Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Methicillin resistant Staphylococcus aureus (MRSA) is a group of S. aureus strains that has acquired resistance to a class of beta lactam antibiotics and is the major cause of hospital associated infections. Their discovery goes back to 1960 when the first cases were identified. Recently community associated MRSA infections have emerged and are caused by strains that are independent of those from the hospital environment, related only because they carry some of the same antibiotic resistance genes. Community associated infections (CA) are more severe, producing pus filled lesions that are painful and capable of invasion of deep tissues. Virulence factors comprised of exported proteins are associated with the invasiveness of CA strains. Most of these proteins are hypothetical in nature with unknown function. The aim of this study is to identify and characterize potential virulence factor proteins that may be involved in the infection pathway of CA-MRSA. This study focuses on a unique gene that encodes an exported protein, SAS1738, found on the chromosome of the CA strain MSSA476. The protein SAS1738 was chosen because it is unique to CA strains and has homology to some proteins identified in other S. aureus strains known for their virulence and host immune evasion. The goal of this work is to characterize SAS1738 and to determine its role in the infection pathway of the organism. The gene of interest has been successfully cloned, expressed, and tested for toxicity in Caenorhabditis elegans, a nematode. The toxicity tests showed that SAS1738 is inhibitory to the growth and development of C. elegans. The actual mode of action of this protein in C. elegans is yet to be established. However, location of SAS1738 using a GFP fusion showed that the highest concentration of the fusion protein was in the gut of the worms. The purified protein when tested in a killing assay against C. elegans, resulted in the death of the worms at an average time point of 8 min after treatment. Microbiological assay results showed that the purified SAS1738 possessed antibacterial activity towards Micrococcus luteus and Proteus vulgaris. This suggests that SAS1738 may play a dual role of antagonizing the commensal flora of the human skin such as Micrococcus luteus and also induce a toxic effect on the human cells as suggested by its toxic effect on C. elegans. Determination of the role of this protein in the infection cycle of CA-MRSA will lead to a better understanding of the pathogenicity of the organism and possible development of new treatment strategies.

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