Too much of a good thing: Adaption to iron (II) intoxication in Escherichia coli

Misty D Thomas; Akamu J Ewunkem; Sada Boyd; Danielle K Williams; Adiya Moore; Kristen L Rhinehardt; Emma Van Beveren; Bobi Yang; Anna Tapia; Jian Han; Scott H Harrison; Joseph L Graves Jr

doi:10.1093/emph/eoaa051

Too much of a good thing: Adaption to iron (II) intoxication in Escherichia coli

Evol Med Public Health. 2021 Jan 18;9(1):53-67. doi: 10.1093/emph/eoaa051. eCollection 2021.

Affiliations

¹ Department of Biology, North Carolina Agricultural and Technical State University, 1601 E. Market St, Greensboro, NC 27411, USA.
² BEACON, Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI 48824, USA.
³ Department of Nanoengineering, Joint School of Nanoscience and Nanoengineering, North Carolina Agricultural and Technical State University and UNC Greensboro, 2907 E. Gate City Blvd., Greensboro, NC 27401, USA.
⁴ Computational Data Science and Engineering, North Carolina Agricultural and Technical State University, 1601 E. Market Street, Greensboro, NC 27411, USA.

Abstract

Background: There has been an increased usage of metallic antimicrobial materials to control pathogenic and multi-drug resistant bacteria. Yet, there is a corresponding need to know if this usage leads to genetic adaptations that could produce more harmful strains.

Methodology: Experimental evolution was used to adapt Escherichia coli K-12 MG1655 to excess iron (II) with subsequent genomic analysis. Phenotypic assays and gene expression studies were conducted to demonstrate pleiotropic effects associated with this adaptation and to elucidate potential cellular responses.

Results: After 200 days of adaptation, populations cultured in excess iron (II), showed a significant increase in 24-h optical densities compared to controls. Furthermore, these populations showed increased resistance toward other metals [iron (III) and gallium (III)] and to traditional antibiotics (bacitracin, rifampin, chloramphenicol and sulfanilamide). Genomic analysis identified selective sweeps in three genes; fecA, ptsP and ilvG unique to the iron (II) resistant populations, and gene expression studies demonstrated that their cellular response may be to downregulate genes involved in iron transport (cirA and fecA) while increasing the oxidative stress response (oxyR, soxS and soxR) prior to FeSO₄ exposure.

Conclusions and implications: Together, this indicates that the selected populations can quickly adapt to stressful levels of iron (II). This study is unique in that it demonstrates that E. coli can adapt to environments that contain excess levels of an essential micronutrient while also demonstrating the genomic foundations of the response and the pleiotropic consequences. The fact that adaptation to excess iron also causes increases in general antibiotic resistance is a serious concern. Lay summary: The evolution of iron resistance in E. coli leads to multi-drug and general metal resistance through the acquisition of mutations in three genes (fecA, ptsP and ilvG) while also initiating cellular defenses as part of their normal growth process.

Keywords: E. coli; experimental evolution; gene expression; iron resistance; pleiotropy.