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 L-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 XU. 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 Dobereiner and her colleagues for "Spirillum" lipoferum; Aquaspirillum peregrinum also was found to possess nitrogenase activity. Investigation of the physiology and deoxyribonucleic acid base composition of strains X, XI, and XI1 has indicated that even though the organisms are straight rods, they are probably members of the genus Aquaspirillum. 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, Aquaspirillum fasciculus, and the type strain was deposited with the American Type Culture Collection under the number 27740.In 1953, Houwink (11) described a "rod-like" bacterium which exhibited a "floundering" rather than a swimming type of motility. In resting specimens, both bipolar flagellar fascicles had the form of a coiled spring. The cells seldom moved more than a cell length, but in Present address:

The bipolar flagellar fascicles of a gram-negative, floc-forming, aquatic rod were studied. Each fascicle consisted of up to 11 individual flagella of 19-nm diameter with separate insertion sites in the cell wall. Fascicles exhibited three modes of behavior as illustrated by darkfield cinematography, (a) Extended helical fascicles exhibited wave propagation from base to tip at varying speeds with temporary stopping. (b) A fascicle could be retracted into the form of a polar coil as if it were a spring, the process beginning at the distal end and proceeding proximally. Fascicles could be reextended by reversal of this process. (c) An extended fascicle could bend at its base until it lay adjacent to the cell wall. This was preceded by an increase in wavelength and amplitude at the distal end which progressively affected the entire fascicle. After bending, a fascicle could swing back out again into the extended configuration. Despite the intense activity of the flagellar fascicles, the cells were found to be unable to swim about. Several possible explanations for this lack of motility are discussed. The unusual features of the flagellar activity are related to current concepts of the mechanisms of flagellar motion.