<p>Over three million infections were reported in the United States of America in 2019. These infections were caused by either antibiotic-resistant pathogens or <i>Clostridioides difficile</i> and resulted in more than 50,000 deaths. Unfortunately, antibacterial agents are rapidly losing their ability to treat infections and the process of discovering new antibiotics is too slow to cope up with bacterial evolution. Repurposing FDA-approved drugs of well-studied safety, pharmacology and pharmacokinetics represents a faster alternative method of antibacterial drug discovery. Repurposing is more successful and less depleting method of drug discovery than classical de novo method in regard to both cost and time. In the following studies, two major pathogens are targeted, vancomycin-resistant <i>Enterococcus</i> (VRE) and <i>C. difficile</i>. Both bacteria are more prevalent in healthcare settings were more vulnerable population of elderly and immunocompromised individuals reside. In addition, healthcare settings are usually associated with higher frequency of receiving antibiotics which in turn, compromises the integrity of normal microbiota responsible for protection against invading pathogens. Furthermore, hospital stays are associated with exposure to bacterial shedding from other patients. Our aim was to identify FDA-approved drugs with novel ability to eradicate these two bacterial pathogens in the gastrointestinal tract (GIT). Notably, the GIT is considered the actual site of infection in case of <i>C. difficile while it is only a transition site for VRE where the bacteria colonize before causing true infections in other tissues. Studies against both bacteria started with an <i>in vitro</i> screening of FDA-approved drugs and clinical molecules to identify potential candidates for further investigation.</i></p><p><i>For VRE, two drugs where identified with potent inhibitory activity and favorable pharmacokinetic profiles, auranofin and ebselen. Auranofin was approved in the 1960s for the treatment of rheumatoid arthritis due to its anti-inflammatory activity. Auranofin was found to exert potent bacteriostatic activity against both vancomycin-sensitive and vancomycin-resistant <i>Enterococcus</i> strains (minimum inhibitory concentration against 90% of the strains, MIC90 = 1 µg/mL). In addition, bacteria could not develop resistant mutants against auranofin upon prolonged exposure. On the other hand, ebselen is an organoselenium compounds currently in clinical trials for several indications. Similarly, ebselen was found to be a potent inhibitor of VRE growth (MIC90 = 2 µg/mL). In addition, ebselen successfully inhibited bacterial biofilm formation and eradicated mature biofilms. In a mouse model of VRE colonization, both drugs inhibited bacterial shedding and reduced bacterial counts in the GIT of the colonized animals.</i></p><p><i>For <i>C. difficile</i>, auranofin was also found to exert potent inhibitory activity against bacterial growth (MIC90 = 2 µg/mL), toxin production and spore formation. Additionally, it was beneficial in protecting colon cells against <i>C. difficile</i> toxin-induced inflammation. Further, auranofin was found to not promote growth of VRE as seen with the current anticlostridial agents. In addition to auranofin, two more antiprotozoal drugs were found to potently inhibit <i>C. difficile</i> growth, ronidazole and secnidazole. Both drugs are 5-nitroimidazoles approved for human (secnidazole) or veterinary (ronidazole) applications. Secnidazole and ronidazole halted <i>C. difficile</i> growth at very low concentrations (MIC90 = 0.5 and 0.125 µg/mL, respectively). Furthermore, both drugs were superior to metronidazole in bacterial killing and had favorable activities against protective gut microbiota. In addition, they demonstrated efficient protection to mice in a <i>C. difficile</i> infection model. </i></p><p><i>Overall, several drugs were presented to possess favorable activities against <i>C. difficile</i> or VRE. These drugs merit more evaluation as potential candidates for the treatment of infection caused by either bacteria. </i></p><div><i><br></i></div>

Multiorgan dysfunction syndrom is the major driving pathophysiological mechanism of morbidity and mortality in critically ill patients. Gastrointestinal dysfunction usually develops as a result critical illness and it is believed to play a key role in the development and progression of multiple organ dysfunction. Moreover, any primary disorder of the gastrointestinal tract, if severe enough, can lead to a critical state and secondary multiorgan dyfunction. Despite intensive experimental and clinical research, reliable tools for monitoring and evaluation of the severity of gastrointestinal dysfunction remain unknown. In the same line, therapy of this complex pathology remains largely supportive. The aim of this thesis was first to explain the severity of the most common and most serious nosocomial infection of the digestive tract, second to elucidate the safety and effectiveness of the endoscopic dual enteral probe insertion in ventilated critically ill patients, and, third to evaluate new diagnostic tools of the gastrointestinal dysfunction. Finally, we present an ongoing project aimed at investigating esophageal dysfunction in mechanically ventilated critically ill patients.