Synthesis and Biological Effects of Silver Nanoparticles on the Bacteria Staphylococcus aureus and Escherichia coli

Anes Abdulhadi, Mohammed Jawad (2018) Synthesis and Biological Effects of Silver Nanoparticles on the Bacteria Staphylococcus aureus and Escherichia coli. PhD thesis, Universiti Malaysia Sarawak (UNIMAS).

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Abstract

Silver nanoparticles (Ag-NPs) have strong antimicrobial potential, which has been exploited in many biomedical applications. However, the interaction between these unique nanomaterials and microorganisms has not been well elucidated. It is necessary to understand the mechanisms of interaction between nanomaterials and microorganisms. Therefore, this study investigates the interaction that occurs between Ag-NPs and the bacteria Staphylococcus aureus (s. aureus) and Escherichia coli (E. coli), with a focus on analysing the influence of size and surface functionalisation of Ag-NPs towards their antibacterial applications. To perform the study, various Ag-NPs were synthesised, characterised and tested for their antibacterial activity; the synergy between laser light and Ag-NPs towards enhanced antibacterial activity was also examined. Three sizes were synthesised via the chemical reduction method, using tri-sodium citrate (TSC) and sodium borohydride (NaBH4) as the reducing and capping agents. These different sizes (5, 10, 20 nm) Ag-NPs were tested for toxicity and antibacterial activity against the Gram-negative bacterium, E. coli and Gram-positive bacterium, S. aureus. The different-sized Ag-NPs all exhibited antibacterial activity against both E. coli and S. aureus strains. However, Ag-NPs with 5 nm displayed the highest antibacterial activity when compared with 10 and 20 nm particles sizes. The 20 nm size particle showed the lowest antibacterial efficiency against both bacterial strains and has therefore been selected for further studies. The photoactivation of 20 nm Ag-NPs using 410 nm laser light demonstrated that photoactivated Ag-NPs at 50 ~glml exposed for 10 min reduced E. coli and S. aureus . populations to 13% and 28%, respectively. S. aureus exhibited lower sensitivity after photoactivation compared to E. coli. Photo activation of 20 nm Ag-NPs by laser revealed that production of reactive oxygen species (ROS) due to oxidative stress, the mode of action against bacteria, causing cell wall cleavage and cell lysis, as shown by the scanning iv electron microscope (SEM). An additional experiment to examine the effect of Ag-NPs functionalised with antibodies was conducted using a 20 om size particle as the antibody carriers for inducing selective bacterial activity. This study utilised Ag-NPs functionalised with antibodies specific to one of the bacterial cell wall components against S. aureus to investigate the Ag-NPs' antibacterial ability to bind and destroy their target bacteria when excited using 410 om irradiation. The conjugation of 20 om Ag-NPs to antibody after photoactivation by laser irradiation led to 100% dead cells after 10 min exposure time. Physical damage to bacterial cells was observed via fluorescence assay, leading to loss of membrane integrity in treated bacterial cells. The proposed mechanism of killing is that, the irradiation of Ag-NPs' surfaces causes oxidative stress, which induces ROS, such as 'OH, 02'- and H202 generation, that attack cell components resulting in loss of membrane permeability and DNA damage, as seen via the SEM and fluorescence assay. This study demonstrates that several factors can influence the microbial effectiveness of Ag-NPs, including the size of particles, photo activation and surface functionalisation. The antibacterial activity of 20 om Ag-NPs can be enhanced using several methodologies outlined within this thesis. The research shows that functionalised Ag-NPs appropriately recognize and bind to their target bacteria, and that the Ag-NPs successfully complete the energy transfer from a visible laser to the target bacteria.

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