"Forever chemicals" could see their days numbered

The idea to eliminate PFAS (per- and polyfluoroalkyl substances, known as “forever chemicals”) via bacteria took seed in laboratories in Strasbourg (northeastern France). This involves using the enzymatic* properties of bacteria to break the chemical bond that makes PFAS so resistant to alteration, namely the carbon-fluorine bond, one of the strongest that exist. Two methods are used: culturing bacteria in the presence of PFAS in a Petri dish (photo); and using microfluidics for culture in microdroplets, which then serve as so many miniature test tubes. *Enzyme: protein that facilitates a chemical reaction in a cell.

Trying to use bacteria to degrade PFAS is one thing, knowing which ones to select is another. Researchers at the RNA structure and reactivity laboratory (headed by Michaël Ryckelynck) rely on microfluidic chips cast from silicone resin. Soil or water containing PFAS is first collected, and the bacteria present in these samples are cultured in microdroplets. These will then be sorted by the chips’ extremely narrow channels (on the order of a micrometre, or 0.001 millimetres).

The silicone block is pierced in different spots to connect the microfluidic chip to a tubing system designed to carry the microdroplets containing the bacteria through the chip’s channels. The droplets can then be screened at high-throughput to determine whether they contain the bacteria that produce the enzymes capable of degrading PFAS.

The last step in the manufacturing of the microfluidic chip involves welding the silicone resin block to a glass plate, in order to seal them both hermetically. To do so, the resin block and plate are placed in a plasma oven for a few minutes, after which the microfluidic chip is ready for use.

The next step is to connect the microfluidic chip to the tubing. Each droplet that is carried through contains bacteria and PFAS from samples of contaminated water or soil. The scientists add a particular RNA to the culture medium: an aptamer that can specifically detect the presence of fluorine (fluoride) ions.

When the presence of fluorine (fluoride) is detected, the droplet turns fluorescent green, signalling the breaking of the carbon-fluorine bond, and hence the degradation of the PFAS. Thanks to microfluidics, the scientists can screen millions of bacteria each day, hoping to find the needle in the haystack.

Once the sorting is complete, the researchers place the fluorescent droplets near a plasma lamp, whose electric field “breaks” them and recovers the bacteria of interest.

In parallel to microfluidics, the more traditional approach of cell culture in a Petri dish is used at the GMGM molecular genetics, genomics, and microbiology laboratory (CNRS / Université de Strasbourg), in the team led by Stéphane Vuilleumier. Samples of water or soil contaminated with PFAS are suspended in a liquid culture medium to select bacteria potentially able to degrade them. These cultures are then transferred to a solid medium. The gel present in the dishes contains the nutrients needed for bacterial growth, in addition to a pH indicator. If the cultured bacteria thrive in this environment by degrading a fluorinated compound, the medium tends to become more acidic, changing from green to yellow.