The Genus Toxothrix

Abstract

The first description of a bacterium that produced twisted bundles of thin filaments which contained oxidized iron was given by Cholodny (1924). He called this organism Leptothrix trichogenes, the filament-producing Leptothrix. The flexible bacterium was observed in a water basin next to the river Dnjepr near Kiev, Russia. Its cells (diameter approximately 0.5 �m) were connected to form a trichome of up to 400 �m in length. A peculiar movement was observed: the U-shaped trichome glided with its rounded part forward; both ends of the trichome left bundles of twisted filaments of polymer as parallel “railroad tracks.” Occasionally, parts of the bundles appeared to be drawn out in a fan-shaped fashion (Figs. 1a, 2). Although this first description was very accurate, as we know now, later scientists failed to notice the presence and importance of the flexible, gliding bacterium (trichome); they only observed the often rigid, brittle, iron-encrusted bundles of filaments. Consequently, these filaments were interpreted to be alive, to comprise the organism. It appears strange that the producer of these structures did not attract greater interest. The explanation for the failure to recognize the true nature of Toxothrix came from Krul, Hirsch, and Staley (1970), who employed a partially immersed, phase-contrast microscope to study the formation of such filament bundles in an iron spring in Michigan. The bacterial trichome was observed to consist of up to 40 rods, each of 0.5–0.75 � 3–6 �m, with a total length of up to 240 �m. The forward movement of the often U-shaped trichome consisted of symmetrical rotation of both trichome ends, which thereby left a track on their attachment surface that consisted of twisted polymer fibers. Occasionally, the torque on the rounded center part of the U was released by an upward twist followed by a downward “printing” of polymer onto the surface of the glass slide. Later these polymer fibers became encrusted with iron oxide. Attempts to study the flexible filaments in the laboratory resulted in an explosive lysis that occurred within a few minutes after the living preparation had been made (Fig. 1b). Thus the absence of trichomes in samples of many earlier observers could be explained (Krul, Hirsch, and Staley, 1970). The fact that the bundle of twisted fibers was different from the “true” sheaths produced by other Leptothrix spp. caused Molisch (1925) to name this organism Toxothrix ferruginea (toxon, Greek noun, a bow). Later, Beger and Bringmann (1953) changed the name partially back into Toxothrix trichogenes, a more proper name (Hirsch and Zavarzin, 1974).