A microscopic battle for iron
Probiotic E. coli Nissle impedes Salmonella intestinal colonization by competing for iron
Written by: Katherine Kelly
Edited by: Jayanth Jawahar, Jonathan Keller
Background
Within the human intestine lies a complex microbial ecosystem. Many scientists aim to understand the complex interactions not only between bacteria and the human body, but also the interactions between different bacterial species. These relationships can range from antagonistic (where both species fight for limited resources) to symbiotic (where both species benefit from each other’s presence). Bacterial interactions at the microscopic level can bring about major changes at the macroscopic level of the human body, where fluctuations in bacterial composition or flora can influence health drastically. One study (Cruz et al. 2012) has shown that a co-infection of a certain fungus and bacterium may actually be less virulent than infection by each microbe alone as their hostile relationship impedes complete pathogenicity of either. The pathogenic fungus, Candida albicans, was less virulent to the host when the competing, antagonistic bacterium Enterococcus faecalis was present, illustrating the macroscopic benefits of microscopic competition.
A similar relationship has been observed between Salmonella and a certain strain of E. coli named E. coli Nissle. Salmonella enterica sv. Typhimurium is one of the leading causes of acute gastroenteritis, which is characterized by inflammatory diarrhea (Scallan et al. 2015). E. coli Nissle was originally isolated from a soldier who seemed resistant to a diarrhea outbreak. This strain has since been put to use as a probiotic, essentially a microbial supplement, to treat or prevent numerous gastrointestinal disorders. Until recently, exactly how E. coli Nissle suppressed Salmonella was a mystery.
One group of researchers from University of California-Irvine hypothesized that the limited iron within the intestine might explain this mystery. Iron is an essential nutrient for bacteria and when iron is scarce, bacteria produce small molecules called siderophores which help the bacterium scavenge for iron. The human immune system counteracts bacteria’s need for iron by producing proteins to sequester the siderophores and inhibit bacterial growth. Some bacteria, such as S. typhimurium, have evolved to produce a different type of siderophore which is not recognized by the human immune system and therefore cannot be removed from action. This confers a benefit to S. typhimurium if it is the only bacterial species present able to acquire iron via this mechanism. However, it is not alone — E. coli Nissle has been shown to produce the same non-recognized siderophore. The UC-Irvine team hypothesized that because E. coli Nissle could also evade the siderophore-sequestering immune response, it would be able to survive and compete alongside S. typhimurium, ultimately limiting S. typhimurium from reaching maximum colonization and peak infection of the host (Deriu et al. 2013).
Summary
In order to investigate this hypothesis, researchers used mutant strains of E. coli Nissle. Mutant strains are bacteria that have a certain gene or set of genes deleted from their DNA. Mutants can help researchers identify how a certain gene is necessary (e.g. for growth, mating, etc.). The team created an E.coli Nissle mutant which lacked the ability to produce the non-recognizable siderophore, called salmochelin. To compete the two bacteria, investigators used mice prone to developing chronic Salmonella colitis as model host for the co-infection. They first infected the mouse with S. typhimurium and measured the amount of iron present in the fecal matter of the mice. The amount of iron found in non-infected mice feces was significantly higher than that found in infected mice feces, which illustrated that limited iron was correlated with colonic infection by S. typhimurium.
The team then compared the amount of S. typhimurium bacterial cells present in the fecal matter of mice that were co-infected with the normal, salmochelin-producing E. coli Nissle to that of mice who were co-infected with the mutant strain of E. coli Nissle. Results showed that more S. typhimurium was present within the fecal matter and the colon when co-infected with the mutant strain of E. coli Nissle than with the normal, wild-type E. coli Nissle. As the only presumed difference between the treatments was whether or not the E. coli Nissle strain could sequester iron, this supported the hypothesis that the colonization by S. typhimurium was attenuated specifically due to competition for limited iron.
Future Applications
Understanding the mechanism of probiotics provides insights into many fields of biology as well as medicine. Within the field of biology, coevolution — the change of a biological object triggered by the change of a related object — is becoming more commonly researched. More efficient and widespread data collection has allowed researchers to find new connections and new relationships between species in a shared environment. Understanding bacterial behavior in different environments under variable conditions offers profound insight into new therapies and treatments for infections within hosts such as humans. It is entirely possible that sometime in the near future, antibiotics might not be the central treatment for infections — your microbial community might just need a boost or change in composition.
References
Cruz, MR; Graham, CE; Gagliano, BC; Lorenz MC & Garsin DA (2012) “Enterococcus faecalis Inhibits Hyphal Morphogenesis and Virulence of Candida albicans.” Infect. Immun. 2013, 81(1):189. DOI: 10.1128/IAI.00914–12.
Deriu, E; Liu, JZ; Pezeshki, M; Edwards, RA; Ochoa, RJ; Contreras, H; Libby, SJ; Fang, FC & Raffatellu, M. (2013) “Probiotic Bacteria Reduce Salmonella Typhimurium Intestinal Colonization by Competing for Iron.” Cell Host & Microbe, vol. 14, no. 1, pp. 26–37.
Guarner, F & Malagelada, JR. (2003) “Gut flora in health and disease.” The Lancet, vol. 361, no 9356, pp. 512–519.
Scallan, E; Crim, SM; Runkle, A; Henao, OL; Mahon, BE; Hoekstra, RM & Griffin, PM (2015) “Bacterial Enteric Infections Among Older Adults in the United States: Foodborne Diseases Active Surveillance Network, 1996–2012.” Foodborne Pathogens and Disease, vol. 12, no. 6, pp. 492–499.
Schultz, M. (2008). “Clinical use of E. coli Nissle 1917 in inflammatory bowel disease.” Inflammatory Bowel Diseases vol. 14, no 7, pp. 1012–1018.
