ASSIGNMENT OF FATTY ACID-BETA-OXIDIZING SYNTROPHIC BACTERIA TO SYNTROPHOMONADACEAE FAM NOV ON THE BASIS OF 16S RIBOSOMAL-RNA SEQUENCE ANALYSES

After enrichment from Chinese rural anaerobic digestor sludge, anaerob ic, sporing and nonsporing, saturated fatty acid-beta-oxidizing syntro phic bacteria were isolated as cocultures with H-2- and formate-utiliz ing Methanospirillum hungatei or Desulfovibrio sp. strain G-11. The sy ntrophs degraded C4 to C8 saturated fatty acids, including isobutyrate and 2-methylbutyrate. They were adapted to grow on crotonate and were isolated as pure cultures. The crotonate-grown pure cultures alone di d not grow on butyrate in either the presence or the absence of some c ommon electron acceptors. However, when they were reconstituted with M . hungatei, growth on butyrate again occurred. In contrast, crotonate- grown Clostridium kluyveri and Clostridium sticklandii, as well as Clo stridium sporogenes, failed to grow on butyrate when these organisms w ere cocultured with M. hungatei. The crotonate-grown pure subcultures of the syntrophs described above were subjected to 16S rRNA sequence a nalysis. Several previously documented fatty acid-beta-oxidizing syntr ophs grown in pure cultures with crotonate were also subjected to comp arative sequence analyses. The sequence analyses revealed that the new sporing and nonsporing isolates and other syntrophs that we sequenced , which had either gram-negative or gram-positive cell wall ultrastruc ture, all belonged to the phylogenetically gram-positive phylum. They were not closely related to any of the previously known subdivisions i n the gram-positive phylum with which they were compared, but were clo sely related to each other, forming a new subdivision in the phylum. W e recommend that this group be designated Syntrophomonadaceae fam. nov .; a description is given.