Genus I. Sphaerotilus, p. 263.
II. Trichomes surrounded by sheaths impregnated with oxides of iron or manganese which dissolve in strong hydrochloric acid. Free-living or sessile.
A. Individual trichomes, each with a sheath.
Genus II. Leptothrix, p. 264.
B. Sheaths contain more than one trichome; the trichomes are sometimes in a fan-like arrangement.
Genus III. Toxothrix, p. 269.
Genus I. Sphaerotilus Kützing, 1833.
(Kützing, Linnaea, 8, 1833, 385; Cladothrix Cohn, Beitr. z. Biol. d. Pflanz., 1, Heft 3, 1875, 185.)
Sphae.ro'ti.lus. Gr. noun sphaera a sphere; Gr. noun tilus anything shredded, flock, down; M.L. mas.n. Sphaerotilus sphere down.
Attached or free-floating, colorless trichomes showing false branching, though this may be rare in some species. When e.xamined under the electron microscope, the sheath shows a homogeneous structure. Sheath may become j'ellowish or brown with the deposition of iron oxide. The deposition of iron is dependent on environmental factors, not on the ph3'siologi- cal ability to store iron. Trichomes consist of rod-shaped or ellipsoidal cells surrounded by a firm sheath. Multiplication occurs both by non-motile conidia and by motile swarm cells, the latter with a subpolar tuft of flagella. Gram-negative so far as known. Found in fresh water.
The systematic positions of the species placed in Sphaerotilus, Leptothrix and related ge- nera are uncertain. Pringsheim (Phil. Trans. Roy. Soc. London, Series B, 233, 1949, 605, and Biol. Reviews, 24, 1949, 200) would combine some of the species now placed in Lepto- thrix with Sphaerotilus nutans and broaden the definition of Sphaerotilus to include other species here placed in Leptothrix and Clonothrix. However, Beger and Bringmann (Zent. f. Bakt., II Abt., 107, 1953, 318) indicate differences in the structures of the sheaths of Sphaero- tilus and Leptothrix and give other reasons why it may be better to keep the earlier groupings as they have been.
The type species is Sphaerotilus natans Kützing.
1. Sphaerotilus natans Kützing, 1833. (Kiitzing, Linnaea, 8, 1833, 385; not (Sp/jae?o- tilus natans Sack, Cent. f. Bakt., II Abt., 65, 1925, 116.) na'tans. L. part. adj. wafa/is swimming. Colorless, slimy trichomes which attain a length of several millimeters. The tri- chomes are ensheathed, show false branching and are either free-floating or attached at one end by means of a small disc. The indi- vidual cells are cylindrical, 1 by 2 to 6 mi- crons, and vacuolated (Lackey and Wattie, U. S. Pub. Health Ser., Pub. Health Repts., 55, 1940, 975). Multiplication occurs through the forma- tion of conidia within the sheath of the vegetative cells, from which thej' swarm out at one end, swim about for a time, then attach themselves to objects and develop into delicate trichomes. Gelatin rapidly liquefied, requires organic nitrogen, does not grow in the ordinary pep- tone solution, grows best with low concen- trations of meat extract (Zikes, Cent. f. Bakt., II Abt., 4S, 1915, 529). See Stokes (Jour. Bact., 67, 1954, 278) for a recent study of the cultural and physiological charac- teristics of this species, Distinctive characters: This species thrives in great tassels on solid substrata covered by dirty running water. These tas- sels are composed of trichomes of bacterial cells held together by slimy, tubular sheaths. The latter may become softened and dissolved, releasing Pseudomonas-like swarm cells. The same organism grows in a quite different state in quiet waters with only a little organic matter, forming branched structures occurring in trichomes, the sheaths of which are not slimy. A third form is produced when ferrous compounds and very little organic substance are pres-
