Design, Synthesis, Evaluation and Mechanistic Elucidation of Antibacterial Agents for Gram-Positive Bacteria

This thesis describes my work on antibiotics against methicillin-resistant Staphylococcus aureus (MRSA) and Clostridioides difficile (C. difficile), which are the leading causes of death associated with antimicrobial resistance in the United States.

Structure-activity relationship (SAR) study of quinazolinones against MRSA was carried out. One featured compound exhibited antibacterial activity of its own and synergized with the piperacillin-tazobactam (TZP) combination both in vitro and in vivo. The synergy is documented and rationalized by the ability of the quinazolinones to bind to the allosteric site of PBP2a, a key penicillin-binding protein that confers methicillin resistance.

C. difficile is an important nosocomial pathogen that causes recurrent diarrhea and colitis. The troublesome high recurrence of CDI results primarily from C. difficile’s ability to produce dormant spores that survive antibiotic treatment and opportunistically germinate in a perturbed microbiota to initiate more infection. Oxadiazoles discovered in our lab were found to kill the bacterial cell and interfere with spore germination, which enable oxadiazoles to halt colonization by vegetative bacteria and spores.

An SAR study of the oxadiazole antibacterial was conducted to assess their bactericidal activity against C. difficile vegetative cells. An oxadiazole-nitroimidazole hybrid was found to show high selectivity against C. difficile over the common gut bacteria and eukaryotic cells. The high selectivity of the antimicrobial spares the gut microbiota avoiding further spore germination after the treatment. This new series of nontoxic, narrow-spectrum antibiotic holds promise for being a better & cheaper alternative for treating rCDI. An additional SAR study of the oxadiazoles that weighed on spore germination inhibition were also explored. Dual-acting analogs or exclusively spore germination inhibitors were discovered.

Based on the previous SAR discovery, oxadiazole probes were designed and synthesized for target identification. Proteomics studies with oxadiazole analogs/probes were carried out. The application of proteomics study leads us to two key proteins involved with C. difficile germination, which showed strong binding with oxadiazole antibacterial by microscale thermophoresis (MST) binding assay.