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Title: Mechanisms of haem acquisition by the black-pigmenting anaerobes, Prevotella intermedia and Porphyromonas gingivalis
Author: Byrne, Dominic Patrick
ISNI:       0000 0004 2733 6647
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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Periodontitis describes a set of related inflammatory diseases that damage the periodontal tissues that support the teeth. It is caused by bacterial infections that, if left untreated, can lead to progressive loss of alveolar bone and subsequent loosening and loss of teeth. Periodontitis is the primary cause of tooth loss in adults and treatment of more severe forms of the disease can be both painful and costly. Black-pigmenting bacterial species, such as Porphyromonas gingivalis and Prevotella intermedia, are among the major periodontal pathogens associated with severe periodontitis. These bacteria form black pigments consisting of iron protoporphyrin IX (haem; Fe(III)PPIX) when grown on blood-containing media. Development of the haem- containing black pigments by these organisms serves an important defensive role as the intrinsic catalase activity of ferrihaem destroys H202 (Smalley et al., 2000). Furthermore, formation of the pigment by strictly anaerobic P. gingivalis is a process which consumes oxygen and promotes anaerobiosis (Smalley et al., 1998; 2002). This study sought to further characterise some of the facets of the mechanisms of haem acquisition by these two bacteria, with particular emphasis on the role of specific virulence factors (InpA of P. intermedia, HmuY and the gingipains of P. gingivalis, and LPS from both species). This resulted in identification of Interpain A (InpA), a 90 kDa cysteine protease, and homologue of Streptococcus pyogenes streptopain (SpeB), which is expressed by P. intermedia as a potential major virulence factor in haem acquisition through its ability to degrade haemoglobin, haemalbumin and haern-haemopexin. InpA-mediated proteolysis of these haemoproteins was highly dependent on experimental factors such as pH and redox potential, which profoundly affected the protease sensitivity of the haem -containing protein substrates. This suggested that InpA activity would be hugely influenced by the changing conditions of the periodontal pocket during progressive periodontitis. A unique syntrophic relationship of haem acquisition from haemoglobin was also identified between the HmuY haemophore and the K- and R- gingipains of P. gingivalis. Furthermore, InpA facilitated greater haem extraction by HmuY by promoting haemoglobin oxidation to the methaemoglobin form. This suggested a degree of mutualism and possible cross-reactivity between the haem acqui sition systems of these two bacteria which has possibly arisen on account of their close proximity within the periodontal pocket biofilm. This study also identified a possible central role for LPS in binding and storage of haem at the cell surface of these black-pigmenting bacteria and presents evidence suggesting that the lipophilic nature of this molecule, in addition to the respective pericellular pHs of P intermedia and P gingivalis cells, may affect the species of haem present in the pigments.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral