is an aerobic, moderately acidophilic bacterium that is also able to grow under microaerophilic conditions but not anaerobically. On the basis of preliminary analyses, the other strains in Table have similar physiological characteristics and are capable of transforming organic compounds, with a preference for protein substrates over polysaccharides. In addition, they were retrieved from different soils having pH values below 6.5 (Table ). The Hawaiian volcanic deposit, from which SOSP1-21T
-related sequences were retrieved, might also have an acidic pH (27
). The method used for isolation of SOSP1-21T
and related strains did not allow estimation of the abundance in soil. However, the relative ease of retrieval suggests that, as in many other instances (8
), the combination of a slightly acidic nutrient-poor medium with long incubation times was critical for isolation of these organisms. At the moment, the role of SOSP1-21T
and related strains in the soil is not known. They do not appear to produce antibiotic substances under the conditions that we tested; the NRPS gene-like sequences in their genomes may encode compounds for metal uptake, which are usually expressed under metal-limiting conditions (5
). Perhaps the ability of these organisms to grow well under microaerophilic conditions may provide a competitive advantage.
Strain SOSP1-21T and the related strains have a unique mixture of features observed in other bacterial lineages. The mycelial growth is reminiscent of the growth of Actinomycetales, while the G+C content and the overall shapes of the spores are consistent with properties of the Bacillaceae. The strains analyzed are gram positive and, like gram-positive bacteria, are sensitive to high-molecular-weight antibiotics, such as ramoplanin. The high level of 2-hydroxy C16:1, the major component of cellular fatty acids, and the menaquinone profile are uncommon, while the amino acid composition of SOSP1-21T peptidoglycan occurs in other gram-positive strains.
Comparison of strain SOSP1-21T with Chloroflexi.
and the related strains phylogenetically related to Chloroflexi
? The highest levels of binary similarity of the 16S rRNA sequences were the levels of similarity with representatives of this phylum (Table ), but the association with Chloroflexi
was not supported by high bootstrap values in phylogenetic trees. In addition, it was difficult to find similarities with characterized members of this phylum. The major morphological peculiarity of strain SOSP1-21T
is the production of a branched mycelium. In contrast, species belonging to the genera Roseiflexus
), and Herpetosiphon
), all described as filamentous, produce unbranched, multicellular filaments and not true mycelia. Moreover, none of them is described as a spore-forming organism. Other characteristics found in most representatives of the Chloroflexi
are the temperature ranges for growth, as all organisms except Oscillochloris
are thermophilic (optimal temperature, more than 50°C), and the pH ranges, as all organisms except Herpetosiphon
grow at pHs higher than 6. All the strains that we describe here are gram positive, while most Chloroflexi
strains are gram negative; the only exception is Sphaerobacter thermophilus
, which has only recently been reclassified as a member of this phylum (21
). The structure of the cell wall, however, clearly differentiates this organism from our strains, as S. thermophilus
has the peptidoglycan type A3β (l
-Orn←β-Ala) and contains the completely unsaturated menaquinone MK-8 (9
In conclusion, morphological, physiological, and chemotaxonomic data suggest that the strains described here are not associated with the Chloroflexi. In addition, the fact that, despite a relatively low level of 16S rRNA gene similarity, strains in clades GER1 and GER3 have similar morphological features that clearly differentiate them from known Chloroflexi support the hypothesis that the strains in Table , together with the phylogenetically related environmental clones, constitute a new bacterial division of filamentous, spore-forming, gram-positive bacteria. However, we cannot exclude the possibility that the Chloroflexi is a highly heterogeneous phylum with respect to phylogenetic, morphological, and physiological characteristics.
The 11 strains represent at least five families (or higher-rank taxa), as deduced from 16S rRNA gene similarities. In addition, they are phylogenetically distant enough from other cultivated Chloroflexi to justify a proposal for a new lineage comprising SOSP1-21T and the other strains in Table . At this time, we do not propose a new bacterial division to accommodate our strains. Strain SOSP1-21T is phylogenetically distinct from previously characterized bacterial strains and represents a new genus and species, for which we propose the name Ktedobacter racemifer.
Description of Ktedobacteria classis nov.
Ktedobacteria (Kte.do.bac.te′ri.a. N.L. masc. n. Ktedobacter, type genus of the class; -ia, suffix denoting a class; N.L. fem. pl. n. Ktedobacteria, the Ktedobacter class).
On the basis of 16S rRNA gene sequence analyses, this group may represent one of the primary lineages in the phylum Chloroflexi. Although we are aware that further analyses might indicate that Ktedobacteria is not part of the phylum Chloroflexi, we do not propose a new phylum at this time. The class Ktedobacteria currently comprises only the order Ktedobacterales.
Description of Ktedobacterales ord. nov.
Ktedobacterales (Kte.do.bac.ter.a′les. N.L. masc. n. Ktedobacter, type genus of the order; -ales, suffix denoting an order; N.L. fem. pl. n. Ktedobacterales, the Ktedobacter order).
The description is the same as that for the genus Ktedobacter. The order contains the family Ktedobacteraceae.
Description of Ktedobacteraceae fam. nov.
Ktedobacteraceae (Kte.do.bac.ter.a′ce.ae. N.L. masc. n. Ktedobacter, type genus of the family; -aceae, suffix denoting a family; N.L. fem. pl. n. Ktedobacteraceae, the Ktedobacter family).
The description is the same as that for the genus Ktedobacter. The family contains the type genus Ktedobacter.
Description of Ktedobacter gen. nov.
Ktedobacter (Kte.do.bac′ter. Gr. n. ktedon, fiber; N.L. masc. n. bacter, rod, bacterium, prokaryote; N.L. masc. n. Ktedobacter, filamentous bacterium). The filamentous, spore-forming bacteria are gram positive. Strains grow as mesophilic aerobic heterotrophs and can also grow under microaerophilic conditions. They contain ornithine, alanine, glutamic acid, serine, and glycine as the peptidoglycan amino acids. C16:1 2OH is the major component of the cellular fatty acids, and MK-9(H2) is the major menaquinone. The G+C content of the genomic DNA of strain SOSP1-21, the type strain of the type species, Ktedobacter racemifer, is 53.9%.
Description of Ktedobacter racemifer sp. nov.
Ktedobacter racemifer (ra.ce′mi.pher. L. adj. masc. racemifer, carrying clusters of grapes). In addition to the properties given in the genus description, this species has the following characteristics.
Gram-positive, non-acid-fast, aerobic, heterotrophic organism that produces branched vegetative mycelium in solid and liquid cultures. It also produces aerial hyphae that can bear spherical spores that are 1.6 to 1.8 μm in diameter. Spores emerge singly on short sporophores, but the dense spores and the interlacing of different spore-forming hyphae result in clusters of spores. Spores are not motile. Colonies are solid with cream to pink or orange pigmentation. Grows at pH 4.2 to 7.2; the optimal pH is around 6, and there is no growth at pH 3.9 and 7.5. The temperatures tolerated range from 17 to 40°C, and the optimum temperatures range from 28 to 33°C; no growth occurs at 14°C and 45°C. Lysozyme at a concentration of 100 μg/ml and 10 g/liter NaCl do not inhibit growth, while 30 g/liter NaCl does inhibit growth. Gelatin, casein, keratin, and starch are hydrolyzed, while chitin, xylan, and cellulose are not hydrolyzed. Catalase positive. Grows well in a microaerophilic atmosphere. Produces H2S but does not reduce nitrates. The type strain is SOSP1-21 (= DSM 44963).