The genera
Cristispira,
Clevelandina,
Diplocalyx,
Hollandina, and
Pillotina are morphologically complex, Gram-negative, motile, spirochetes (helical bacteria) in which the flagella are always entirely
periplasmic (i.e., located between the inner “plasma” and the outer membrane typical of Gram-negative bacteria) (Fig. 1). (For a general discussion of the morphology of spirochetes, see Free-Living Saccharolytic Spirochetes: The Genus
Spirochaeta in this Volume). All are obligate symbionts in the digestive system of mollusks or arthropods. These morphologically complex
spirochetes have greater than 10 and sometimes as many as 300 flagella inserted at both ends of the cell and overlapping in
the middle. If n is the number of flagella at one end of the cell and 2n the number of overlapping flagella in the middle
of the cell, then the characteristic array is n:2n:n (e.g., 10:20:10 or 300:600:300). The coated membranes, distinctive cytoplasmic
structures (including the sillon, a cell-length invagination or groove of the outer membrane in contact with the inner membrane),
and relative proportions that distinguish these genera are depicted in Fig. 2, based on the morphometric analyses summarized in Table 1.
The habitats of these organisms are predictable (e.g., the crystalline style of bivalve mollusks for Cristispira and the intestine of dry wood-eating cockroaches and termites for the others). None has been grown axenically. As molecular
biological data are not yet available, species have been determined morphologically. Five species of large, symbiotic spirochetes
have been described in the modern bacteriological literature and reverified, revised, or named as: Clevelandina reticulitermitidis, Cristispira pectinis, Diplocalyx calotermitidis, Hollandina pterotermitidis, and Pillotina calotermitidis. (For genera description, including an explanation of the morphometric analysis of spirochetes, see Bermudes et al., 1988.)
Although often classified on the basis of size and light microscopic morphology in the family Spirochaetaceae (e.g., Bermudes et al., 1988), these spirochetes are ultrastructurally distinct from all other members of the Spirochaetaceae (see Free-Living Saccharolytic
Spirochetes: The Genus Spirochaeta in this Volume). Therefore, we classify the large symbiotic spirochetes in the family Pillotinaceae. (This family was first
suggested by Hollande and Gharagozlou, 1967, who used the incorrect Latin derivative “Pillotaceae.”) The spirochetes most similar morphologically to any member of the
Pillotinaceae are the tick-borne symbionts, e.g., Borrelia persica, (flagella formula 25:50:25) (Karimi et al., 1979); the free-living microbial mat spirochete Mobilifilum chasei (10:20:10) (Margulis et al., 1990a); and the pectinolytic rumen spirochete Treponema saccharophilum strain PB, which is reported to have approximately 32 flagella (whether 32:64:32 or 16:32:16 is not clear) (Paster and Canale-Parola, 1985). All these spirochetes are significantly smaller than any member of the Pillotinaceae. No free-living Pillotina-like spirochete has ever been reported—none, at least, larger than 0.5 µm in diameter bearing at least 30 flagella.
A summary of the characteristics of the large, symbiotic spirochetes is presented in Table 1. Although only five genera are in the formal taxonomic literature, hundreds of symbiotic spirochetes from a large number
of animals have been reported. For a more extensive discourse on Cristispira species in a variety of mollusks, see Kuhn (1981) and Breznak (1984a). Large spirochetes in the hindguts of termites and wood-eating cockroaches are more thoroughly described by To et al. (1980).
Cristispira pectinis (Gross) was first described as the trypanosome Trypanosoma balbiani (a eukaryote) by Certes (1882). Thought to have a multicellular, chambered body, Cristispira was renamed by Gross (1910), the name Cristispira being derived from the unusually prominent flagellar bundle or “crest.” The modern understanding of the organism comes from
Noguchi (1921), who, detecting them in oysters (Crassostrea), clams (Venus), and mussels (Modiola), recognized Cristispira as a spirochete bacterium. This identification was confirmed when the crest was shown to be a flagellar bundle by Ryter and Pillot (1965).
Leidy (1850) first described spiral-shaped microbes from termite intestines and called them Spirillum undula, later renaming them Vibrio termites (Leidy, 1881). Dobell (1912) recognized these organisms as spirochetes; he called the larger ones Treponema termites and the smaller ones Treponema minor. The larger spirochetes were transferred to the genus Cristipira by Hollande (1922). They were further described by Damon (1925), Duboscq and Grassé (1929), and Kirby (1941). In an early electron microscopic study, Grimstone (1967) described a large unnamed spirochete from the wood-eating cockroach Cryptocercus punctulatus. Electron microscopy and the use of glutaraldehyde as a fixative reinvigorated the study of the termite hindgut microbiota
by providing a means of distinguishing morphologically the uncultivable symbiotic bacteria. After the genus Pillotina and the family Pillotinaceae were proposed by Hollande and Gharagozlou (1967) for large spirochetes from the Madeiran termite Incisitermes praecox, large insect gut spirochetes were informally called “pillotinas.” The discovery of a separate spirochete morphotype from
the termite Incisitermes flavicollis led Gharagozlou (1968) to establish the genus Diplocalyx. To et al. (1978) described a third genus, Hollandina, from the hindgut of the Sonoran desert termite Pterotermes occidentis. Known informally from the micrographs of D. G. Chase, Clevelandina was fixed in the microbiological literature by Bermudes et al. (1988).