Meet Rhodococcus ruber - the plastic digesting bacterium

 Image by Hvonwerd (Creative Commons: https://microbewiki.kenyon.edu/index.php/File:DSM_44190.jpg)

The bacterium Rhodococcus ruber eats and actually digests plastic. This has been shown in recent laboratory experiments conducted at Royal Netherlands Institute for Sea Research (NIOZ).

 

Based on a model study with plastic in artificial seawater in the laboratory, researchers (led by Maaike Goudriaan) have calculated that bacteria can break down about one percent of the fed plastic per year into carbon dioxide and other substances. This finding may explain what happens to some of the plastic in the oceans.

 

The selected bacterium R. ruber can form a biofilm on plastic in nature. It had also been measured that plastic disappears under that biofilm.

 

R. ruber initially forms a branching mycelium that breaks into shorter rods and cocci as it transitions through different growth phases. R. ruber is characteristically a Gram positive, non-motile, non-spore forming bacteria. It has diverse metabolic and nutritional capabilities depending on the strain, such as utilizing gaseous hydrocarbons, aromatic hydrocarbons, complex polymers, and steroids for carbon and energy sources.

 

To demonstrate the effect, the researchers used a special plastic manufactured with a distinct form of carbon (13C) in it. The plastic was introduced to the bacteria after pretreatment with a UV lamp in a bottle of simulated seawater. The treatment with UV light was necessary because sunlight partially breaks down plastic into bite-sized chunks for bacteria.

 

Sunlight breaks down plastics on the ocean's surfaces. Floating microplastic is broken down into ever smaller, invisible nanoplastic particles that spread across the entire water column, that can then be completely broken down by bacteria. About two percent of visibly floating plastic may disappear from the ocean surface in this way each year.

 

R. ruber is an aerobic chemoorganotroph, meaning the bacterium uses organic compounds as its carbon and energy source through oxidative metabolic pathways. The organism is able to utilize gaseous hydrocarbons, aliphatic hydrocarbons, aromatic hydrocarbons, and xenobiotic substances, such as crude oil and plastics.

 

In time it was noted that a special version of carbon appear as carbon dioxide appeared above the water.

 

While it has been long suspected, this is the first time it has been experimentally demonstrated that bacteria can digest plastic into other molecules. R. ruber is able to use inert polymers, such as polyethylene and polystyrene, as its sole carbon source.

 

Calculations suggest that the total breakdown of plastic into carbon dioxide by bacteria per year is about one percent of the available, although this is estimate is not robust and further research is required to assess the microbial impact.

 

However, while it may appear that microbial digestion is a possible solution to the huge problem of all the plastic floating on and in our oceans the best solution is for humanity to reduce the use of plastic.

 

See:

 

Maaike Goudriaan, Victor Hernando Morales, Marcel T.J. van der Meer, et al. A stable isotope assay with 13C-labeled polyethylene to investigate plastic mineralization mediated by Rhodococcus ruber. Marine Pollution Bulletin, 2023; 186: 114369 DOI: 10.1016/j.marpolbul.2022.114369

 

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)