Page:Bergey's manual of determinative bacteriology.djvu/876

 ORDER VIII. MYXOBACTERALES JAHN, 1915.

Myx.o.bac.te.ra'les. M.L. mas.n. Myxobacter name of the genus first described by Thaxter; -ales ending to denote an order; M.L. fern. pi. n. Myxobacterales the order based upon the type genus, Myxobacter.

Common or trivial name. The myxobacters.

Brief characterization of the order. The vegetative cells are flexible rods of low refractility which exhibit gliding movement on solid surfaces and which multiply by binary, transverse fission to produce a thin, flat, rapidly extending colony. Actively motile cells at the periphery of the colony commonly occur as groups of 2 or 3 to several hundred individuals in the form of tongue-like extensions or isolated islands whose presence is virtually diagnostic of the order. The moving cells may pave the substrate with a thin layer of slime on which they rest.

Resting cells are formed by all myxobacters except members of the genus Cyiophaga. In the family Myxococcaceae the resting cell is a spherical or oval body, thick-walled and highly refractile; in the remaining groups it is merely a shortened vegetative cell. Except in the genus Sporocytophaga, resting cells are borne in or on spatially localized, larger structures known as fruiting bodies. In the simplest case, the fruiting body consists of a uniform mass of resting cells held together by slime. Some groups produce more complex fruitingstructures: the resting cells may be enclosed in cysts and may be raised above the substrate on stalks, either simple or branched. Fruiting bodies are usually brightly colored and often sufficiently large to be visible to the naked eye.

The vegetative state. In the vegetative condition, m3'xobacters consist of unicellular rods which occur in two characteristic shapes. Members of the iamily Sorangiaceae, together with some representatives of the families Archangiaceae and Polyangiaceae, have cylindrical vegetative cells with blunt, rounded ends; in extreme cases the cell is broader at the tips than at the center. All other myxobacters, with the possible exception of some Cytophaga species, have vegetative cells which taper towards the tips. The cells are not surrounded by a demonstrable wall and, perhaps as a consequence, are flexible and very weakly refractile. Division is always by binary, transverse fission. Motility is universal. Movement occurs only in contact with a solid surface and is of the gliding type also found in the Cyanophyta and in filamentous, colorless organisms such as the Beggiatoaceae and Vitreoscillaceae. There are no demonstrable means of locomotion, and the actual physical mechanism of movement is not understood, although many authors have offered speculations, which were recently reviewed by Meyer-Pietschmann (Arch. Mikrobiol., 16, 1951, 163). Discrete nuclear structures similar to those of true bacteria can be demonstrated by appropriate cytological procedures. Their appearance and behavior during division and formation of resting cells are described and illustrated by Badian (Acta Soc. Bot. Poloniae, 7, 1930, 55), Krzemieniew-