STRUCTURAL-ANALYSIS OF THE LEPTOSPIRACEAE AND BORRELIA-BURGDORFERI BYHIGH-VOLTAGE ELECTRON-MICROSCOPY

Spirochetes are an evolutionary and structurally unique group of bacte ria. Outermost is a membrane sheath (OS), and within this sheath are t he protoplasmic cell cylinder (PC) and periplasmic flagella (PFs), The PFs are attached at each end of the PC and, depending on the species, may or may not overlap in the center of the cell, The precise locatio n of the PFs within the spirochetal cells is unknown. The PFs could li e along the cell axis, Alternatively, the PFs could wrap around the PC in either a right- or a left-handed sense. To understand the factors that cause the PFs to influence cell shape and allow the cells to swim , we determined the precise location of the PFs in the Leptospiraceae (Leptonema illini) and Borrelia burgdorferi. Our approach was to use h igh-voltage electron microscopy and analyze the three-dimensional imag es obtained from thick sections of embedded cells. We found that a sin gle PF in L. illini is located in a central channel 29 nm in diameter running along the helix axis of the right-handed PC, The presence of t he PFs is associated with the end being hook shaped, The results obtai ned agree with the current model of Leptospiraceae motility, In B. bur gdorferi, which forms a flattened wave, the relationship between the P Fs and the PC is more complicated, A multistrand ridge 67 nm in diamet er, which was shown to be composed of PFs by cross-sectional and mutan t analysis, vias found to extend along the entire length of the cell, We found that the PFs wrapped around the PC in a right-handed sense. H owever, the PFs formed a left-handed helix in space. The wavelength of the cell body and the helix pitch of the PFs were found to be identic al (2.83 mu m) The results obtained were used to propose a model of B. burgdorferi motility whereby backward-propagating waves, which gyrate counterclockwise as viewed from the back of the cell, are generated b y the counterclockwise rotation of the internal PFs, Concomitant with this motion, the cell is believed to rotate clockwise about the body a xis as shown for the Leptospiraceae.