Spirochaetes are constructed in a far more complicated manner than ordinary bacteria. They are slender spiral cells that have an intrinsic active undulating motility although they lack ﬂagella. They may consist of regularly spaced tight coils or loose irregular spirals of varying amplitude. Some propel themselves with rapid lashing movements, others by slow twisting and bending. They differ in length from 4 to l4μm and in thickness from 0-1 to 0-6μm. Larger spirochaetes can easily be seen in stained ﬁlms but the very thin ones are barely detectable by this means and are best demonstrated by dark ﬁeld microscopy in wet preparations.
The protoplasmic core of these spiral cells is enclosed within a cell wall and an inner cytoplasmic membrane; between these two layers there are overlapping sets of ﬁne ﬁbrils which are anchored by knobs at the two poles of the organism. The number of ﬁbrils varies according to the genus from a single pair to six or more pairs. It is probable that the spiral shape of the organisms depends on the tension in the ﬁbrils for if they have been ruptured the coiled appearance is lost and the cell straightens out. The serpentine movement of spirochaetes may also depend on the integrity of the ﬁbres.
Spirochaetes belong to the order Spirochaetales which is divided into two familes:
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