NOVELTY - Isolated or purified polypeptide (I) that is active in degradation of methyl tert-butyl ether (MTBE) and/or at least one of its catabolic intermediates, preferably tert-butanol, 2-methyl-1,2-propanediol (X), hydroxyisobutyraldehyde (X) or hydroxybutyric acid (HIBA).    USE - Cells that express (I) are useful for treating aqueous effluents that are contaminated by MTBE (a petrol additive) and/or its catabolic intermediates, including treatment of aquifers and soil. Also kits that contain probes/primers derived from sequences that encode (I) are useful for detection/quantification of bacteria (particularly Mycobacterium austroafricanus) at sites contaminated by MTBE and/or its catabolic intermediates (all claimed).    DETAILED DESCRIPTION - Isolated or purified polypeptide (I) that is active in degradation of methyl tert-butyl ether (MTBE) and/or at least one of its catabolic intermediates, preferably tert-butanol, 2-methyl-1,2-propanediol (X), hydroxyisobutyraldehyde (X) or hydroxybutyric acid (HIBA). (I) is (a) any of Seq. (2), (4), (6), (8) or (10) (552, 215, 552, 223 and 410 amino acids, respectively); (b) a polypeptide at least 70, preferably 99, % identical with (a); (c) a derivative of (a) and (b) with one or more amino acid substitutions, deletions, insertions, additions and mutations; or (d) biologically active fragments of (a)-(c) containing at least 15 amino acids (aa). INDEPENDENT CLAIMS are included for:    (1) purified or isolated nucleic acid (II) that encodes (I);    (2) vector that contains (II);    (3) isolated host cell that contains (II) or the vector of (2);    (4) probes that are complementary to a fragment of at least 15 nucleotides (nt) from specific (II) or Seq. (12);    (5) oligonucleotide primers that contain at least 15 nt from specific (II) or Seq. (12);    (6) method for identifying a cell, or nucleic acid, able to degrade MTBE and/or its catabolic intermediates;    (7) the strain Mycobacterium austroafricanum CNCM I-3401;    (8) use of Seq. (2) as aldehyde dehydrogenase for converting (Y) to HIBA;    (9) use of Seq. (6) as alcohol dehydrogenase for converting (X) to (Y);    (10) method for treating aqueous effluents contaminated by MTBE and/or its catabolic intermediates by seeding with cells of (3) or (6); and    (11) kit for detecting and/or quantifying bacteria at sites contaminated by MTBE and/or its catabolic intermediates that includes probes/primers of items (4) or (5).