Antibiotics are small molecules that kill bacteria by inhibiting essential processes. However, the concentrations used to kill bacteria in a clinical setting are typically much higher than the concentrations generated in nature, where most antibiotics are secreted by microbes. This discrepancy in concentrations, combined with a recognition that the human use of antibiotics bears little resemblance to the role of antibiotics in nature, prompted questions about whether growth inhibition was the primary function of antibiotics. Studying the effects of antibiotics at sub-lethal concentrations on bacteria could provide new insights into the natural role of antibiotics. One striking effect of bacterial encounters with sub-lethal antibiotics is the stimulation of biofilm formation. Biofilms are surface-adhered communities of bacteria. The biofilm lifestyle confers many benefits for bacteria and is a major mode of bacterial growth. Therefore, the ability of sub-lethal antibiotics to cause a transition from planktonic to biofilm growth indicates that antibiotics could be a driving force behind the assembly and abundance of bacterial communities in nature. Chapters Two and Three investigate the underlying mechanisms of this response in Escherichia coli and Pseudomonas aeruginosa, and suggest that sub-lethal antibiotics perturb central metabolism and respiration, changes that are sensed and relayed into increased biofilm formation to provide population-level protection. Chapters Four and Five investigate the effects of sub-lethal antibiotics on peptidoglycan metabolism in P. aeruginosa and E. coli. Peptidoglycan is an essential macromolecule for bacterial survival and is deeply integrated into their physiology. Furthermore, peptidoglycan synthesis is among the most favoured targets of antibiotics. Chapter Four investigates interactions between peptidoglycan-targeting antibiotics and folate metabolism-targeting antibiotics, and characterizes an overlooked connection between folate and peptidoglycan metabolism. Based on this work, we rationally designed a new inhibitor that potentiates folate and peptidoglycan-targeting antibiotics. Chapter Five sheds new light on peptidoglycan recycling by leveraging a pathway in P. aeruginosa for sensing and responding to sub-lethal doses of PG-targeting antibiotics. Finally, Chapter Six summarizes the understanding gained from Chapters Two through Five and synthesizes this information for broader insights on the possible roles of antibiotics in nature. / Thesis / Doctor of Philosophy (PhD) / The ability to cure infections with antibiotics revolutionized modern medicine and kick-started decades of research into the growth inhibitory properties of antibiotics. Although the therapeutic role of antibiotics as anti-bacterials is clear, the natural role of antibiotics is not. In nature, microbes export antibiotics, allowing them to interact with surrounding microbes that import those antibiotics, changing the physiology of the recipient. Understanding how antibiotics affect bacterial physiology at concentrations below the lethal dose can provide information about the natural role of antibiotics, which in turn can inform future antibiotic discovery. This thesis investigates two major effects of sub-lethal antibiotics on pathogenic bacteria. The first is the ability of antibiotics to induce the formation of surface-attached clusters of bacteria called biofilms. We showed that sub-lethal antibiotics have a common effect of disrupting cell metabolism, and this effect is translated into a signal for increased biofilm formation. The second is the effects of antibiotics on bacterial cell wall metabolism. We discovered that sub-lethal antifolate antibiotics impact cell wall metabolism in at least two different ways, and used this information to rationally design an inhibitor that overcomes antibiotic resistance. Further, sub-lethal antibiotics were used to identify new features of a cell wall recycling pathway. Overall, this work furthers our understanding of bacterial physiology at scales ranging from sub-cellular to multi-cellular, and reveals new impacts of sub-lethal antibiotics.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/29310 |
| Date | January 2024 |
| Creators | Yaeger, Luke |
| Contributors | Burrows, Lori, Biochemistry and Biomedical Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0014 seconds