title: Analysis of the inhibitory activity and mode of action of novel antimicrobial organic nanoparticles
Creators: name: Michael  Tatham, Lee
Publisher: name: University of Liverpool Repository
Year: 2023

description: Nanoparticles are difficult to define specifically hut usually encompass engineered particles ranging in size from 1 to 1000 nm. The physical and chemical properties. of nanoparticles can vary significantly from those of their bulk counterparts largely due to their large surface area to volume ratios. Approximately 40% antimicrobial agents emerging from development programs exhibit low solubility. This results in inadequate bioavailability, pharmacokinetics and stability. The use of appropriate nano-carriers has been shown to improve the efficacy of antimicrobial agents with the' explanation that the biodistribution of the antimicrobial follows that of the carrier rather than being dependent on the physiochemical properties of the compound itself. Therefore characteristics such as solubility and bioavailability can be enhanced. Here, a range of poorly water-soluble antifungal agents, biocides and an antibiotic were processed using a novel emulsion-evaporation technique to produce organic nanoparticles. These preparations were characterised on the basis of size and zeta potential and tested for inhibitory activity against relevant microorganisms including: C. albicans, E. coli, S. aureus and MRSA. Nanoparticle formulated antimicrobials were usually more inhibitory than the equivalent eo-solvent dissolved antimicrobials or water dissolved salt equivalents where available. However, efficacy was dependent on nanoparticle composition. Optimisation of nanoparticle dichlorophen inhibitory activity was attempted using a generic polymer and surfactant screen. The results were subsequently utilised in a computer modelling design approach. Due to formulation problems, predictive optimisation was not possible. However, nanoparticles of dichlorophen were usually most inhibitory when increased loading ratios of sodium dodecyl sulphate and hydroxy propyl methyl cellulose and reduced loading ratios of dichlorophen and gelatin were used in the preparation. No correlations between particle size, zeta potential and inhibitory activity were identified. No correlation between the inhibitory activities of blank nanoparticles and active equivalents were identified. A detailed series of controls prepared for one formulation usually produced low MIC values. However, the nanoparticle formulation exhibited the greatest efficacy. This suggested that enhanced activity due to nanoparticle formulation of the antimicrobial was not simply attributed to a synergistic effect between the different materials. The molecular response of S. aureus SH1000 to nanoparticle-formulated ciprofloxacin was investigated using RNA-Seq. All 5 investigated treatments induced differential gene expression. Moreover, comparative analysis between nanoparticle formulated and DMSO dissolved ciprofloxacin treated S. aureus SH1000 revealed the differential expression of 61 transcripts. No significant differential expression in DNA repair and replication targets was observed. This suggested that ciprofloxacin may not be more bioavailable to S. aureus SH1000 and therefore enhanced efficacy is not attributed to increased bioavailability. However, genes involved in stress response and cell division were shown to be up regulated in response to nanoparticle delivery. The results also revealed that 39 transcripts were differentially expressed due to nanoparticle exposure alone and these included stress response, cell division and virulence-associated genes. The identified differentially expressed transcripts are unlikely to account for the enhanced efficacy associated with nanoparticle delivery. Nanoparticles represent a novel approach to the delivery of hydrophobic anti microbials in aqueous dispersions. The advantageous features of nanoparticles are discussed throughout this thesis. The study used a variety of approaches with the aim of elucidating the mechanisms underpinning the observed enhancement in antimicrobial activity. The improved efficacy observed could not be correlated with any physical characteristics of the particles used. The transcriptional profiling results suggested that the improved antimicrobial activity observed was not associated with differential molecular targeting, and challenges current concepts that link enhanced efficacy with increased bioavailability.