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

Strain MC^T 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 <i>Acetobacterium</i> as '<i>Acetobacterium dehalogenans</i>'. 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 MC^T as most closely related to <i>Acetobacterium malicum</i> MuME1^T, 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 <i>A. malicum</i>, 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 '<i>Acetobacterium dehalogenans</i>' MC^T (=DSM 11527^T=NBRC 117038^T) as <i>A. malicum</i> subsp. <i>dehalogenans</i> subsp. nov. This automatically establishes <i>A. malicum</i> subsp. <i>malicum</i> subsp. nov., with MuME1^T (=DSM 4132^T=ATCC 51201^T) as the type strain.