The contribution of stress-induced mutagenesis to evolvability in Escherichia coli : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Biological Sciences at Massey University, Albany, New Zealand

Abstract

The emergence of antibiotic resistance is a significant public health concern. Stress-induced mutagenesis (SIM) in bacteria has been identified as a potential mechanism contributing to antibiotic resistance evolution. SIM is a process that occurs when cells are under stress and struggling to adapt to their environment. This stress induces the production of enzymes that can increase mutation rates. In bacteria, SIM relies on two stress responses: the SOS response and the RpoS stress response, which control a regulatory switch. This switch changes the type of DNA polymerase used for replication from high-fidelity (Pol III) to error-prone (Pol IV, Pol V, Pol II) DNA polymerases. The genes that encode these error-prone DNA polymerases play a vital role in maintaining fitness and generating genetic diversity by allowing the replication of damaged DNA and bypassing various DNA lesions. In this study, I investigated whether stress-induced mutagenesis in Escherichia coli contributes to evolvability, specifically the ability of a population to adapt to changing environments. I compared the evolvability of a SIM- mutant, which lacked error-prone DNA polymerases Pol IV and Pol II, to a wild-type SIM+ strain from a long-term evolution experiment and ancestral strains. I examined both ancestral strains and those from a long term evolution experiment in environments with and without mitomycin C, an SOS inducer. My results showed that the presence of both stress and error-prone polymerases enhances evolvability. I observed a more significant increase in fitness and mutation rates in the SIM+ strain compared to the SIM- mutant or when only one or neither factor was present. I also found that the SIM-mutant decreased the frequency of mutations and fitness in evolved populations, demonstrating that error-prone polymerases are crucial for SIM to enhance evolvability. My study highlights the significant role of SIM and error-prone polymerases in the evolution of antibiotic resistance, providing valuable insights into the SOS response and suggesting potential avenues for developing new drugs and treatments to combat this growing threat. Limiting the impact of SIM through targeted inhibition of error-prone polymerases and reducing stress levels may improve the management and prevention of antibiotic resistance.