title: Pathogenicity factors of the Streptococcus milleri group
Creators: name: McStay, Lynnzie S.
Publisher: name: University of Glasgow
Year: 1997
Subjects: subject: Microbiology, pathology.

description: The "Streptococcus milleri group" (SMG) are part of the commensal flora, inhabiting the mouth, respiratory and gastrointestinal tracts. They are now recognised as significant pathogens with a proclivity for pyogenic infections such as dental, liver and brain abscesses, as well as being involved in endocarditis and other diseases - Much confusion had surrounded the nomenclature and taxonomy of the group until recently, when three distinct species S.anginosus, S.constellatus and S.intermedius were identified. This led to greater characterisation of the group, but its pathogenicity is less well understood. The aim of the work in this thesis was therefore to examine some of the factors of the SMG which contribute to causation of infection. A group of strains for analysis were collected by isolating organisms from dental abscesses, clinical infections of non-oral source and dental plague. These represented the three species and were tested for Lancefield grouping, haemolysin, hyaluronidase and DNase production and ability to agglutinate red blood cells. The majority of strains were Lancefield non-groupable, but where a grouping could be determined, group F predominated. Toxin and enzyme activity varied and could not be attributed to species or isolate type, and no strains agglutinated RBCs. Therefore, a heterogenous group of isolates was collected for further investigation. Encapsulation had been suggested as a virulence factor for the SMG by previous investigators (Brook and Walker 1985 : Lewis et al 1988), although the mechanism was unclear. Therefore, the capsule of the SMG was investigated by negative staining and electron microscopy. All but one strain were encapsulated with small and large capsule types revealed by both methods. Hyaluronic acid was shown to be a component of the capsule as treatment with hyaluronidase removed capsule to varying degrees. Bacterial cell surface hydrophobicity was measured using two methods, with results of each showing good correlation. Strains exhibited both hydrophobic and hydrophilic characteristics, with possession of capsule or species and isolate type having no bearing on this property. Willcox and Knox (1990) found that dental abscess isolates adhered to BEC to a greater extent than other SMG organisms, and this was investigated. This was indeed found to be true, with dental abscess isolates adhering more readily than both clinical and plaque isolates. It was found that capsule hindered the adherence of the SMG to BEC, as treatment of strains with hyaluronidase caused increased adherence. Cell surface hydrophobicity had no effect on ability to adhere to BEC. It was suggested that the capsule masked specific adhesins on the SMG surface which adhered to receptors on the BEC. The opsonic requirements of the SMG were investigated as was their ability to elicit a chemiluminescent response from PMNL. Opsonization was found to increase chemiluminescence, with serum concentrations of 15% or more proving optimum. A range of chemiluminescence values were obtained, but these could not be related to either species or isolate type. In addition, capsule and cell surface hydrophobicity played no part in either opsonization or chemiluminescence. In addition, phagocytosis of the SMG was further studied to determine if they were able to resist ingestion. Capsule was found to be antiphagocytic, strains with large capsules being ingested less readily than those with small capsules. This property was confirmed, as treatment with hyaluronidase to remove capsule resulted in increased ingestion efficacy. Again no relevance could be attributed to either isolate type or species in the level of ingestion produced, and cell surface hydrophobicity again appeared to play no role. Therefore the aims of the work were achieved, with a greater understanding of some of the pathogenicity factors of the SMG having been obtained. However, these are still only a small part of the whole picture as to why the SMG frequently cause purulent infection, either as the sole infecting organism or within a mixed infection, and much further work is required. It is hoped that an increased understanding of the group will in turn lead to better disease management of patients infected with this clinically significant group of bacteria.