is evolution by natural selection, it can also have profound immediate effects even on the level of individual organisms. In humans, the accumulation of mutations can cause many types of cancer; in bacteria, mutations can lead to dangerous antibiotic resistance and other phenotypic changes; and in viruses, mutations can cause drastic changes in the pathogenesis or modes of transfer. For these reasons, among others, a thorough understanding of mutagenesis is extremely important. One of the fundamental properties of the mutagenesis is its rate-the probability of a mutation occurring within a defined time frame. Despite the lengthy history of studies on mutagenesis and mutation rates, new and exciting findings continue to emerge. This review briefly summarizes the state-of-the-art in mutation rate analysis and continues with a discussion of some recent compelling discoveries on the mutational topology of the E. coli chromosome.