In 1971, Strength and Krieg reported the isolation of a gram-negative freshwater rod which exhibited bipolar flagellar fascicles clearly visible by dark-field microscopy. The flagellar fascicles exhibited helical wave propagation, basal bending, and an ability to coil up like springs. Despite the flagellar activity, the cells were apparently unable to swim freely. Such organisms appeared to be similar morphologically to an organism previously described by Houwink in 1953 and Jarosch in 1969. The present report describes a reliable isolation method for such organisms based on the use of -proline and semisolid agar. Upon isolation, the organisms grew in flocs, from which a highly viscous matrix could be separated by high-speed centrifugation. After many transfers, the growth gradually became homogeneous and turbid, and the viscous substance could no longer be demonstrated. Under certain conditions of growth, steady straight-line motility could be observed and photographed within viscous flocs. Straight-line, free-swimming motility occurred in viscous suspensions of cells prepared by homogenization of flocs. In 8- to 12-h-old cultures in the nonviscous homogeneous condition, some cells could swim slowly in irregular, circular paths; other could move about on surfaces. When the viscosity of the medium was increased, nearly every cell could swim freely and steadily in straight paths. A viscosity of 200 centipoise was optimal for strain XI, whereas 10 centipoise was optimal for strains X and XII. These results suggest that the organisms may be highly adapted to life within viscous flocs. The organism exhibited nitrogenase activity when tested by methods developed by Döbereiner and her colleagues for “ also was found to possess nitrogenase activity. Investigation of the physiology and deoxyribonucleic acid base composition of strains X, XI, and XII has indicated that even though the organisms are straight rods, they are probably members of the genus Important taxonomic considerations include: Coccoid body or “microcyst” formation, possession of a “polar membrane” similar to that occurring in certain spirilla, bipolar tufts of flagella, a strictly respiratory metabolism, inability to attack carbohydrates, positive catalase and oxidase reactions, and a deoxyribonucleic acid base composition of 62 to 65 mol% guanine plus cytosine. The organisms were assigned to a new species, , and the type strain was deposited with the American Type Culture Collection under the number 27740.


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