Cationic antimicrobial peptides (CAPs) offer a viable alternative to conventional antibiotics as they physically disrupt the bacterial membranes, leading to cell lysis and death. However, colonized bacteria often form “biofilms” – characterized by the overproduction of exopolysaccharides - that restrict the penetration of antibiotics; successful antimicrobial agents must evade this exopolysaccharide ‘matrix’ to reach the bacterial membrane. Since the Pseudomonas aeruginosa biofilm alginate traps CAPs by forming peptide-alginate complexes, the aim of this thesis is to better understand the mechanisms of interaction of CAPs with bacterial membranes and biofilm alginate. Using a series of CAPs designed in our lab derived from the sequence KKKKKK-AAFAAWAAFAA-NH2, we found that hydrophobicity, charge distribution, and amino acid composition of CAPs play important roles in their membrane disruptive power, bioactivities, alginate-binding and alginate-diffusion abilities. These findings suggest routes to an optimal balance of factors in CAP design to allow both biofilm penetration and bacterial membrane destruction.

Cationic antimicrobial peptides (CAPs) offer a viable alternative to conventional antibiotics as they physically disrupt the bacterial membranes, leading to cell lysis and death. However, colonized bacteria often form “biofilms” – characterized by the overproduction of exopolysaccharides - that restrict the penetration of antibiotics; successful antimicrobial agents must evade this exopolysaccharide ‘matrix’ to reach the bacterial membrane. Since the Pseudomonas aeruginosa biofilm alginate traps CAPs by forming peptide-alginate complexes, the aim of this thesis is to better understand the mechanisms of interaction of CAPs with bacterial membranes and biofilm alginate. Using a series of CAPs designed in our lab derived from the sequence KKKKKK-AAFAAWAAFAA-NH2, we found that hydrophobicity, charge distribution, and amino acid composition of CAPs play important roles in their membrane disruptive power, bioactivities, alginate-binding and alginate-diffusion abilities. These findings suggest routes to an optimal balance of factors in CAP design to allow both biofilm penetration and bacterial membrane destruction.

Thèse doctorat : Ecologie Microbienne : Villeurbanne, INSA : 2005. / Titre provenant de l'écran-titre. Bibliogr. p. 107-121.

Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 117-136).

Thesis (Ph. D.)--University of Hong Kong, 2010. / Includes bibliographical references (leaves 176-213). Also available in print.

Thesis supervisor: David R. Soll. Includes bibliographic references (p. 280-316).

Thesis (Ph. D.)--State University of New York at Binghamton, Department of Biological Sciences, 2009. / Includes bibliographical references.

Thesis (doctoral)--University of Helsinki, 2000. / Includes bibliographical references. Also available on the World Wide Web.

Bacteria, in the form of biofilm, has been shown to play a critical role in the etiology and pathogenesis of periodontal and peri-implant infectious diseases. Studies have shown that distinctively different dental plaque is commonly found in healthy versus inflamed gingivae and mucosa. It should be noted, however, that in most of these studies, the healthy and diseased plaque samples were collected from different individuals. To address this important issue, in the two studies described within this thesis, I recruited subjects who were periodontally involved and/or had inflamed peri-implant tissues, and also had equivalent healthy control sites. In the first study, I analysed the subgingival microbiota of a cohort of tea labourers from Sri Lanka, who had never performed any oral hygiene practices. Within each of the 32 subjects, one ‘shallow’ (healthy) site and one ‘deep’ (diseased) site were chosen for subgingival plaque sampling. A 16S ribosomal RNA (16S rRNA) gene sequencing method was applied to investigate the diversity of the subgingival microbiome, and to compare the microbial composition between healthy and diseased sites. A taxonomically diverse subgingival microbiota was identified, with 318 operational taxonomic units (OTUs; 98% identity cut-off) from 1,887 cloned full-length 16S rRNA gene sequences. The subgingival microbiota was dominated by the phyla Firmicutes, Proteobacteria and Fusobacteria. A significant difference in the overall composition of microbial communities between shallow and deep sites was found ((-Libshuff, p<0.001) while pairwise comparisons within each subject revealed no significant differences. The absence of oral hygiene resulted in a highly diverse subgingival microbiota in this cohort. In the second study, 22 subjects who had both implants and teeth surrounded by healthy and inflamed tissues, were included for subgingival/submucosal microbiological sampling. Quantitative real-time polymerase chain reaction (q-PCR) was used to detect and to quantify six species, including putative periodontal pathogens, i.e., Porphyromonas gingivalis (P.g.), Treponema denticola (T.d.), Aggregatibacter actinomycetemcomitans (A.a.), Fusobacterium nucleatum (F.n.), Prevotella intermedia (P.i.), and Staphylococcus aureus (S.a.). Within the same subjects, putative periodontal pathogens were common to both periodontal and peri-implant sites irrespective of health status. The detection frequencies for each of the six target species at diseased tooth or implant sites were either equal to, or higher than, the respective detection frequencies at the corresponding healthy sites. Both periodontal and peri-implant sites, irrespective of their health status, were revealed to harbour S. aureus. Even though the target organisms were found in all clinical conditions, there were differences in the involvement of some of the pathogens for the diseased conditions. The prevalence and levels of P. gingivalis and F. nucleatum were significantly associated with periodontitis, but not with peri-implantitis. A. actinomycetemcomitans was associated with both disease conditions, periodontitis and peri-implantitis, but not with either gingival or mucosal health. In conclusion, results from my two studies indicated that the differences between the composition of subgingival microbial communities present in single sites within two different individuals, were always greater than the differences in microbial community composition present in two subgingival sites of differing health status within the same individual. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

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