Scientists from Israel were successful in creating a strain of Escherichia coli bacteria that consumes carbon dioxide and not organic compounds for energy. This is considered to be quite a feat in the field of microbiology and synthetic biology because it sheds some light to the incredible plasticity of bacterial metabolism and may provide the backbone for carbon-neutral bioproduction in the future.
What is E.coli?
We often hear of E.coli in the news as some harmful bacteria that can infect humans, and while some of that is true, there are types of E.coli that are harmless (READ: Why Romaine Lettuce Keeps Getting Called Out for Outbreak of E.coli). According to the Centers for Disease and Prevention Control, Escherichia coli is a diverse group of bacteria that normally lives in the intestines of humans and animals, and the pathogenic strain of this bacteria can cause intestinal problems like diarrhea.
However, for this study, scientists developed E.coli to help address the excess carbon dioxide emissions, especially recently when the rates of the world's carbon emissions are at an all-time high. Senior author Ron Milo, a systems biologist at the Weizmann Institute of Science explains, "our main aim was to create a convenient scientific platform that could enhance carbon dioxide fixation, which can help address challenges related to the sustainable production of food and fuels and global warming caused by carbon dioxide emissions." He also explains that by converting the bacteria's carbon source from organic carbon into carbon dioxide is a major step in achieving their goals.
HOW WILL THE BACTERIA CONSUME CARBON DIOXIDE?
To get a clearer picture of how everything works, think of our world divided into two types of organisms: the autotrophs or the ones that convert inorganic carbon dioxide into biomass. A perfect example for autotrophic organisms are plants, algae, some bacteria, and fungi. On the other hand, organisms that consume organic compounds are called heterotrophs. One of the greatest challenges faced by synthetic biology is to develop synthetic autotrophy within a model of heterotrophic organisms. Unfortunately, even in the middle of widespread interest in renewable energy, attempts that were made to generate autotrophy often ended in failure.
In previous experiments, model heterotrophs will always require the addition of multi-carbon organic compounds for stable growth. With this in mind, Shmuel Gleizer, Weizmann Institute of Science postdoctoral fellow, and the first author of the study explains, "from a basic scientific perspective, we wanted to see if such a major transformation in the diet of bacteria -- from dependence on sugar to the synthesis of all their biomass from carbon dioxide -- is possible." He says that the team wanted to know how extreme an adaptation is needed in terms of the changes to the bacterial DNA blueprint upon testing the feasibility of such a transformation in the laboratory.
For the experiment to succeed, the researchers used metabolic rewiring to finally convert E.coli to become autotrophs that harvest energy formate (a product of the deprotonation of formic acid, this was produced using renewable materials). However, formate is an organic one-carbon compound. That is why it did not serve as a good carbon source for E.coli as it cannot support heterotrophic growth. To answer this dilemma, the scientists engineered the engineered E.coli strain to produce non-native enzymes for carbon fixation and the reduction and harvesting of energy formate (a monocarboxylic acid anion that is the conjugate base of formic acid).
The scientists inactivated central enzymes on the E.coli strain that are directly involved in heterotrophic growth, making the E.coli strain dependent on autotrophic pathways for its growth. The cells were also kept in chemostats with a limited supply of sugar xylose (which inhibits heterotrophic pathways) for 300 days. This was necessary to support cell proliferation and to start the strain's evolution. Milo explains, "in order for the general approach of lab evolution to succeed, we had to find a way to couple the desired change in cell behavior to a fitness advantage -- that was tough and required a lot of thinking and smart design."
In sequencing the genome and plasmids of the evolved strain, they discovered that there are 11 mutations were acquired through the process. The full study is published in Cell.
