The Order Cytophagales
In the past decade much has been learnt about the order Cytophagales and about some of the organisms belonging to it, although the majority of its members are as unfamiliar as ever. On the basis of 16S rRNA studies, we can now delimit the group with some
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The Order Cytophagales HANS REICHENBACH
In the past decade much has been learnt about the order Cytophagales and about some of the organisms belonging to it, although the majority of its members are as unfamiliar as ever. On the basis of 16S rRNA studies, we can now delimit the group with some confidence and have a well-founded idea of its phylogenetic position (Paster et al., 1985; Woese et al., 1985). Accordingly, the Cytophagales appear to be distantly related to the Bacteroides group, and these two together comprise one of the main branches, perhaps a phylum, in the bacterial phylogenetic system. The substructure of the Cytophaga branch of the phylum is more difficult to evaluate. There is a main line on which we find unicellular gliders—Cytophaga (Cy.*) johnsonae, Cy. lytica, Cy. aquatilis = Flavobacterium (Fv.) aquatile, and Sporocytophaga (Sp.) myxococcoides—but at a lower level unicellular nonmotile bacteria (Fv. breve, i.e., low GC, true flavobacteria) are also found. At a still lower level, a cluster branches off which comprises the unicellular gliders—Flexibacter (Fx.) filiformis = Fx. elegans Fx el, Cy. heparina, and Taxeobacter = Myx 2105) unicellular nongliding flavobacteria (Fv. ferugineum), but also filamentous, multicellular, gliding (Saprospira) and nonmotile bacteria (Haliscomenobacter). It is obvious from these data that our present definition of genera does not reflect the phylogenetic situation and also that the grouping in families and perhaps orders needs to be reconsidered. Before that is done, however, 16S rRNA sequences of further species should be determined. From what has been said above it is clear that only a preliminary characterization of the order is possible at present. In this chapter, the order Cytophagales is restricted to unicellular gliding bacteria; the relevant genera are listed in Table 1. Filamentous, gliding bacteria of the genus Saprospira may or may not belong to the order, and they are sufficiently different to justify a separate treatment (see The Order Cytophagales in this Volume); contrary to earlier suggestions, these organisms are not apochlorotic cyanobacteria (Reichenbach et al., 1986). Unicellular, gliding bacteria of the genus Lysobacter have been found to be closely linked to the xanthomonads and thus belong to the gamma branch of the Proteobacteria (Woese et al., 1985); consequently they are discussed separately (see The Genus Lysobacter in Volume 6) It should be mentioned that, in the past, lysobacters have often erroneously been classified as cytophagas; this is discussed in The Genus Lysobacter in Volume 6. On the other In this chapter, the following abbreviations sometimes are used for the genera of the order Cytophagales: CLB, Cytophaga-like bacteria; Cp., Capnocytophaga; Cy., Cytophaga; Ft., Flexithrix; Fv., Flavobacterium; Fx., Flexibacter, Mc., Microscilla; Sa., Saprospira; Sp., Sporocytophaga; Tx., Taxeobacter. This chapter was taken unchanged from the second edition.
hand, the cytophagas have been regarded as myxobact
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