We investigated the supramolecular structure of the SHIGELLA: type III secretion machinery including its major components. Our results indicated that the machinery was composed of needle and basal parts with respective lengths of 45.4 +/- 3.3 and 31.6 +/- 0.3 nm, and contained MxiD, MxiG, MxiJ and MxiH. spa47, encoding a putative F(1)-type ATPase, was required for the secretion of effector proteins via the type III system and was involved in the formation of the needle. The spa47 mutant produced a defective, needle-less type III structure, which contained MxiD, MxiG and MxiJ but not MxiH. The mxiH mutant produced a defective type III structure lacking the needle and failed to secrete effector proteins. Upon overexpression of MxiH in the mxiH mutant, the bacteria produced type III structures with protruding dramatically long needles, and showed a remarkable increase in invasiveness. Our results suggest that MxiH is the major needle component of the type III machinery and is essential for delivery of the effector proteins, and that the level of MxiH affects the length of the needle.