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Co-metabolism is defined as the simultaneous degradation of two compounds, in which the degradation of the second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate). This shouldn’t be confused with simultaneous catabolism, where two substrates are catabolized concomitantly by different enzymes.Co-metabolism occurs when an enzyme has the ability to degrade a second compound in addition to it's original, intended substrate, that is used by the organism that synthesizes the enzyme to derive energy and carbon from them. The degradation of the second compound, however, does not provide the bacteria energy or carbon, in other words, it is a non-growth-supporting substrate.

As some of the molecules that are the substrates of these reactions xenobiotic, persistent compounds, such as  PCE, TCE and MTBE, that have harmful effects on several types of environments, co-metabolism is thus used as an approach to biologically degrade hazardous solvents.

Co-metabolism in Bioremediation

A promising method of bioremediation of chlorinated solvents involves co-metabolism of the contaminants by aerobic microorganisms in groundwater and soils. Several aerobic microorganisms have been demonstrated to be capable of doing this, including methane oxidizers, phenol-degraders, and toluene-degraders.

One of them, Pseudomonas stutzeri OX1, can degrade hazardous chlorinated solvents, such as tetrachloroethylene (PCE), with the enzyme that they originally produce with the intent of deriving energy and carbon from methane and propane. The enzyme the organism synthesize that is capable of both oxidizing methane and chlorinated solvents is methane monooxygenase.

Another example of the use of co-metabolism to degrade pollutants would be the biodegradation of methyl-tert-butyl ether (MTBE): a chemical synthesized by the use of fossil fuels and that is toxic to both ground and underground aqueous environments. Pseudomonas aeruginosa and Pseudomonas citronellolis was shown to be able to carry out the co-metabolic degredation of MTBE and fully mineralize it by using their enzymes that has a physiological role of oxidizing short, n-alkanes to utilize them as growth sources.

The difficulties and high costs of maintaining substrate and an oxic environment, have led to limited field-scale application of co-metabolism for solvent degradation. Recently, this method of remediation has been improved by the substitution of cheap, nontoxic plant secondary metabolites in the place of synthetic, toxic aromatics like toluene.