SummarySpiroplasma melliferum BC3 are wall-less bacteria with internal cytoskeletons. Spiroplasma , Mycoplasma and Acholeplasma belong to the Mollicutes , which represent the smallest, simplest and minimal free-living and self-replicating forms of life. The Mollicutes are motile and chemotactic. Spiroplasma cells are, in addition, helical in shape. Based on data merging, obtained by video dark-field light microscopy of live, swimming helical Spiroplasma cells and by cryoelectron microscopy, unravelling the subcellular structure and molecular organization of the cytoskeleton, we propose a functional model in which the cytoskeleton also acts as a bacterial linear motor enabling and controlling both dynamic helicity and swimming. The cytoskeleton is a flat, monolayered ribbon constructed from seven contractile fibrils (generators) capable of changing their length differentially in a co-ordinated manner. The individual, flat, paired fibrils can be viewed as chains of tetramers ª ª ª ª 100 Å in diameter composed of 59 kDa monomers. The cytoskeletal ribbon is attached to the inner surface of the cell membrane (but is not an integral part of it) and follows the shortest helical line on the coiled cellular tube. We show that Spiroplasma cells can be regarded, at least in some states, as near-perfect dynamic helical tubes. Thus, the analysis of experimental data is reduced to a geometrical problem.

A consistent small-perturbation theory for the aligned-fields, steady, magnetogasdynamic two-dimensional and axisymmetric flow of an inviscid conductor in the sonic, Alfvénic, and hypercritical regimes is developed, and approximate shock relations derived by a simple method. This results in nonlinear relatively simple systems of equations and shock relations for mixed flows around bodies in the transition regimes. It is found that transonic magnetogasdynamic problems can be reduced to gasdynamic ones; each gasdynamic transonic flow around a given body implies an infinite number of magnetogasdynamic patterns around the same body for all values of Alfvén numbers different from zero and one, and each of these, in turn, yields another family of flows around similar bodies, according to a modified von Kármán similarity rule. The hypercritical and transonic systems are mathematically similar and obey the same similarity rule. An explicit correspondence between specific transonic and hypercritical flows does not exist, but transonic arguments and methods can be used to infer qualitative properties of hypercritical flows and to obtain exact solutions of hypercritical flow patterns.

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