D. Streicher2, G. An1 1University Of Chicago,Surgery,Chicago, IL, USA 2University Of Michigan,College Of Literature, Science And Arts,Ann Arbor, MI, USA
Introduction: Non-Toxigenic Clostridium difficile (NTCD) is a generic term for several naturally occurring strains of C. difficile that lack the Pathogenicity Locus (PaLoc) Region, which contains the DNA sequences for Toxins A and B, which are responsible for the pathologic changes in Clostridium difficile Infection (CDI). Otherwise, NTCD shares behavioral properties with other strains of C. difficile, and depletion of commensal microbes by systemic antibiotics affect the suppression of NTCD spores and can trigger their shift from spore to germinated state. Pre-administration of NTCD has been suggested as a prophylaxis for CDI in high-risk population, with the initiation of early stage clinical trials for its study. However, safety concerns have been raised due to recent reports that NTCD is capable of undergoing horizontal gene transfers of the PaLoc region from toxigenic C. difficile. We investigate the feasibility and safety of NTCD prophylaxis with an Agent-based model (ABM) of the gut microbial ecology.
Methods: A previously produced agent-based model of CDI (CDIABM) was expanded by adding NTCD with both its Germinated (NTCD-G) and Sporulated (NTCD-S) forms. Horizontal Gene Transfer of the PaLoc region was implemented as a stochastic contact effect with toxigenic C. difficile in its germinated form, resulting in the acquisition of toxin producing ability in the target NTCDs. Simulation experiments were performed to examine the prophylactic suppressive and treatment efficacy of NTCD on the development of CDI following systemic antibiotic administration. PaLoc gene transfer events were examined with respect to their impact on persistence and recurrence of CDI.
Results: NTCDs were successfully implemented into the existing CDIABM, and demonstrated qualitatively validated sporulation and germination dynamics. Simulations of different prophylactic NTCD regimens produced a dose dependent suppressive effect on the development of subsequent CDI; however, there was an inflection point identified in the dosing of NTCD and a late phase worsening of CDI. This infection point was associated with the number of horizontal gene transfer events of PaLoc, and demonstrated an exponential effect in low nutrient conditions. Post CDI administration of NTCD did not qualitatively change the trajectory of the CDI.
Conclusion: The modified CDIABM plausibly reproduced the behavior of NTCD in the gut. Prophylactic NTCD appeared to slow the rate and severity of CDI, though it does not prevent damage entirely. The risk of virulizing NTCD through horizontal gene transfer of PaLoc is low, though present with a NTCD dose dependent effect. NTCD was not demonstrated to be effective as a treatment of active CDI. As NTCD prophylaxis is currently being considered in clinical trials, computational investigations of this type can help to augment research and trial design by identifying potential negative effects, characterizing safety concerns, and establishing potential efficacy ranges.