DNA Base Excision Repair: a Network of Defense, Mutagenesis, and Tolerance in Streptococcus mutans

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Microbiology and Immunology, 2012.The oral pathogen Streptococcus mutans possesses inducible DNA repair defenses for protection against pH fluctuations and reactive oxygen metabolites, such as hydrogen peroxide (H2O2), which occur in the oral cavity. DNA base excision repair (BER) plays a critical role in maintaining genomic integrity by preventing the accumulation of genetic mutations associated with environmental insult. In the present study, we examined the biological consequences that result from compromising the DNA glycosylases (Fpg and MutY) and endonucleases (Smx and Smn), of the BER pathway, and their relative role in stress-adaptation and virulence of the organism. Enzymatic characterization of the BER system revealed its protective role in dealing with oxidized DNA damage; specifically for detecting and repairing oxidized bases and AP sites. In vitro analysis revealed a complex interaction of the BER enzymes in protecting the cell against mutagenic lesions and the plethora of biological responses to DNA damage. S. mutans strains lacking a functional Fpg, MutY, or Smn showed elevated spontaneous mutation frequencies and these mutator phenotypes correlated with the ability of the strains to survive killing by acid and oxidative agents. In addition, use of the G. mellonella virulence model showed that loss of BER resulted in elevated virulence, as compared to other strains tested. The data indicate that, for S. mutans, mutator phenotypes arising from the loss of BER enzymes may confer advantage to the organism’s virulence potential. Finally, we determined the effects of acidification on genomic stability in S. mutans. The results indicated that chronic exposure to acid results in increased genomic instability, abnormal mutability, and temporary resistance to oxidative stress environments due to functional loss of DNA repair defenses. In summary, the present work demonstrates that the BER system can contribute to the regulation of many aspects of the biological networks in S. mutans that include DNA repair, mutagenesis, biofilms, adaptation, survival, persistence, tolerance to DNA-damaging agents, and virulence. This study provides a more complete perspective of the contributions of DNA repair in S. mutans, facilitating our understanding of the role of these enzymes in adaptation and virulence