Characterization, toxicity, and biological activities of organometallic compounds and peptide nucleic acids for potential use as antimicrobials
Bacterial antibiotic resistance is a globally recognized problem that has prompted extensive research into novel antimicrobial compounds. This dissertation describes research focusing on two types of potential antimicrobial molecules, organometallic compounds (OMC) and peptide nucleic acids (PNA). Organometallic compounds show promise as antimicrobial drugs because of their structural difference from conventional antibiotics and antimicrobials, and because of the ability to "tune" their chemical and biological properties by varying ligand attachments. Peptide nucleic acids, when linked to a cell-penetrating peptide (CPP), can suppress bacterial gene expression by an antisense mechanism and are attractive candidates for antimicrobial drugs because they bind strongly to target nucleic acids and are resistant to nucleases. Chapters 1 and 2 of the dissertation provide an introduction and broad literature review to frame the experimental questions addressed. Chapter 3 describes work to test the cytotoxicity and cellular penetration capabilities of novel OMCs by evaluating their effects on J774A.1 murine macrophage-like cells that were either uninfected or were infected with Mycobacterium bovis BCG. Results indicate that OMCs with an iridium (Ir) metal center and an amino acid ligand show minimal cytotoxicity against eukaryotic cells but likely do not penetrate the intracellular compartment in significant amounts. Chapter 4 presents research into in vitro effects of CPP-PNAs targeting the tetA and tetR antibiotic resistance genes (CPP-anti-tetA PNA and CPP-anti-tetR PNA, respectively) in tetracycline-resistant Salmonella enterica ssp. enterica serovar Typhimurium DT104 (DT104). Through the use of modified minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays it was shown that both the CPP-anti-tetA PNA and CPP-anti-tetR PNA increase tetracycline susceptibility in DT104. Chapter 5 explores the molecular mechanism of the CPP-anti-tetA PNA and CPP-anti-tetR PNA through the use of reverse transcriptase quantitative polymerase chain reaction (RT-qPCR). Results indicate good specificity of the CPP-anti-tetA PNA for its nucleic acid target as evidenced by suppression of tetA mRNA expression in DT104 cultures treated with a combination of tetracycline and the PNA. Chapter 6 describes the development of a mouse model of DT104 infection using BALB/c mice, followed by implementation of that model to test in vivo antimicrobial effects of the CPP-anti-tetA PNA and the CPP-Sal-tsf PNA, which targets expression of the essential tsf gene. An optimal dose of DT104 was identified that causes clinical illness within 2-4 days. At the doses tested, concurrent treatment of infected mice with tetracycline and the CPP-anti-tetA PNA or with the CPP-Sal-tsf PNA alone did not have a protective effect. Final conclusions are 1) that further research with the OMCs should focus on compounds with an Ir center and an amino acid ligand, and should explore ways to enhance intracellular penetration, 2) that the in vitro results of the PNA studies suggest that PNAs targeting expression of antibiotic resistance genes could allow for repurposing of antibiotics to which bacteria are resistant, and 3) additional study of the behavior of PNAs in vivo is advised.