Features of Escherichia coli mutant colonies appearing on nonlethal conditions of selection

Abstract

Spontaneous mutagenesis in bacteria is understood as the product of genome-wide mutations occurring in the absence of selection during active cell proliferation in which mutant cells survive lethal selection while non-mutants cells die. However, when non-lethal selection is used to identify mutants, cells that have not yet mutated are still alive but unable to actively proliferate. This report addresses whether the first mutant colonies appearing on non-lethal selection plates are the product of spontaneous mutations occurring during exponential growth or whether they are produced by mutagenesis during selection. We infer, based on evidence obtained by using two unique methods, that most of these colonies are the product of mutation events happening soon after saturated bacterial cultures are deposited on plates. This mutability takes place irrespective of the selective allele location, whether chromosomal or extrachromosomal. This result demonstrates the fallibility of fluctuation analysis under non-lethal conditions of selection because this method assumes that the first mutants on a selection plate have occurred during exponential growth. A process of mutagenesis, such as the one observed here, occurring during starvation and in the absence of cell division, provides insights into how bacteria may quickly adapt to an ever-changing environment, and rapidly evolve new traits, such as antibiotic resistance. Furthermore, this is a demonstration of the strength of mutagenic processes, perhaps evolutionary conserved, that occur in the absence of active cell proliferation that might partly explain the acquisition of chemotherapy resistance in transformed cells.