Visualizing multiple constrictions in spheroidal Escherichia coli cells.

An Escherichia coli cell grows by elongation and divides in a perpendicular plane. Alternating planes of successive divisions in three dimensions can only be ascertained when multiple constrictions exist simultaneously in large, spheroidal cells (with extended constriction process), if the division signals are enhanced. Large, spheroidal cells are obtained by a brief mecillinam treatment, and more frequent divisions are achieved by manipulating the rate of chromosome replication without affecting cell mass growth rate. Such a procedure has recently been performed by thymine-limitation of E. coli K12 strain CR34 (Zaritsky et al., Microbiology 145 (1999), 1052-1022). Enhancing the replication rate in cells with multi-forked replicating chromosomes (by addition of deoxyguanosine) shortens the intervals between successive terminations and thus triggers divisions more frequently. Monoclonal antibodies against FtsZ were used to visualize the rings of secondary constrictions, but apparent shortage of FtsZ to complete rings over wide cells allowed assembly of arcs only. The arcs observed were not parallel nor perpendicular; the tilted constriction planes are consistent with our 3-D 'nucleoid segregation'model for division under conditions which relieve the cylindrical constraint for nucleoid segregation by the bacillari peptidoglycan sacculus (Woldringh et al. , J. Bacteriol. 176 (1994) 6030-6038). The shortage in FtsZ may explain the longer time required to complete the division process in wide cells with long circumferences, observed during thymine step-up. Overexpression of fusion protein FtsZ-GFP on a multi-copy plasmid should circumvent the shortage.