Too much of a good thing

Abstract

Background There has been an increased usage of metallic antimicrobial materials to control pathogenic and multidrug 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-hour optical densities compared to controls. Furthermore, these populations showed increased resistance towards 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.

Experimental Evolution of Magnetite Nanoparticle Resistance in Escherichia coli

“…In addition, we found that iron (II)-and iron (III)-resistant populations showed unanticipated correlated resistances to a range of antibiotics including ampicillin, chloramphenicol, rifampicin, sulfanilamide, and tetracycline. Our results suggested that E. coli K-12 could respond to excess iron by both physiological acclimation and evolutionary adaptation [13]. This was manifested by both patterns of genomic and gene expression changes exhibited in the iron (II)-, iron (III)-, and Gallium (III)-resistant populations compared to their ancestors and controls (grown in the absence of excess iron or gallium).…”

“…In addition, if they can, what correlated traits will result from the adaptation? We have already answered some of these questions in a series of experiments utilizing ionic iron II, iron III, and the iron analog gallium III [13][14][15]. These studies utilized experimental evolution and examined mechanisms of iron and gallium resistance through the evaluation of phenotypic and genomic changes in E. coli K-12 MG1655.…”

Experimental Evolution of Magnetite Nanoparticle Resistance in <em>Escherichia coli</em>

“…Our results suggested that E. coli K-12 could respond to excess iron by both physiological acclimation and evolutionary adaptation [13]. This was manifested by both patterns of genomic and gene expression changes exhibited in the iron (II), iron (III), and Gallium (III)-resistant populations compared to their ancestors and controls (grown in the absence of excess iron or gallium).…”

“…Bacterial strains E. coli MG1655 (ATCC #47076) was used in this study because it does not have plasmids and its circular chromosome is composed of 4,641,652 nucleotides (GenBank: NC_000913.3; Riley et al 2006). All of our previous studies of ionic and nanoparticle resistance have used this strain [13][14][15]. This strain is the ancestor of all the selection treatments (FeNP and Controls) evaluated in this study.…”

“…We have already answered some of these questions in a series of experiments utilizing ionic iron II, iron III, and the iron analog gallium III [13][14][15]. These studies utilized experimental evolution and examined mechanisms of iron and gallium resistance through the evaluation of phenotypic and genomic changes in E. coli K-12 MG1655.…”