A new study by Princeton University researchers has found that bacteria are able to adapt to changes in their environment despite not having a nervous system.
In the study, the researchers used lab experiments to demonstrate this phenomenon in common bacteria and also used computer simulations to explain how a microbe species' internal network of genes and proteins could evolve over time to produce such complex behaviour.
"What we have found is the first evidence that bacteria can use sensed cues from their environment to infer future events. The two lines of investigation came together nicely to show how simple biochemical networks can perform sophisticated computational tasks," said Saeed Tavazoie, an associate professor in the department of Molecular Biology.
The findings would help scientists understand how bacteria mutate to develop resistance to antibiotics and also aid in the development of specialized bacteria to perform useful tasks such as cleaning up environmental contamination.
In one part of the study, the researchers focused on Escherichia coli and wanted to determine how its genes respond to the temperature and oxygen changes that occur when the bacterium enters the gut of warm-blooded vertebrates, which is conventionally attributed to its switch from aerobic (oxygen) to anaerobic (oxygen-less) respiration. But this was a little doubtful according to Tavazoie, who said: "This kind of reflexive response would not be optimal."
Thus the researchers exposed a population of E. coli to different temperatures and oxygen changes, and measured the gene responses in each case. The results showed that an increase in temperature had nearly the same effect on the bacterium's genes as a decrease in oxygen level. In fact, upon transition to a higher temperature, many of the genes essential for aerobic respiration were practically turned off.
To prove that this is not just genetic coincidence, the researchers then grew the bacteria in a biologically flipped environment where oxygen levels rose following an increase in temperature. Remarkably, within a few hundred generations the bugs partially adapted to this new regime, and no longer turned off the genes for aerobic respiration when the temperature rose.
"This reprogramming clearly indicates that shutting down aerobic respiration following a temperature increase is not essential to E. coli's survival. On the contrary, it appears that the bacterium has "learned" this response by associating specific temperatures with specific oxygen levels over the course of its evolution," said Tavazoie.
The findings challenge the prevailing notion that only organisms with complex nervous systems have this ability. Tavazoie explained that while higher animals can learn new behavior within a single lifetime, bacterial learning takes place over many generations and on an evolutionary time scale.
For a deeper understanding of this phenomenon, researchers developed a virtual microbial ecosystem, called Evolution in Variable Environment. Each microbe in this novel computational framework is represented as a network of interacting genes and proteins. An evolving population of these virtual bugs competes for limited resources within a changing environment, mimicking the behaviour of bacteria in the real world.
In this virtual world, microbes are more likely to survive if they conserve energy by mostly turning off the biological processes that allow them to eat. The challenge they face then is to anticipate the arrival of food and turn up their metabolism just in time. To help them along, the researchers gave the bugs cues before feeding them, but the cues had to appear in just the right pattern to indicate that food was on its way.
"To predict mealtimes accurately, the microbes would have to solve logic problems," said Ilias Tagkopoulos.
After a few thousand generations, an ecologically fit strain of microbe emerged which did exactly that. This happened for every pattern of cues that the researchers tried. Tagkopoulos said that the feeding response of these gastronomically savvy bugs peaked just when food was offered. When the researchers examined a number of fit virtual bugs, they could at first make little sense out of them.
"Their biochemical networks were filled with seemingly unnecessary components. That is not how an engineer would design logic-solving networks," said Tagkopoulos.
Pared down to their essential elements, however, the networks revealed a simple and elegant structure. The researchers could now trace the different sequences of gene and protein interactions organisms used in order to respond to cues and anticipate mealtimes.
"It gave us insights into how simple organisms such as bacteria can process information from the environment to anticipate future events," said Tagkopoulos.
The study is published in the latest issue of Science.