We present evidence in support of revival of the genus Hyphomonas Pongratz 1957 and emend the description of this genus. The type species is Hyphomonas polymorpha nom. rev., with type strain PS728 (= ATCC 33881 = IFAM PS728). A second strain of Hyphomonas polymorpha, strain PR727 (= ATCC 33880 = IFAM PR727), is also recognized. Transfer of a related organism, Hyphomicrobium neptunium strain LE670 (= ATCC 15444 = IFAM LE670), to the genus Hyphomonas is formally proposed.

In 1961 Aristovskaya described the new hyphal budding bacteria Pedomicrobium ferrugineum and Pedomicrobium manganicum. Only a limited amount of information concerning these bacteria was obtained from enrichment cultures, and these cultures no longer exist. In an effort to revive investigations on pedomicrobia, six new iron-depositing strains were isolated from podzolic soils. One of these isolates, strain S-1290 (=ATCC 33119 =DSM 1540), was investigated thoroughly and is cited in the Approved Lists of Bacterial Names as the type strain of P. ferrugineum. Four other isolates (strains P-1196, Q-1197, R-1198, and T-1130) showed sufficient similarity to strain S-1290 to be placed in the same species. The taxonomic position of the sixth isolate, strain F-1225, is not yet certain. One additional isolate, strain E-1129 (=ATCC 33121 =DSM 1545), which deposits manganese, was isolated from a quartzite rock pool. This isolate closely resembles the original description of P. manganicum and is cited in the Approved Lists as the type strain of this species. I present an emended description of the genus Pedomicro bium and compare the generic properties of Pedomicro bium with those of other hyphal budding bacteria. To encourage further work on pedomicrobia, I also describe enrichment and isolation procedures for these bacteria. Cultures of all of the new strains are available from the American Type Culture Collection and from the Deutsche Sammlung von Mikroorganismen.In 1961, Aristovskaya (1) described budding Sakhalin, USSR. He described oval cell types bacteria with threadlike outgrowths (hyphae); ("Pedomicro bium manganicum subsp. sachathese organisms were found predominantly in Zinicum") and other cells which were spherical; podzolic soils in the Karelian Isthmus, USSR. both types accumulated oxidized manganese Enrichment cultures on agar media containing compounds. "organomineral complexes of Mvic acids"The aquatic environment is also a habitat of served as bases for descriptions of these new Pedomicrobium-like organisms. Hirsch (13) isobacteria, which were named Pedomicro bium fer-lated several iron-depositing pedomicrobia from rugineum and Pedomicrobium manganicum be-seawater, and Hirsch and Rheinheimer (16) used cause they were able to accumulate oxidized iron seawater samples containing 2.5% salt for enrichand manganese compounds, respectively, on ment cultures of other Pedomicrobium strains. their cell surfaces.Kutuzova (20) found pedomicrobia in the sediSubsequently (2), a third species "Pedomicro-ment of a freshwater lake. Tyler and co-workers bium podsolicum" was described (names in quo-isolated Pedomicro bium-like bacteria, described tation marks are not on the Approved Lists of as Hyphomicrobium strains, from hydroelectric Bacterial Names [35]; hence, they are without pipelines in Tasmania, Australia (6, 7, 40-43). standing in nomenclature). This organism was These organisms caused thick manganese decharacterized by the ability to deposit both iron posits in various freshwater pipelines in Engand manganese oxides...

A total of 65 strains of appendaged or prosthecate, budding bacteria from our culture collection were selected for a study of deoxyribonucleic acid (DNA) base composition and nucleotide distribution. These strains represented 11 genera, including 4 genera of hyphal, budding bacteria which have not been formally described yet. The DNA species were thermally denatured, and absorbance-temperature profiles were recorded. The midpoints, widths, and asymmetries of the melting transitions were determined. When the DNA base compositions and nucleotide distributions were plotted on a dissimilarity map, it became evident that the strains of each genus occupied a distinct area. The distribution of strains within such an area indicated the degree of heterogeneity of a genus. When 16 Hyphomicrobium strains were analyzed, they formed five clusters within their generic area. These clusters correlated well with groups which had been previously established by DNA base composition analyses, by DNA-DNA homology studies, and by numerical taxonomy. Nine of the strains investigated were distinguished by melting profiles which were skewed uniquely to the left.

The levels of genetic relatedness of 19 Hyphomicrobium strains which utilize one-carbon compounds were determined by deoxyribonucleic acid (DNA)-DNA hybridization in solution under optimal conditions (S1 nuclease technique). Most of these hyphomicrobia fell into four groups with high levels of relatedness (level of homology within each group, 86 to 110%). These groups were only distantly related to each other (levels of homology between groups, 1 to 9%). Three additional groups of C1-utilizing hyphomicrobia were represented by only one strain each. In addition, the levels of DNA-DNA homology of four Hyphomonas species and 11 Hyphomicrobium-Hyphomonas-like isolates were determined. Seven of these isolates formed three groups containing two or three strains each; the level of homology within each group was 94 to 120%. These groups of peptide-utilizing strains were related to Hyphomonas spp. at a DNA-DNA homology level of 13 to 43 % ; thus, they represented new species of Hyphomonas. Four of the isolates had less than 10% DNA homology with either Hyphomicrobium reference strains or Hyphomonas spp. Strain B-1408 was distinguished by its DNA base composition of 46.19 mol% guanine plus cytosine, which is 14 mol% below the average base composition of Hyphomicrobium spp. or Hyphomonas spp. The levels of genetic relatedness of other hyphal, budding bacteria, such as Pedomicrobium spp., genus F, Rhodomicrobium vannielii, and genus T, to Hyphomicrobium sp. strain MC-750 were too low to be evaluated by DNA-DNA hybridization techniques.The genus Hyphomicrobium was described in 1898 as the first of a group of hyphal, budding bacteria (35). Later, morphologically similar organisms belonging to the genera Rhodomicrobium (3, Hyphomonas (29), and Pedomicrobium (1, 2) were discovered. While Rhodomicrobium spp. were clearly distinguished by their ability to produce photosynthetic pigments ( 5 , 23, 28), the remaining three genera could not be differentiated with certainty by morphological and physiological properties (6, 28). However, Pedomicrobium spp. could be distinguished from other budding and hyphal bacteria by means of deoxyribonucleic acid (DNA) base composition and nucleotide distribution (7, lo), genome size (19), and DNA-DNA homology (8, 9). The differentiation of Hyphomicrobium spp. from Hyphomonas spp. by means of morphology, DNA base composition (9, 10,21), nucleotide distribution (lo), and genome size (19,27) was still afflicted with uncertainty. However, DNA-DNA hybridization experiments (9, 26) performed with 20 Hyphomicrobium strains and 3 strains of Hyphomonas spp. revealed 2% homology, at most. Ribosomal ribonucleic acid-DNA hybridization between Hyphomicrobium strain B-522 and three Hyphomonas strains detected high thermal denaturation (T,) values, which indicated distant relatedness between these representatives of the two genera (25).Unfortunately, the first description of Hyphomicrobium vulgare (35) did not provide information about the utilization of one-carbon compounds by this organism. On the other hand,...

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