Interactions of Oral Bacteria with Host Tissues and Allochthonous Microorganisms

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Abstract

THE UNIVERSITY OF MANCHESTER ABSTRACT OF THESIS submitted by Raja Moftah Moman for the Degree of Doctor of Philosophy entitled Interactions of Oral Bacteria with Host Tissues and Allochthonous Microorganism. 15th June 2017. The oral microbiome is a taxonomically diverse microbial community situated principally upon the hard and soft tissue surfaces of the mouth. It represents a readily accessible biofilm community for the investigation of bacteria-bacteria and bacteria-host interactions, which are responsible for some of the main features of oral biofilms in health and disease (colonisation resistance, antimicrobial tolerance, metabolic cross feeding, and other cooperative phenomena). In the oral cavity these relate specifically to cariogenesis and interactions with soft tissue that are responsible for periodontal disease. This doctoral thesis presents a series of investigations that consider processes for which growth in the biofilm phenotype or bacterial-bacteria or bacteria-host cell interaction are responsible. Four distinct methods were used to assess the effect of the biofilm phenotype on susceptibility of eight distinct oral hygiene actives with various modes of action. Bisphenol microbicide triclosan and the bis-biguanide chlorhexidine were most effective. All were markedly more effective against bacteria grown planktonically than the same organisms grown as biofilms illustrating antimicrobial tolerance, an important biofilm characteristic. In studies of interactions between oral isolates, bacteria previously isolated from the saliva and different oral sites of the oral cavity were tested using a modified cross streak method, in all possible pair-wise combinations. The frequency and strength of physical interactions (coaggregation) between these isolates was also assessed. The incidence of positive interactions was higher than the incidence of negative interactions (15.21% vs. 1.04%) and the incidence of coaggregation in bacteria isolated from saliva was significantly lower than for bacteria isolated from oral biofilms. Together, these data suggest that bacterial cooperation plays a greater role in oral biofilm development and maintenance than competition. With respect to putatively beneficial interactions between bacteria and host, the potential of the candidate dental probiotics L. rhamnosus GG, L. reuteri and S. salivarius to protect host tissues from damage by three Gram negative periodontal pathogens were investigated using human oral cells culture and the (invertebrate) G. mellonella model system. All probiotics inhibited the growth of the test pathogens when applied simultaneously, and significantly decreased toxicity (p<0.05, in most mixtures). The rank order of cytopathic effect for the pathogens was F. nucleatum> P. gingivalis > A. actinomycetemcomitans in two distinct cell lines. Whilst all probiotics conferred protection against the periodontal pathogens, L. rhamnosus GG, had the greatest protective effect, regardless of probiotic or pathogen used, followed by L. reuteri. S. salivarius was the least effective. Prophylactic treatment with probiotics conferred greater protection than treatment concomitant with pathogen challenge. The data presented in this doctoral thesis demonstrate the functional significance of interactions between taxonomically distinct bacteria and between bacteria and host tissues. Such interactions may determine the outcome of exposure to antimicrobials and are, particularly significant in health and through further research, may be harnessed for prevention and treatment of oral disease.

Keyword(s)

Bacterial interactions; Oral host tissues; Oral pathogens; Probiotics

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