The structure and occurrence of pili on were investigated with the electron microscope. Piliation depended on the growth of the organism. The number and length of pili reached a maximum in the logarithmic phase and then declined during the stationary phase. A change in colonial morphology from a dry to a moist colony was correlated with the decline from maximum piliation. The distribution of pili as mono- or bipolar was not a function of the growth phase of the culture. Moreover, the number of pili per cell was not uniform within the culture during the growth cycle. Pili, observed by negative staining, had mean diameters of 6·0±2·8 nm. The wide variation of diameter on individual filament was thought to be due to the flexible nature of the pili. This interpretation was supported by the observation that filaments were capable of extreme coiling and bending. When negatively stained the pili did not appear hollow and seemed to differ from the rigid tube-like structure of type I pili (observed in ) by being flexible and rod-like.


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