EFFECTS OF BACTERIAL HOST AND DICHLOROMETHANE DEHALOGENASE ON THE COMPETITIVENESS OF METHYLOTROPHIC BACTERIA GROWING WITH DICHLOROMETHANE

Methylobacterium sp. strain DM4 and Methylophilus sp. strain DM11 can grow with dichloromethane (DCM) as the sole source of carbon and energ y by virtue of homologous glutathione-dependent DCM dehalogenases with markedly different kinetic properties (the k(cat) values of the enzym es of these strains are 0.6 and 3.3 s(-1), respectively, and the K-m v alues are 9 and 59 mu M, respectively). These strains, as well as tran sconjugant bacteria expressing the DCM dehalogenase gene (dcmA) from D M11 or DM4 on a broad-host-range plasmid in the background of dcmA mut ant DM4-2cr, were investigated by growing them under growth-limiting c onditions and in the presence of an excess of DCM. The maximal growth rates and maximal levels of dehalogenase for chemostat-adapted bacteri a were higher than the maximal growth rates and maximal levels of deha logenase for batch-grown bacteria. The substrate saturation constant o f strain DM4 was much lower than the K-m of its associated dehalogenas e, suggesting that this strain is adapted to scavenge low concentratio ns of DCM. Strains and transconjugants expressing the DCM dehalogenase from strain DM11, on the other hand, had higher growth rates than bac teria expressing the homologous dehalogenase from strain DM4. Competit ion experiments performed with pairs of DCM-degrading strains revealed that a strain expressing the dehalogenase from DM4 had a selective ad vantage in continuous culture under substrate-limiting conditions, whi le strains expressing the DM11 dehalogenase were superior in batch cul ture when there was an excess of substrate. Only DCM-degrading bacteri a with a dcmA gene similar to that from strain DM4, however, were obta ined in batch enrichment cultures prepared with activated sludge from sewage treatment plants.