Development of Promising Antimicrobial Materials Against Different Pathogenic Bacteria and Fungi


Student thesis: Doctoral Thesis

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Award date2 Sep 2021


The development of multi-drug resistance (MDR) in bacterial strains, including Enterococci, Staphylococci, Klebsiella, Acinetobacter, Pseudomonas, Enterobacter species, etc., has become a severe challenge. These bacteria are displaying resistance to common antibiotics, including Cephalosporins, Carbapenems, Vancomycin, and Methicillin. Furthermore, some fungi, for example, Candida species, are also showing resistance to various antifungal drugs, such as azole. These pathogenic microbes are causing life-threatening diseases, such as aspergillosis, candidiasis, pneumocystis pneumonia, sepsis, osteomyelitis, meningitis, cholecystitis, severe bacteremia, diarrhea, tuberculosis. Moreover, they are also involved in contaminating contact lenses and lens cases and causing various ocular diseases (bacterial keratitis, fungal keratitis, etc.), and inflammatory events (contact lens-related acute red eye, contact lens peripheral ulcer, infiltrative keratitis, etc.). Numerous antimicrobial drugs have been developed; however, because of the emergence of MDR in pathogens, the clinical efficiency of existing drugs is vulnerable. According to the World Health Organization (WHO), current deaths due to microbial diseases are ~0.7 million per year. If we could not develop efficient drugs to control or destroy these pathogenic microbes, annual death caused by microbial infections may rise to ~10 million by 2050. Therefore, this has become obligatory to find out alternate routes to tackle these MDR pathogenic microbes. Hence, this work mainly emphasizes the development of efficient antimicrobial materials with antioxidant and biocompatibility properties against MDR bacterial and fungal strains.

Firstly, plant extract was proposed to be used as active biomedical components for nanoparticles (NPs) synthesis, aiming to obtain NPs inheriting the biomedical functions of the plants. By using leaves extract of Clerodendrum inerme (C. inerme) as both a reducing agent and a capping agent, we have synthesized gold (CI-Au) and silver (CI-Ag) NPs covered with biomedically active functional groups from C. inerme. The synthesized NPs were evaluated for various biological activities such as antibacterial and antimycotic against different MDR pathogenic microbes (B. subtilis, S. aureus, Klebsiella, and E. coli) and (A. niger, T. harzianum, and A. flavus), respectively. The antimicrobial propensity of the NPs further assessed with reactive oxygen species (ROS) glutathione (GSH) and FTIR analysis. Biofilm inhibition activity was also carried out using colorimetric assays. They exhibit excellent antibacterial, antimycotic, and biofilm inhibition against pathogenic microbes comparing to commercial Au and Ag NPs functionalized with dodecanethiol and PVP, respectively. Biocompatibility tests further confirm that CI-Ag and CI-Au NPs were more biocompatible. Hence, this work opens a new environmentally-friendly path for synthesizing nanomaterials with enhanced and/or additional biomedical functionalities inherited from their herbal sources.

Secondly, multifunctional gallic acid (GA), phytomolecules-coated zinc oxide NPs (ZN), and phytomolecules-coated zinc oxide NPs + gallic acid + tobramycin (ZGT) coated contact lenses were developed using a single-step sonochemical technique. The coated contact lenses present excellent antibacterial (>log10 5.60), antifungal (>log10 5.50), and biofilm inhibition performance against bacterial keratitis (BK), causing multi-drug resistant bacteria (S. aureus, P. aeruginosa, E. coli) and fungal keratitis (FK) related pathogenic fungal strains (C. albicans, A. fumigates, and F. solani). The gallic acid, tobramycin, and phytomolecules-coated zinc oxide nanoparticles have various functional groups (-OH, -NH2, -COOH, -COH, etc.) that enhanced wettability of the coated contact lenses compared to uncoated and further enabled them to exhibit remarkable antifouling property by prohibiting adhesion of platelets and protein. The coated contact lenses also present significant antioxidant activity by scavenging DPPH and good cytocompatibility to human corneal epithelial cells and keratinocytes cell lines. These findings proposed that coated contact lenses can be beneficial for treating ocular surface infections such as bacterial keratitis, fungal keratitis, and inflammatory events.

Thirdly, zwitterionic and antimicrobial metal oxide-phenolic networks (MOPNs) (PSBDA@CeO2 NPs) based on cerium oxide (CeO2) nanoparticles (NPs) and poly (sulfobetaine-co-dopamine methacrylamide) copolymer (PSBDA) were developed that could be conveniently coated onto CLs owing to the adherent properties of dopamine's catechol groups. The zwitterionic and antifouling sulfobetaine methacrylate (SBMA) groups in the PSBDA@CeO2 NPs coating can substantially enhance the hydrophilicity of CLs while decreasing protein, platelets, and live/dead microbial's adsorption, resulting in a more water-retentive and biomolecules-free lens surface that prevents tear film evaporation and eye dryness. Additionally, since the MOPNs contain immobilized cerium ions, the zwitterionic PSBDA@CeO2 NPs coating exhibits potent and broad-spectrum antimicrobial activity against BK and FK-associated pathogenic microbes. In comparison to the uncoated CL, the PSBDA@CeO2 NPs coated CL prevents biofilm formation efficiently after 14 days of long-term exposure with the above microbiota environment. Importantly, the zwitterionic PSBDA@CeO2 NPs coating produced in this study is biocompatible, retaining >90% cell viability upon direct contact with human corneal epithelial cells (HCECs), keratinocytes, and 293T cells. Consequently, the zwitterionic PSBDA@CeO2 NPs coating has significant application potential for creating a multifunctional surface coating for CLs that improves wear comfort and prevents BK and FK.