f Taxonomic Analysis of Thermophilic Strains of the Genus Methanobacterium: Reclassification of Methanobacterium thermoalcaliphilum as a Synonym of Methanobacterium thermoautotrophicum

DNA reassociation, a comparative analysis of whole-cell protein patterns, and indirect immunofluorescence methods were used to determine the taxonomic relationships of thermophilic Methanobacterium strains. On the basis of the results dendrograms showing degrees of similarity among the organisms were constructed. The organisms studied are members of three different groups. One group, whose members exhibit 10 to 40% cross-hybridization, includes Methanobacterium wolfeii, “Methanobacterium defluvium” ADZ, and “Methanobacterium thermoflexum” IDZ. The second group contains Methanobacterium thermophilum MT (T = type strain) and Methanobacterium thermoaggregans (levels of DNA relatedness, 30 to 45%). The third group, whose members exhibit 65 to 99% cross-hybridization, includes Methanobacterium thermoautotrophicum ΔHT, F-1, and DV; Methanobacterium thermoformicicum Z-245T; and Methanobacterium thermoalcaliphilum AC60T. The combination of three independent taxonomic methods showed that the group of strains studied is phenotypically, genotypically, and antigenically diverse. The most distinct organisms are M. wolfeii, M. thermoaggregans, M. thermophilum, “M. defluvium,” “M. thermoflexum,” and M. thermoautotrophicum. These species names should be adopted. The results of a DNA-DNA hybridization study (level of hybridization, 99%), an immunological analysis (+3 reaction), and a protein similarity study (level of similarity, 75%) indicate that M. thermoalcaliphilum should be reclassified as a synonym of M. thermoautotrophicum.