Date of Degree
PhD (Doctor of Philosophy)
Kim A. Brogden
Periodontal disease is a chronic inflammation of the gingiva and periodontium that leads to progressive destruction and irreversible damage to the supportive structures of the teeth. It affects nearly half of the United States population and is a particular risk factor in adults older than 65 years of age. Oral microorganisms assemble in plaque as a polymicrobial biofilm and Porphyromonas gingivalis, an important secondary colonizer in oral biofilms, has been implicated in periodontal disease. Although the protective functions of various salivary molecules such as antimicrobial proteins have been delineated, lipids present in saliva and on the oral mucosa have been largely ignored and there is growing evidence that the role of lipids in innate immunity is more important than previously realized. In fact, recent studies suggest that sphingoid bases and fatty acids, which exhibit potent broad spectrum antimicrobial activity against a variety of bacteria and fungi, are likely important innate immune molecules involved in the defense against oral bacterial and fungal infections. However little is known about their spectrum of activity or mechanisms of action. In addition, the effects of these lipids that are endogenous to the oral cavity have not been explored against oral bacteria. In this study I hypothesized that oral mucosal and salivary lipids exhibit dose-dependent antimicrobial activity against P. gingivalis and alter cell morphology and metabolic events. To test this hypothesis, I first examined the effects of two fatty acids: sapienic acid and lauric acid, and three sphingoid bases: sphingosine, dihydrosphingosine, and phytosphingosine, against a variety of gram-positive and gram-negative bacteria including P. gingivalis. Using broth microdilution assays to determine minimum inhibitory and minimum bactericidal concentrations, I show that antimicrobial activity against bacteria is dose-dependent, lipid specific, and microorganism specific. Kill kinetics were also variable across each bacteria-lipid combination. Upon examination of select bacteria-lipid combinations via scanning and transmission electron microscopy, different morphologies were evident across all treatments, demonstrating differential activity of each lipid for a particular bacterium as well as for each bacterium across different lipids. In addition, all sphingoid bases and fatty acids were taken up and retained in association with P. gingivalis cells and could be extracted along with bacterial lipids and separated using thin layer chromatography. Using a combination of two-dimensional in-gel electrophoresis and Western blots followed by mass spectroscopy and n-terminus degradation sequencing, I show that sapienic-acid treatment induces a unique stress response in P. gingivalis, as evidenced by the ability of P. gingivalis to upregulate a set of proteins involved in fatty acid biosynthesis metabolism and energy production, protein processing, cell adhesion, and virulence. Finally, utilizing flow cytometry and confocal microscopy, I assessed the effects of oral antimicrobial lipids against a representative host cell and describe oral lipid concentrations that are both antimicrobial to P. gingivalis cells and non-cytotoxic to the representative host cells tested. Combined, these data strongly suggest that sphingoid bases and fatty acids found within the saliva and on oral mucosa likely do contribute to the innate antimicrobial activity of saliva, mucosal surfaces, and skin and this dose-dependent activity is both lipid specific and bacteria specific. This information adds to current knowledge of the innate functions of endogenous lipids in the oral cavity. With bacterial resistance to current antibiotics increasing, the exploration of new antimicrobial agents is important and these lipid treatments may be beneficial for prophylactic treatments or therapeutic intervention of infection by supplementing the natural immune function of endogenous lipids on skin and other mucosal membranes.
Antimicrobial lipids, Fatty Acids, Innate Immunity, Porphyromonas gingivalis, Sphingoid Bases
xi, 136 pages
Includes bibliographical references (pages 126-136).
Copyright 2013 Carol Lea Fischer