Although oral vancomycin (VAN) treatment failures in Clostridioides difficile infection (CDI) are common,
antibiotic susceptibility testing is not conducted in the clinical setting. This has resulted in, and is in part due to,
the lack of established susceptibility measure(s) corresponding with clinical outcomes. Our long-term goal is to
develop clinically relevant microbiologic susceptibility measures that are predictive of patient outcomes in
patients with CDI. The objective of this proposal is to determine if VAN inhibitory or bactericidal concentrations
against C. difficile isolates are predictive of sustained clinical cure. The central hypothesis is that bactericidal
concentrations will be more discriminatory and clinically significant than inhibitory concentrations in predicting
outcomes. The rationale underlying this proposal is well supported by CDI pathophysiology, VAN drug
characteristics, and preliminary data. As CDI is a toxin-mediated disease, ongoing production of toxins
worsens clinical outcomes. Toxins are predominately produced during the stationary growth phase in which
cell wall synthesis slows, thereby decreasing VAN effectiveness and allowing for continued toxin production at
concentrations around the MIC. VAN MICs are also relatively non-discriminatory and generally cluster between
0.5-2 µg/mL. Isolates with the most common resistance mechanism, vanG-mediated resistance, are also
known to have disproportionately increased MBCs compared to MICs. The central hypothesis will be tested by
pursuing two specific aims: 1) Determine the variance of VAN bactericidal concentrations stratified by C.
difficile MICs; 2) Determine if VAN bactericidal measures or MIC better predicts sustained clinical cure in
hospitalized patients given VAN for CDI. This project will prove innovative through challenging the current
clinical practice paradigm that VAN susceptibility testing is not needed for patients with CDI, through the
expansion of our valuable biobank of 600 patients with CDI that will enable future in-depth mechanistic studies,
and development of a high-throughput ATP assay to perform bactericidal concentration testing for future
studies. The proposed research is significant as CDI is the most common healthcare-acquire infection and
VAN is the most common antibiotic used to treat it; a VAN susceptibility measure corresponding with outcomes
is critical to ensuring optimal VAN use and desirable patient outcomes. The expected outcome will provide
valuable preliminary data allowing for a future stream of research into molecular diagnostics and point-of-care
susceptibility tests incorporate VAN susceptibility. Compared to the CLSI-recommended but labor-intensive
broth microdilution MBC method, the high-throughput ATP bioluminescence MBC assays will enable future
research studies and implementation into clinical microbiology labs. These studies also lay the foundation for
the PI to further expand into CDI translational research and become an independent researcher.