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Evidence for a Role for Coagulase-Negative Staphylococci and their Biofilms in the Historic and Future Evolution of Staphylococcus Aureus Including MRSA
Budri, Paulo Eduardo
<p>Rates of methicillin-resistant <em>Staphylococcus aureus</em> (MRSA) infections in Ireland are decreasing since 2006. However, MRSA and methicillin-susceptible <em>S. aureus</em> (MSSA) remain a major cause of healthcare-associated infections in Europe. Recent evolutionary trends indicate that MRSA lineages associated with infections in the community are causing infections in the healthcare setting with increasing frequency.</p> <p>It is therefore important to investigate potential reservoirs in the community that may contribute to MRSA and <em>S. aureus</em> evolution. The methicillin resistance gene, <em>mecA</em>, is located within a transferable genetic element, staphylococcal Cassette Chromosome (SCC), referred to as SCC<em>mec</em>, which is also frequently found in coagulase-negative staphylococci (CoNS) including <em>Staphylococcus epidermidis.</em> <em>S. epidermidis</em> is the predominant human skin colonizer and clinically, its most important virulence trait is biofilm production. Indirect evidence suggests that MRSA clones arose from genetic transfer of SCC<em>mec</em> from CoNS to MSSA. Adjacent to SCC<em>mec,</em> other antibiotic resistance genes, most notably <em>fusC</em>, encoding fusidic acid resistance, have also been found associated with SCC. It is thought that CoNS may be a reservoir of these antibiotic-resistances and the close proximity of these staphylococcal species in the human nasopharynx provides opportunity for such gene acquisitions.</p> <p>In this thesis, we describe the investigation of community-associated colonizing staphylococcal species as a reservoir that may underpin the evolution of <em>S. aureus</em>. In Chapter 3, DNA microarray analysis of nasal colonising staphylococcal isolates revealed a greater frequency of the antimicrobial resistance genes <em>mecA, fusC, fusB, ileS2, qacA/qacC </em>and the arginine catabolite mobile element (ACME) among colonising CoNS compared to <em>S. aureus</em> among 137 <em>S. aureus</em>-carriers with no previous healthcare exposure, identified from 444 medical students in their pre-clinical years. Among <em>S. aureus</em> carriers, MRSA and methicillin-resistant CoNS (MRCoNS) rates were 6.5% (9/137) and 13.1 % (18/137), respectively. Simultaneous carriage of MRSA and MRCoNS was found in 1/137 students studied. The clonal lineages of colonizing <em>S. aureus</em> included several clonal complexes (CC)s associated with the acquisition of SCC<em>mec</em>.</p> <p>In Chapter 4 an <em>in-vitro</em> study is described in which the efficiency of three mechanisms of horizontal gene transfer (transformation, transduction and conjugation) were investigated when applied in a staphylococcal biofilm model, for the transfer of clinically relevant antibiotic resistance genes identified in Chapter 3. Within a staphylococcal biofilm of the <em>S. aureus</em> strain RN4220, transduction was the most efficient genetic transfer mechanism, followed by conjugation and transformation. Transfer of the SCC<em>mec-fus </em>element, identified among <em>S. aureus </em>carriers, was successful in this model when exposed to sub-inhibitory concentrations of fusidic acid.</p> <p>In Chapter 5, the further detailed analysis of a subset of isolates from <em>S. aureus</em>-carriers (MRSA, MSSA and methicillin-susceptible <em>S. epidermidis</em> (MRSE)) using a combination of DNA microarray and whole genome sequence (WGS) analyses, is described. Using this approach, a high homology of SCC elements and <em>fusC</em> among <em>S. aureus</em> and <em>S. epidermidis</em> isolates from healthy people was revealed including 100% sequence similarity among the SCC<em>fus</em> elements.</p> <p>Overall, investigation of the staphylococcal species among 30.8% of medical students positive for <em>S. aureus</em>, prior to their exposure to the clinical environment, indicates a low rate of MRSA but the presence of several lineages into which SCC<em>mec</em> is capable of inserting. Furthermore, the finding of the composite element SCC<em>fus </em>carried in association with SCC<em>mec </em>V in the CC1 background, is concerning as it suggests the recent expansion of this clone in the community possibly against the selective pressure of un-regulated fusidic acid use. The stable transfer potential of this element with its associated phenotypic antibiotic resistance traits, determined in an <em>in-vitro</em> biofilm model, particularly by transduction further supports this evolutionary trend.</p> <p>Data presented support the...
Keyword(s): MRSA; Horizontal Genetic Transfer; Biofilm; Staphylococcus Aureus; CoNS; Nose Colonization; Bacteriology; Environmental Microbiology and Microbial Ecology; Medicine and Health Sciences; Microbial Physiology; Organismal Biological Physiology
Publication Date:
Type: Doctoral thesis
Peer-Reviewed: Yes
Institution: Royal College of Surgeons in Ireland
Citation(s): Budri P. Evidence for a Role for Coagulase-Negative Staphylococci and their Biofilms in the Historic and Future Evolution of Staphylococcus Aureus Including MRSA [PhD Thesis]. Dublin: Royal College of Surgeons in Ireland; 2018.
Supervisor(s): Dr Deirdre Fitzgerald-Hughes
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First Indexed: 2018-12-23 07:19:09 Last Updated: 2018-12-23 07:19:09