Bio-oligomers as antibacterial agents and strategies for bacterial detection

Abstract

In this thesis I examined the potential of Bio-Oligomers such as peptoids, peptides and aptamers, as therapeutic and diagnostic entities. Therapeutic Bio-Oligomers; A series of peptoid analogs have been designed and synthesised using solid phase synthesis. These peptoids have been subjected to biological evaluation to determine structure-activity relationships that define their antimicrobial activity. In total 13 peptoids were synthesised. Out of 13 different peptoids, only one peptoid called Tosyl-Octyl-Peptoid (TOP) demonstrated significant broad-spectrum bactericidal activity. TOP kills bacteria under non-dividing and dividing conditions. The Minimum Inhibitory Concentrations (MIC) values of TOP for S. epidermidis, E. coli and Klebsiella were 20 µM, whereas Methicillin-resistant Staphylococcus aureus (MRSA) and Methicillin-sensitive Staphylococcus aureus (MSSA) were 40 µM. The highest MIC values were observed for Pseudomonas aeruginosa (PAO1) at 80 µM. The selectivity ratio (SR) or Therapeutic index (TI) was calculated, by dividing the 10% haemolysis activity (5 mM) by the median of the MIC (50 µM) yielding a TI for TOP as 100. This TI is well above previously reported peptidomimetics TI of around 20. TOP demonstrates selective bacterial killing in co-culture systems and intracellular bacterial killing activity. Diagnostic Bio-Oligomers; In the second part of my thesis, I investigated aptamer and peptide-based molecular probes to detect MRSA. As well as screening aptamers and peptide probes against whole MRSA, I over-expressed and purified PBP2A protein. This purified protein was used as a target for aptamer and peptide probes to detect MRSA. Two different aptamer libraries were initially screened for utility. In-vitro conditions for SELEX were optimised. Biopanning with a phage derived peptides was also performed. Target sequences for both methods were identified and chemically synthesised. Evaluation of fluorescently labelled sequences with flow cytometry and confocal imaging showed no specificity for MRSA detection with either method. The Bio-Oligomers and the in-vitro selection methodology require further refinement to improve diagnostic utility.