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Title: Small colony variants in Staphylococcus aureus and other species : antibiotic selection, antimicrobial susceptibility, and biofilm formation
Author: Norville, Phillip
ISNI:       0000 0004 2732 3379
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2011
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Staphylococcus aureus is one of the leading causes of hospital acquired infections. The ability of S. aureus to acquire resistance to a diverse range of antimicrobial compounds, results in limited treatment options, particularly in methicillin-resistant S. aureus. A mechanism by which S. aureus develops reduced susceptibility to antimicrobials is through the formation of small colony variants (SCVs). Reduced antimicrobial susceptibility in S. aureus SCVs is not related to ‘classical’ mechanisms of resistance, but occurs as a direct result of the development of the SCV phenotype. S. aureus SCVs are frequently associated with defects in the bacterial electron transport chain and these defects are responsible for the characteristics associated with the SCV phenotype. This study aimed to investigate and characterise the selection of S. aureus SCVs in the presence of various antibiotics and also to examine their biofilm forming capabilities. Four members of the aminoglycoside family of antibiotics were shown to select for S. aureus SCVs. In addition, a broad range (X 0.25 MIC – X 4 MIC) of aminoglycoside concentrations were shown to select for S. aureus SCVs. Characterisation of these isolates revealed that differences in auxotrophy, biochemical profiles, carotenoid production, haemolysis, levels of intracellular ATP, mutation frequency and reversion rate were present. Members of the tetracycline family of antibiotics were also shown to select for S. aureus SCVs. Tetracycline selected S. aureus SCVs show attenuated catalase, coagulase and heamolysis activity and reduced production of extracellular DNase and lipase and reduced susceptibility to various antimicrobial agents. As SCVs have been linked to persistent and recurrent infections their ability to form biofilms was also investigated. A range of S. aureus SCVs isolated from various backgrounds were shown to form greater biofilms in comparison to parent strains, which was attributed to increased production of polysaccharide intracellular adhesin. In addition S. aureus SCV biofilms displayed a more pronounced reduction in antimicrobial susceptibility, which was attributed to a reduction in antimicrobial penetration through SCV biofilms. Limited discovery of novel antibiotics in recent years and the observation that S. aureus SCVs can be selected for by various antimicrobial compounds highlights the need for novel antimicrobial compounds. Accordingly, an investigation into the susceptibility of S. aureus to various plant compounds was undertaken. Both S. aureus SCVs and parent strains showed susceptibility to five plant antimicrobials tested, of which SCVs were more susceptible to cinnamon bark, green tea and oregano. Resistance to these plant antimicrobials could not be induced and synergistic relationships between certain plant antimicrobials and antibiotics were demonstrated. Finally, formation of SCVs in bacterial species other than S. aureus was examined. Gentamicin induced SCV selection in Escherichia coli, Pseudomonas aeruginosa and S. epidermidis as well as chloroamphenicol and ciprofloxacin in E. coli and tetracycline in S. epidermidis. SCVs from these bacterial species shared common characteristics associated with the SCV phenotype including altered growth and biochemical profiles, auxotrophy for compounds involved in electron transport, reduction in expression of virulence factors and reduced antimicrobial susceptibility. Additionally all SCVs showed an increased capacity to form biofilms. The ability of certain antibiotics to select for SCVs and their increased capacity to form biofilms suggest that SCV are an important adaptation to aid survival and persistence in times of stress. Reduced susceptibility to commonly used antibiotics in SCVs signifies that the development of new antimicrobial compounds is required. Harnessing naturally occurring plant antimicrobials and their synergistic relationship with antibiotics may offer a novel approach to treating antibiotic resistant infections whilst overcoming antibiotic selection for SCVs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QR180 Immunology