Purdue University Graduate School
Final version of PhD thesis_Nader S. Abutaleb.pdf (2.32 MB)

Development of new antibacterial agents for treatment of vancomycin-resistant enterococci and Clostridioides difficile infections

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posted on 2020-12-11, 04:23 authored by Nader Shawky AbutalebNader Shawky Abutaleb

The discovery of penicillin marked the beginning of the golden era of antibiotics. Antibiotics have been the valuable weapons combating an array of bacterial infections. However, each year, millions of bacterial infections occur which lead to thousands of deaths. For instance, over three million infections were reported in the United States in 2019, that were caused by antibiotic-resistant pathogens and Clostridioides difficile. These infections resulted in more than 50,000 deaths. Clostridioides difficile and vancomycin-resistant enterococci (VRE) are listed as top-threat bacteria that urgently require prompt action to fight the infections caused by both of them. Both are highly prevalent in the healthcare settings, ranking as the first and second bacterial species causing nosocomial infections. The increased incidence and severity of diseases caused by both bacteria in addition to the dearth of effective anticlostridial and anti-VRE agents have created an urgent need for development of new therapeutic agents. The process of discovering new antibiotics is time-consuming and associated with high costs and risk. Repurposing FDA-approved drugs represents an attractive venture of antibacterial drug discovery. FDA-approved drugs possess well-studied safety, pharmacology and pharmacokinetics. Hence, drug repurposing saves time and costs, and reduces the risk associated with the de novo drug discovery. In the following studies, new drugs discovered utilizing the drug repurposing approach, were investigated C. difficile and VRE.

Against C. difficile, diiodohydroxyquinoline (DIHQ) and auranofin were investigated. DIHQ exhibited potent activity against C. difficile isolates inhibiting growth of 90% of these isolates at the concentrations of 2 µg/mL. It demonstrated superior activity to vancomycin and metronidazole, in its killing kinetics. Furthermore, it reacted synergistically with vancomycin and metronidazole against C. difficile in vitro. Moreover, at subinhibitory concentrations, DIHQ was superior to vancomycin and metronidazole in inhibiting two key virulence factors of C. difficile, toxin production and spore formation. Additionally, DIHQ did not inhibit growth of intestinal normal microbiota. In addition, the in vivo activity of auranofin was investigated in C. difficile infection (CDI) mouse and hamster model. It efficiently protected mice and hamsters against CDI. Remarkably, at clinically achieved concentrations, auranofin was capable of preventing CDI recurrence.

For VRE, three drugs where identified with potent inhibitory activities both in vitro and in vivo, auranofin, acetazolamide and dorzolamide. Auranofin’s antibacterial activity was not affected when evaluated against a higher inoculum size of VRE and it successfully reduced the burden of stationary phase VRE cells. In addition, auranofin reduced VRE production of key virulence factors including proteases, lipase and hemagglutinin. In a lethal mouse model of VRE septicemia, auranofin-treated mice were protected from the lethal VRE challenge. Interestingly, auranofin successfully reduced VRE count below the limit of detection in murine internal organs after only four days of oral or subcutaneous treatment. In addition to auranofin, two carbonic anhydrase inhibitors, acetazolamide and dorzolamide, were investigated against VRE. Acetazolamide exhibited potent activity against a wide panel of different enterococcal strains. Moreover, it outperformed linezolid in two in vivo VRE mouse models; murine colonization-reduction and VRE septicemia. Additionally, dorzolamide exhibited potent activity against VRE isolates. Remarkably, in combination with gentamicin, dorzolamide interacted synergistically reducing gentamicin’s MICs by several folds. Moreover, dorzolamide significantly reduced the VRE burden in mice fecal samples by 99.9% after 3 days of treatment. Furthermore, dorzolamide significantly surpassed linezolid in reducing the VRE count in the cecal and ileal contents of mice.

Collectively, several drugs exhibited promising activities against C. difficile and VRE, in vitro and in vivo. The results presented suggest that these drugs merit further evaluation as potential candidates for treatment of infections caused by either bacterium.


Degree Type

  • Doctor of Philosophy


  • Comparative Pathobiology

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Mohamed N. Seleem

Additional Committee Member 2

Herman O. Sintim

Additional Committee Member 3

Lynn F. Guptill-Yoran

Additional Committee Member 4

Sanjeev K. Narayanan

Additional Committee Member 5

Suresh K. Mittal