PROJECT SUMMARY
Little is known about how bacteria prevalent in the digestive tract, most of which are anaerobes, take up and
process the host’s dietary iron. How this activity influences the efficiency with which iron is absorbed by the
host is also unclear. Closing this gap in the knowledge is of both fundamental and biomedical interest. Iron-
deficiency and associated anemia are the most prevalent nutritional disorders worldwide, shared by nearly a
third of the human population. At the same time, unmetabolized heme iron from red meat diets that remains in
the colon has been associated with the development of diseases ranging from inflammation to colon cancer,
with microbial activity postulated to play a key role. The long-term goal of this work is to understand how
commensal bacteria commonly found in the healthy mammalian gut metabolize iron under the low/no
O2 conditions that are prevalent in this ecosystem. The proposed work focuses our group’s knowledge and
infrastructure – accrued over 15 years of studying aerobic heme/iron biochemistry at the level of the catalyst,
cell, and ecosystem – on this ambitious long-term goal, which we have divided into two overlapping parts.
First, we will examine how common gut microbes, most of which are anaerobic, heme auxotrophic
bacteria (HAB), metabolize heme. We are focusing on three experimentally tractable HAB which are
abundant in humans and which either require heme for respiration (Bacteroides thetaiotaomicron), are capable
of but not dependent on heme-mediated respiration (Lactobacillus rhamnosus), or are obligately fermentative
but still have limited uses for heme (Clostridium scindens). We will examine genes (via the generation of
knock-outs) and gene products that are predicted to play important roles in heme metabolism in these species,
but which belong to metabolic pathways that are typically incomplete. At the same time, we will employ
discovery-based approaches to identify members of the heme-proteome, using chemically defined growth
media, stable-isotope-labeled heme, and spectroscopic analyses with which we have a depth of expertise.
Second, we will define how gut bacterial species work together and with the animal host to metabolize
heme iron. As part of our experimental approach, we will use knock-out strains and isotopically labeled heme
to examine heme metabolism by co-cultures, using subsets of the three HAB above and a common enteric
heme heterotroph (Escherichia coli). Cocultures will be studied both in the flask and in mice with defined
(gnotobiotic) microbiomes, in collaboration with Prof. Seth Walk (MSU). Understanding anaerobic heme
metabolism by commensal bacteria serves the long-term biomedical goal of manipulating the microbiome to
facilitate host metabolism of iron, thereby remediating diseases associated with iron deficiency (anemia) or
excess (infection, colitis, inflammation, colon cancer).
