Ingestion exposure estimates typically use a default bioavailability of 100%, thereby assuming that the entirety of an ingested dose is absorbed into systemic circulation. However, the actual bioavailability of ingested contaminants is oftentimes lower than 100%. The research described herein investigates the use of the Simulator of the Human Intestinal Microbial Ecosystem (SHIME) for the calculation of <i>in vitro</i> bioaccessibility (IVBA), a conservative predictor of bioavailability, of mercury (Hg) from traditional northern foods and arsenic (As) from soils. The primary objective of the research described herein is to address data-gaps which have hindered attempts to incorporate IVBA into risk assessment on more than a case-by-case basis. The hypotheses of this thesis are that (1) the bioaccessibility of contaminants is dependent upon concentration due to kinetic limitations on dissolution, (2) gastrointestinal (GI) microbes in the ileum and colon alter contaminant bioaccessibility and/or speciation, (3) the GI microbial effect on bioaccessibility is toxicologically relevant, and (4) metal bioaccessibility is predictable according to dissolution kinetics.<p>
Mercury bioaccessibility from country food samples was independent of total Hg concentration (F=0.5726, P=0.578) whereas As bioaccessibility was inversely related to total As concentration for Nova Scotia mine tailings, synthesized ferrihydrite with adsorbed AsV, and synthesized amorphous scorodite (P=2 x 10-10). Isotherm analysis indicated that, at high soil As concentrations, saturation of simulated GI fluids limited As bioaccessibility under gastric conditions whereas kinetic limitations constrained As bioaccessibility under intestinal conditions. Additionally, we demonstrated that GI microbes may affect Hg bioaccessibility, either increasing or decreasing bioaccessibility depending upon the type of food. For example, the bioaccessibility of HgT decreased in the presence of GI microbial activity for caribou kidney, caribou tongue, seal blood, seal brain, seal liver, and walrus flesh. In contrast, HgT bioaccessibility from Arctic char and seal intestine increased in the presence of GI microbial activity. Similarly, colon microbial activity increased (Fishers Protected LSD, P<0.05) As bioaccessibility from synthesized amorphous scorodite (56 110%), Nova Scotia mine tailings (140 300%), an agricultural soil (53%) and an ironstone soil (350%) containing elevated arsenic concentrations. However, under small intestinal conditions, this microbial effect was transient and demonstrated a small effect size.
The toxicological relevance of microbial effects upon As bioaccessibility was assessed using a juvenile swine model with co-administration of oral antibiotics (neomycin and metronidazole). This study research indicated that microbial effects on As bioaccessibility are not reflected in the juvenile swine model. For example, the microbial communities present in the pigs proximal colon clustered according to antibiotic treatment (e.g. microbial communities of antibiotic treated pigs differed from non-treated pigs). Despite this, the urinary arsenic excretion (and hence arsenic bioavailability) of antibiotic-treated juvenile swine orally exposed to soil-borne arsenic was equivalent (Holm-Sidak, P=0.930) to the urinary arsenic excretion of juvenile swine not treated with antibiotics. Therefore, in vitro GI models may not need to include a microbially active intestinal stage when measuring As IVBA.<p>
Metal bioaccessibility from soils appears predictable according to fundamental chemical properties of the metal-of-concern. Specifically, metal bioaccessibility of 7 of the 13 metals (V, Ni, Zn, Cu, U, Cd, & Ba but not Tl, Pb, As, Se, Cr, and Hg) regulated according to Canadian Council of Ministers of the Environment Soil Quality Guidelines (CCME SQG) were strongly dependent (R2 = 0.7) on water exchange rate constants of metal cations (kH20) indicating that desorption kinetics may serve as the foundation of a predictive model of metal bioaccessibility.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:SSU.etd-11292010-165216 |
| Date | 30 November 2010 |
| Creators | Laird, Brian Douglas |
| Contributors | Hamilton, Don, Van Kessel, Andrew, Basta, Nicholas, Wickstrom, Mark, Blakley, Barry, Siciliano, Steven |
| Publisher | University of Saskatchewan |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://library.usask.ca/theses/available/etd-11292010-165216/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Saskatchewan or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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