Free-Living Saccharolytic Spirochetes: The Genus Spirochaeta
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Free-Living Saccharolytic Spirochetes: The Genus Spirochaeta SUSAN LESCHINE, BRUCE J. PASTER AND ERCOLE CANALE-PAROLA
Introduction The genus Spirochaeta includes anaerobic and facultatively anaerobic spirochetes that are indigenous to aquatic environments such as the mud and water of ponds and marshes. These spirochetes occur in nature as free-living forms; that is, their existence does not depend on physical associations with other organisms (CanaleParola, 1984a). Spirochaeta cells are helically shaped (Fig. 1–12) and possess the typical ultrastructural features of spirochetes (Canale-Parola, 1984b; Fig. 13). The outermost structure of the cells is an “outer membrane,” or “outer sheath,” which encloses the coiled cell body (“protoplasmic cylinder”) consisting of the cytoplasm, the nuclear region, and the peptidoglycan-cytoplasmic membrane complex (Fig. 13). Organelles ultrastructurally similar to bacterial flagella are located in the area between the outer membrane and the protoplasmic cylinder (Fig. 13). These organelles are essential components of the motility apparatus of spirochetes (Paster and Canale-Parola, 1980) and are usually called “periplasmic flagella.” Other names used to designate these motility organelles are “periplasmic fibrils,” “axial fibrils,” “axial filaments,” and “endoflagella.” One end of each periplasmic flagellum is inserted near a pole of the protoplasmic cylinder, while the other end is not inserted (Fig. 13A). Individual periplasmic flagella extend for most of the length of Spirochaeta cells so that those inserted near opposite ends overlap in the central region of the organism (Fig. 13A). The Spirochaeta cell illustrated in Figure 13 has two overlapping periplasmic flagella, each inserted near a cell pole in a “1-2-1” arrangement. With one exception, all the known Spirochaeta species have two periplasmic flagella per cell. The exception is the large spirochete Spirochaeta plicatilis, which has as many as 18–20 periplasmic flagella inserted near each end of the protoplasmic cylinder (Blakemore and Canale-Parola, 1973). In contrast to flagella of other bacteria, the periplasmic flagella of spirochetes are perma-
nently wound around the cell body and are entirely endocellular (Fig. 13B). Thus, the motility mechanism of spirochetes is different from that of other bacteria, which have flagella that function in direct contact with the external environment and are not wound around the cell body.
Phylogeny Based on 16S rRNA gene sequence comparisons, most members of the genus Spirochaeta form one of the nine phylogenetic clusters of the spirochetes (Fig. 14). All spirochetes are presently classified in the class or phylum Spirochaetes in the order Spirochetales and are divided into three families. The first family, Spirochaetaceae, contains species of the genera Borrelia, Brevinema, Cristispira, Spirochaeta, “Spironema” and Treponema. The second proposed family, Serpulinaceae, contains the genus Brachyspira (Ser
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