Unexpected 16S rRNA heterogeneity in 'Acetobacterium dehalogenans' and reclassification as Acetobacterium malicum subsp. dehalogenans subsp. nov.

Strain MCT is a strictly anaerobic, homoacetogenic bacterium with the ability to utilize methyl chloride as the sole energy source. It was tentatively assigned to the genus Acetobacterium as 'Acetobacterium dehalogenans'. Due to sequence ambiguities, it was not possible to determine the 16S rRNA gene sequence of this strain by direct sequencing of a PCR-amplified DNA segment. Whole-genome sequencing revealed significant heterogeneity amongst the five rRNA operons detected in this strain, with maximum sequence differences between the individual 16S rRNA genes exceeding 1.4%, compared to <0.8% in related species. Genome comparisons identified strain MCT as most closely related to Acetobacterium malicum MuME1T, with a digital DNA-DNA hybridization value of 71.9% and an average nucleotide identity score of 96.59%, indicating that the strains belong to the same species. Both strains share the ability to utilize malate, a key feature of A. malicum, but differ in the utilization of methanol and glucose. Chemotaxonomic analyses also revealed distinct fatty acid and polar lipid patterns. Based on these findings, we propose the classification of strain 'Acetobacterium dehalogenans' MCT (=DSM 11527T=NBRC 117038T) as A. malicum subsp. dehalogenans subsp. nov. This automatically establishes A. malicum subsp. malicum subsp. nov., with MuME1T (=DSM 4132T=ATCC 51201T) as the type strain.