Evolution of Radio-Resistant Strains of Bacteria Revealed More Complex Mutations

A team of researchers set out to understand how some organisms develop resistance to radiation within the Earth, a place naturally protected from solar radiation by its own magnetic field.

The term extremophiles have been used to describe a class of organisms that survive in extreme environments previously thought unable of sustaining life - intense heat or cold, places of extreme pressure or acidity levels, and those with abnormally high radiation levels. One species of bacteria, in particular, Deinococcus radiodurans, can withstand radiation levels fatal to humans a thousand times over.

Researchers led by Dr. Michael M. Cox from the University of Wisconsin-Madison have decided to "let the cells tell them," according to an article from Frontiers. They have published the results of their study in the journal Frontiers in Microbiology, September 22.

Gradual Development of Radio-Resistance

In their study, the researchers noted that ionizing radiation can be fatal to most organisms at high doses, with IR causing cellular damage through the induction of ROS, or reactive oxygen species.

Starting with naturally nonresistant cultures of Escherichia coli, researchers exposed the samples to iterative cycles containing high levels of radiation. After repetitive exposures to radiation, radio-resistant populations started to appear.

The genetic alterations in the now-resistant populations were observed through whole-genome sequencing, narrowing down on the mutation that made the bacteria species resistant to radiation.

A previous study, also from the same team, began with the exposure of E. coli populations to 50 iterations of ionizing radiation. After ten rounds or so, the radio-resistant population started appearing and by the fiftieth round, the population revealed a genetic profile that contained three particular mutations that turned the E. coli sample resistant to ionizing radiation. These mutations were all linked to genes responsible for the bacteria's DNA repair mechanisms.

Adapting New Mechanisms in Response to Increasing IR Levels

The new study continues on their previous works by exposing the bacteria to an additional 50 iterations of radiation exposure, selection, and genome sequencing. Certain parts of the population continued adapting to the increasing levels of radiation, resulting in new sub-populations more resistant to IR.

While in the previous study, radio-resistance in E. coli was mostly attributed to the three mutations found by researchers, the recent study revealed a new adaptation strategy from the emerging sub-population. Portions of E. coli after the second set of exposures showed that they now lacked a number of genes while duplicating others.

Cox noted that the four subpopulations that appeared after the second set of exposures are now approaching resistance levels comparable to Deinococcus radiodurans. He added: "the genomic alterations have proven to be much more complex than anticipated."

Based on experimental evolution data and previous work on the matter, researchers generated the most radiation-resistant culture of Escherichia coli to date. The surviving E. coli lineages, according to the researchers, are 100 times more resistant to IR compared to their ancestors back at the post-50 cycles of exposure.

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