Sphaerochaeta globosa gen. nov., sp. nov. and Sphaerochaeta pleomorpha sp. nov., free-living, spherical spirochaetes

Abstract

Free-living bacteria with spherical cells 0.5–2.5 µm in diameter were isolated from freshwater sediment. 16S rRNA gene sequence analysis placed the new isolates within the phylumSpirochaetes(‘spirochaetes’). The isolates never displayed a helical morphology or motility. Growth occurred in the presence of 100 mg ampicillin l−1in complex and defined mineral salts medium amended with vitamins, yeast extract and monosaccharides, disaccharides or soluble starch as fermentable substrates. Two distinct isolates, designated BuddyTand GrapesT, exhibited doubling times of 21±2 and 15±1 h in glucose-amended medium and grew at 15–37 and 15–30 °C. Optimum growth was observed between 25 and 30 °C and pH 6.5–7.5, with no growth below pH 5 or above pH 10. Hexose and pentose fermentation yielded ethanol, acetate and formate as major end products. Growth was strictly fermentative and anaerobic, but the isolates tolerated brief oxygen exposure. Nitrate, sulfate, thiosulfate and carbon dioxide were not used as electron acceptors, but soluble Fe(III) was reduced to Fe(II) in glucose-amended medium. The DNA G+C base contents of isolates BuddyTand GrapesTwere 45.5–46.4 and 47.0–49.2 mol%, respectively. Phospholipid fatty acid (PLFA) profiles contained large proportions of C14 : 0and C16 : 0straight-chain saturated fatty acids; C16 : 1ω7cand C16 : 1ω9cdominated the mono-unsaturated PLFAs in isolate GrapesT, whereas isolate BuddyTalso possessed C18 : 1ω5c, C18 : 1ω7cand C18 : 1ω9cfatty acids. Branched monoenoic acids accounted for up to 12.4 and 30 % of the total PLFA in isolates GrapesTand BuddyT, respectively. Based on their unique morphological features and the phylogenetic distance from their closest relatives, we propose the new genus,Sphaerochaetagen. nov., to accommodate the new isolates within the novel speciesSphaerochaeta globosasp. nov. (type strain BuddyT = DSM 22777T = ATCC BAA-1886T) andSphaerochaeta pleomorphasp. nov. (type strain GrapesT = DSM 22778T = ATCC BAA-1885T).Sphaerochaeta globosais the type species of the genus.