A combination of light-microscopy and image processing was used to elaborate on the fluctuation in the width of the cylindrical part of Escherichia coli at sub-pixel-resolution, and under in vivo conditions. The mean-squared-width-difference along the axial direction of the cylindrical part of a number of bacteria was measured. The results reveal that the cylindrical part of Escherichia coli is composed of multi-domain morphological structures. The length of the domains starts at 150 nm in newborn cells, and linearly increases in length up to 300 nm in aged cells. The fluctuation in the local-cell-widths in each domain is less than the fluctuation of local-cell-widths between different domains. Local cell width correlations along the cell body occur on a length scale of less than 50 nm. This finding could be associated with the flexibility of the cell envelope in the radial versus longitudinal directions.