What Is Ralstonia eutropha and Why Is It Important?

Ralstonia eutropha is a Gram-negative bacterium found in soil and water. Though officially reclassified as Cupriavidus necator in 2004, the name Ralstonia eutropha remains widely used. This microorganism is a subject of interest in biotechnology and environmental microbiology due to its diverse metabolic functions. Its adaptability allows it to thrive in various environments, making it a candidate for several technological applications.

Unique Metabolic Capabilities

The utility of Cupriavidus necator stems from its metabolic flexibility, allowing it to acquire energy and carbon in multiple ways. It can function as a chemolithoautotroph, oxidizing hydrogen gas (H₂) for energy while using atmospheric carbon dioxide (CO₂) to build cellular components via the Calvin-Benson-Bassham cycle, the same pathway used by plants.

This autotrophic capability is complemented by its heterotrophic lifestyle, consuming a wide range of organic compounds like sugars, organic acids, and alcohols. This versatility allows the bacterium to adapt its metabolism based on available nutrients, making it resilient and widespread in soil ecosystems.

The genetic basis for this flexibility is its large genome, divided into two chromosomes and a megaplasmid. Many genes for specialized functions, like hydrogen oxidation and breaking down certain aromatic compounds, are on these secondary genetic elements. This structure suggests the bacterium acquired some capabilities through horizontal gene transfer.

Production of Bioplastics

One of the most studied applications of C. necator is its ability to produce bioplastics, which are biodegradable polyesters known as polyhydroxyalkanoates (PHAs). PHAs are synthesized and stored inside the cell as granules, serving as a carbon and energy reserve much like fat in animals. These polymers have properties similar to conventional plastics but are fully biodegradable.

The bacterium initiates PHA production as a survival mechanism. When a carbon source is abundant but an essential nutrient like nitrogen or phosphorus is limited, the cell cannot grow. It redirects the excess carbon into synthesizing PHA granules, which can comprise up to 80% of the cell’s dry weight.

This process has attracted industrial interest for sustainable plastic production. In a controlled setting, bacteria are cultivated in large bioreactors and fed low-cost carbon sources like waste glycerol or agricultural sugars. The cells are then harvested, and the PHA is extracted and processed into a resin to manufacture biodegradable products.

Role in Environmental Bioremediation

Cupriavidus necator also plays a role in breaking down environmental pollutants. Its versatile metabolism allows it to degrade hazardous organic compounds in soil and water, using them as a food source. This capability makes it a tool for bioremediation, the use of living organisms to clean up contaminated sites.

The bacterium can degrade various chlorinated aromatic compounds, such as the herbicide component 2,4-dichlorophenoxyacetic acid, and industrial byproducts like chlorophenols. Some strains can also break down components of crude oil. This degradative capacity is often encoded by genes on its megaplasmid.

In addition to breaking down organic pollutants, C. necator is resistant to high concentrations of toxic heavy metals. This resistance is a feature for bioremediation, as industrial sites are often co-contaminated with both organic chemicals and heavy metals. Strains have shown tolerance to:

• Copper
• Zinc
• Arsenic
• Cadmium

A Platform for Synthetic Biology

The well-understood genetics and metabolism of Cupriavidus necator have made it an attractive “chassis organism” for synthetic biology. A chassis is a base microbe that scientists can reliably modify and engineer to perform new functions. Because its biological systems are well-characterized, researchers can introduce new genetic pathways to program it to produce substances it does not naturally create.

This engineering approach extends the bacterium’s utility. By inserting genes from other organisms, scientists have engineered C. necator to produce valuable chemicals, including advanced biofuels like isobutanol and other industrial specialty chemicals. This turns the microbe into a programmable factory powered by simple feedstocks.

The ability of C. necator to use H₂ and CO₂ makes it an interesting platform for sustainable bioproduction. Engineering strains to convert these gases into high-value liquid fuels or chemicals represents a path toward carbon-neutral manufacturing. This positions the bacterium as a tool for a circular bio-economy, where waste gases are upcycled into useful products.