The function of iron in E. Coli generational memory

By Bushra Bowen

Image by Geralt via Pixabay

Scientists from the University of Texas in Austin carried out a number of studies on Escherichia coli, or E. coli, in 2023 with the goal of illuminating the mechanisms underlying particular bacterial behaviors. Many of the behaviors exhibited by bacteria, particularly those that are visible in bacterial colonies, have unclear causes. However, these scientists were able to identify a crucial mechanism behind E. coli's swarming habit and demonstrate how the bacteria might inherit their memory of swarming from mother cells.

Bacterial swarming is defined as the collective migration of a large population of bacteria in which thousands of cells can colonize new surfaces. Swarming can be advantageous to bacteria populations, as swarms are able to better resist antibiotics due to their high cell density and are able to collectively find nutrient-rich environments under high-stress conditions.

An Indiana University researcher has previously investigated generational memory in E. coli in 2010. This study revealed that E. Coli were better swarmers when they had either swarmed before or had mother cells that had swarmed before, indicating that bacteria could "remember" their prior swarming experiences. However, the mechanisms underlying this generational memory remained a mystery. It was also unclear for how many generations bacteria could "remember" their swarming experience.

In an attempt to determine the cause of the E. coli outbreak, the University of Texas at Austin researchers evaluated possible genetic variables. Separately, the Souvik Bhattacharyya-led team inserted plasmids containing the genes for mechanosensing (mscL and mscS), iron regulation (fepA and fur), efflux (evgA and marA), redox (katG and sodB), and porins (ompA and ompF). This led the scientists to conclude that the bacterial strain carrying the fepA gene (ΔfepA) was a poorer swarmer than wild-type E. coli, whereas the strain carrying the fur gene (Δfur) did not swarm at all. Iron outer membrane transporter FepA and iron uptake gene repressor Fur are different. Intracellular iron levels are a key factor in E. coli swarming, as demonstrated by the overexpression of fepA and fur, which reduces the ability to swarm. Higher levels of iron result in poorer swarming, whereas lower levels result in better swarming.

Bhattacharyya et al. determined the swarming capability of wild-type (WT), wild-type in iron-starved conditions (WT+DFO), wild-type in iron-rich conditions (WT+FeCl3), and ΔfepA (ΔfepA) E. coli (ΔfepA), ΔfepA in iron-starved conditions (ΔfepA+DFO), and ΔfepA in iron-rich conditions (ΔfepA+FeCl3). The experiment's results are displayed in the graphs below.

Better swarming is shown by a peak on the graph's right side, and worse swarming is indicated by a peak on the graph's left side. It should be noted that ΔfepA E. coli strains were not significantly impacted by an alteration in the environmental iron content, corroborating the theory that fepA prevents the cell from absorbing iron. These findings corroborate the theory that an E. Coli cell's capacity to swarm is reduced as it absorbs more iron; in other words, an E. Coli bacterium is more likely to swarm in an environment with low iron levels.

The researchers observed the swarming ability of wild-type E. coli daughter cells in environments with varying iron levels up to the 12th generation (G12) in order to investigate the potential impact of iron deficiency on generational memory. Bhattacharyya and associates discovered that swarming memory remained till G7 and vanished by G12. Furthermore, they discovered that limited swarming potential continued throughout G12.

These findings, which offer a fresh perspective on the evolution of bacteria, highlight the significance of memory in bacterial decision-making as well as the part environmental cues play in bacterial generational memory.

References

Be’er, A., Ilkanaiv, B., Gross, R., Kearns, D. B., Heidenreich, S., Bär, M., & Ariel, G. (2020). A phase diagram for bacterial swarming. Communications Physics, 3(66). https://doi.org/10.1038/s42005-020-0327-1 

Bhattacharyya, S., Bhattarai, N., Pfannenstiel, D. M., Wilkins, B., Singh, A., & Harshey, R. M. (2023). A heritable iron memory enables decision-making in Escherichia coli. Proceedings of the National Academy of Sciences, 120(48). https://doi.org/10.1073/pnas.2309082120 

Kearns, D. B. (2010). A field guide to bacterial swarming motility. Nature Reviews Biology, 8(634-644). https://doi.org/10.1038/nrmicro2405 

Parshall, A. (2024). Bacteria make decisions based on generational memories. Scientific American. https://www.scientificamerican.com/article/bacteria-make-decisions-based-on-generational-memories/