Fletcher, Julie Maxine; (1999) Response of periodontopathogens to environmental changes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Periodontal diseases are associated with inflammation, loss of gingival attachment and erosion of alveolar bone. More than 300 species of bacteria have been isolated from human subgingival plaque samples. Of these, certain species including Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis and Fusobacterium nucleatum have been specifically implicated in the aetiology of one or more of the periodontal diseases. The microenvironment of the periodontal pocket is extremely complex and it is likely that there will be continual variation in the environmental conditions operating in this habitat. The aim of this thesis was to study the effect of environmental factors on the growth and virulence potential of periodontopathogenic bacteria. The redox potential (Eh) of the environment was found to profoundly affect the growth and survival of P. gingivalis and F. nucleatum. Addition of the redoxmodifying agent, methylene blue (MB), to suspensions of P. gingivalis and F. nucleatum caused an increase in the Eh of the medium and this was associated with killing of both organisms. In a small-scale clinical trial, application of MB to the periodontal pocket was associated with changes in the subgingival microflora which represented a shift towards a microflora more compatible with gingival health. The effect of growth medium and incubation atmosphere on the acid (AcP) and alkaline (AIP) phosphatase activity of A. actinomycetemcomitans was studied. AcP activity was greater under anaerobic conditions compared to CO2-enriched aerobic conditions. The effect of IL-1β and IL-6 on the growth of P. gingivalis and A. actinomycetemcomitans and the ability of these two organisms to degrade IL-1β, IL-6 and IL-1ra was determined. Neither IL-1β nor IL-6 affected the growth of either organism. However, P. gingivalis was shown to rapidly degrade these cytokines both in the absence and presence of serum. A. actinomycetemcomitans exhibited no cytokine-degrading activity. The effect of a range of environmental conditions (growth in a CO2-enriched aerobic atmosphere versus anaerobic growth, presence of serum or blood, biofilm versus planktonic mode of growth and iron depletion) on the surface protein and antigen profiles of A. actinomycetemcomitans was examined. Up-regulation and down-regulation of a number of proteins was observed when growth conditions were altered. This study has shown that environmental conditions do affect the growth and virulence potential of periodontopathogenic bacteria and furthermore, that environmental modification may represent an alternative to traditional antimicrobial agents in eliminating these microorganisms from the periodontal pocket.

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