Ideonella sakaiensis is a bacterium from the genus Ideonella and family Comamonadaceae capable of breaking down and consuming the plastic polyethylene terephthalate (PET) as a sole carbon and energy source. The bacterium was originally isolated from a sediment sample taken outside of a plastic bottle recycling facility in Sakai, Japan.
Ideonella sakaiensis was first identified in 2016 by a team of researchers led by Kohei Oda of Kyoto Institute of Technology and Kenji Miyamoto of Keio University after collecting a sample of PET-contaminated sediment near a plastic bottle recycling facility in Japan. The bacterium was isolated from a consortium of microorganisms in the sediment sample, including protozoa and yeast-like cells. The entire microbial community was shown to mineralize 75% of the degraded PET into carbon dioxide once it had been initially degraded and assimilated by I. sakaiensis.
Ideonella sakaiensis is Gram-negative, aerobic, and rod-shaped. It does not form spores. Cells are motile and have a single flagellum. I. sakaiensis also tests positive for oxidase and catalase. The bacterium grows at a pH range of 5.5 to 9.0 (optimally at 7 to 7.5) and a temperature of 15–42 °C (optimally at 30–37 °C). Colonies of I. sakaiensis are colorless, smooth, and circular. Its size varies from 0.6-0.8 μm in width and 1.2-1.5 μm in length. The bacterium was shown to grow on PET surfaces in a community with other I. sakaiensis cells by adhering to the PET and other cells with thin appendages. These appendages may also function to secrete PET-degrading enzymes onto the PET surface.
Through phylogenetic analysis, the species was shown to be a part of the genus Ideonella, but possessed a significantly different genome than other known species in the genus, including Ideonella dechloratans and Ideonella azotifigens, thus justifying its classification as a new species.
Degradation and assimilation of PET
Ideonella sakaiensis cells adhere to the PET surface and use a secreted PET hydrolase, or PETase, to degrade the PET into mono(2-hydroxyethyl)terephthalic acid (MHET), a heterodimer composed of terephthalic acid (TPA) and ethylene glycol. The I. sakaiensis PETase functions by hydrolyzing the ester bonds present in PET with high specificity. The resulting MHET is then degraded into its two monomeric constituents by a lipid-anchored MHET hydrolase enzyme, or MHETase, on the cell’s outer membrane. Ethylene glycol is readily taken up and used by I. sakaiensis and many other bacteria. Terephthalic acid, a more recalcitrant compound, is imported into the I. sakaiensis cell via the terephthalic acid transporter protein. Once in the cell, the aromatic terephthalic acid molecule is oxidized by terephthalic acid-1,2-dioxygenase and 1,2-dihydroxy-3,5-cyclohexadiene-1,4-dicarboxylate dehydrogenase into a catechol intermediate. The catechol ring is then cleaved by PCA 3,4-dioxygenase before the compound is integrated into other metabolic pathways (e.g. TCA cycle). As a result, both of the molecules derived from the PET are used by the cell to produce energy and to build necessary biomolecules. Eventually, the assimilated carbon may be mineralized to carbon dioxide and released into the atmosphere.
Impact and applications
The discovery of Ideonella sakaiensis has potential importance for the degradation of PET plastics. Prior to its discovery, the only known degraders of PET were a small number of bacteria and fungi, including Fusarium solani, and no organisms were definitively known to degrade PET as a primary carbon and energy source. The discovery of I. sakaiensis spurred discussion about PET biodegradation as a method of recycling and bioremediation.
The wild-type bacterium is able to colonize and break down a thin (0.2 mm thickness) film of low-crystallinity (soft) PET in approximately six weeks, and the responsible PETase enzyme was shown to degrade high-crystallinity (hard) PET approximately 30-fold slower than low-crystallinity PET. A large amount of manufactured PET is highly crystalline (e.g. plastic bottles), so it is thought that any prospective applications of the I. sakaiensis PETase enzyme in recycling programs will need to be preceded by genetic optimization of the enzyme. The MHETase enzyme could also be optimized and used in recycling or bioremediation applications in combination with the PETase enzyme. It degrades the MHET produced by the PETase into ethylene glycol and terephthalic acid. Once formed, these two compounds can be further biodegraded into carbon dioxide by I. sakaiensis or other microbes, or they can be purified and used to manufacture new PET in an industrial recycling plant setting.
The PET degrading enzyme of Ideonella sakaiensis, PETase, has been genetically modified and combined with MHETase to break down PET faster, and also degrades PEF. This may possibly, along with other approaches, be useful in efforts for recycling and upcycling of mixed plastics.