CELL ELONGATION AND SEPTATION ARE 2 MUTUALLY EXCLUSIVE PROCESSES IN ESCHERICHIA-COLI

Bacterial rod morphogenesis was studied in synchronously growing cells of Escherichia coli C600 during the reshaping process that follows th e removal of mecillinam, a beta-lactam antibiotic that specifically in hibits lateral wall formation of gram-negative rods and causes transit ion to coccal shape. Removal of mecillinam after 30 min of action did not affect the timing of subsequent cell division, but removal after 5 0 min delayed resumption of cell division for approximately one genera tion time. In order to study the interplay between lateral wall elonga tion and septum formation in determining and maintaining the bacterial rod shape, we evaluated the effect of re-adding mecillinam or of addi ng aztreonam (a specific inhibitor of septum formation) at various sta ges of the reshaping process. We conclude that mecillinam was active o nly during the reshaping process. while aztreonam was active only late r when the cells were close to dividing again. These results provide f urther evidence for our previous proposal according to which elongatio n and septation are two alternating and competing events of the cell c ycle and are linked to each other to force bacterial rods to grow to a given length.