Dehalobacter – a genus of anaerobic bacteria dedicated to organohalide degradation

Abstract The genus Dehalobacter embraces bacterial populations that seem to exclusively degrade organohalides. All isolates in pure culture and highly enriched strains are obligate organohalide-respiring bacteria that use hydrogen as energy and electron source, acetate as carbon source, and an organohalide as terminal electron acceptor. Depending on the strain, they are restricted to the use of only one or two organohalides from the same chemical group (i.e. aliphatic or aromatic organohalides), a few strains however can use several compounds and from different groups. Organohalides used by Dehalobacter are chlorinated methanes, ethanes, ethenes, cyclohexanes, benzenes, phenols, and phthalides. However, two enrichments dominated by Dehalobacter spp. indicate another metabolic pathway with a specific organohalide, namely fermentation of dichloromethane. No particular habitat can be defined for this bacterial genus since the different strains have been enriched and isolated from various matrices such as sediments, aquifers, and anaerobic sludge from waste treatment processes. The small motile rods (0.5 μm in diameter, 2-3 μm long) usually stain Gram-negative, contain, however, peptidoglycan features of Gram-positives, menaquinones, and cytochrome b, and are surrounded by proteinaceous S-layer. Phylogenetically Dehalobacter is affiliated to low GC Gram-positive Firmicutes. Recently available genome sequences revealed that Dehalobacter spp. harbour an unexpected large number of putative reductive dehalogenase genes (10-27 paralogs) showing a relatively high diversity, several hydrogenases of different types, an 11-subunit respiration complex I, all necessary genes for the Wood-Ljungdahl pathway and the biosynthesis pathway of corrinoids, and seemed to confirm that Dehalobacter spp. cannot carry out any other respiration process than organohalide respiration. Hence, the hydrogen and carbon metabolisms seem to be more complex than anticipated, and also the observed restriction to few organohalides as electron acceptor is perhaps not reflecting the real dechlorination capabilities of Dehalobacter strains with the numerous putative reductive dehalogenase genes in their genomes.