Bacterial Adaptations to Extended Time in Microgravity

Adaptations Include Biofilm Formation Mutations

On long-duration spaceflights, what will happen to life aboard the spacecraft?

George Fox (left) and Madhan TirumalaiGeorge Fox (left) and Madhan Tirumalai in University of Houston’s Department of Biology and Biochemistry examined E. coli bacteria grown in simulated microgravity conditions for 1,000 generations.Life on Earth has evolved in the presence of a specific gravitational force. In space, minus this strong pull, life responds in different ways, affecting astronauts’ bodies as well as the bacteria tagging along for the ride.

Some of these changes are obvious. For example, when astronauts go into space, blood concentrates more in the upper body, which tricks the body into shedding 10-15 percent of its water volume.

Not as obvious are the effects on bacteria. In space, bacteria have been shown to develop additional virulence. Add in the fact that living in space compromises the immune system, and suddenly the problem of keeping astronauts healthy on long spaceflights becomes that much more complicated.

Growing Bacteria in Microgravity for 1,000 Generations

In an article recently published in the journal ‘npj Microgravity,’ Madhan Tirumalai, a postdoctoral researcher at the University of Houston, and collaborators reported the results of experiments on Escherichia coli grown for 1,000 generations in simulated microgravity conditions.

Microgravity is when people or objects appear to be weightless, and is commonly (and erroneously) referred to as zero-gravity, in spite of the fact there is always a small amount of gravity wherever you go in space.

Tirumalai conducted this research in the lab of George Fox, John and Rebecca Moores Professor of Biology and Biochemistry in the College of Natural Sciences and Mathematics. Fox is one of the co-authors of this paper, along with collaborators from the NASA Johnson Space Center and the NASA Ames Research Center.

Simulating Microgravity in the Lab

To run this experiment, Tirumalai used a rotating vessel that simulates microgravity, which offers an effective way to run experiments without the high costs of spaceflight. For 1,000 generations, he grew E. coli in microgravity conditions. Then, he compared these microgravity-exposed bacteria to samples grown in Earth’s gravity.

This experiment differs from previous experiments in that Tirumalai was able to look at a longer span of generations. This difference is more applicable to long-duration spaceflight, where astronauts will find themselves living in space for months or years.

Microgravity Promotes Biofilm Formation

“When people are in a free fall, their first instinct is to grab onto something,” Tirumalai said. “When bacteria are in conditions of microgravity, one can expect something similar to happen at the microscale level, which is to grab onto a substrate and to grab onto other bacteria.”

In space, bacteria have a higher propensity to form biofilms, linking up and working together, exchanging nutrients and dividing up important tasks. Biofilm formation is an example of different bacteria working together to increase their chances of survival and can form under many circumstances.

Bacteria Exposed to Microgravity Had Mutations Related to Biofilm Formation

After 1,000 generations, Tirumalai and his collaborators did a broad genetic analysis, finding 16 different mutations, most of which appear to be in genes related to biofilm formation. This suggests that in microgravity, mutations that enhance biofilm formation help bacteria survive better.

Afterwards, Tirumalai mixed the microgravity-exposed E. coli strain with regular E. coli and grew these bacteria together in microgravity. Mixed together, the adapted strain outgrew the control strain, suggesting that these bacteria had evolved to cope under the new conditions.

Some Changes Permanent, Others Short-Lived

However, when the microgravity-exposed strain grew under normal gravity for 10 generations, and was then grown together with the control strain, it lost some of its competitive advantage.

This suggests that some of the adaptation was a short-term change, most likely related to changes in gene expression rather than a permanent change to DNA sequence, while the remaining changes were permanent.

No Increased Resistance to Antibiotics

Due to the concern about bacteria developing additional virulence in microgravity, Tirumalai also checked to see if E. coli grown in microgravity conditions developed any additional resistance to antibiotics.

Much to the relief of astronauts and NASA personnel everywhere, he found that after 1,000 generations of living in microgravity, these bacteria hadn’t developed any additional antibiotic resistance.

“Evolution is a strangely complex phenomenon,” Tirumalai said. “It’s difficult to predict. You can never know how many generations it would take for adaptations to become irreversible.”

This research was funded by the Institute of Space Systems Operations at the University of Houston and the NASA University Research Center at Texas Southern University.

- Rachel Fairbank, College of Natural Sciences and Mathematics