The reshaping process of Klebsiella pneumoniae cells after removal of mecillinam, an antibiotic that causes transition from rod to coccal shape.

The process of bacterial morphogenesis that leads to rod shape formation was studied in synchronous cells during the reshaping process after removal of mecillinam, a beta-lactam antibiotic which, by specifically inhibiting lateral wall formation of rods, cause rod-to-sphere transition in Gram-negative rods. The addition of mecillinam for 50 min of the cell cycle made the cells to skip a division, while the addition of the antibiotic for 30 min (or less), allowed the cells to divide regularly. In order to study the interplay between lateral wall elongation and septum formation in reacquisition of rod shape, we evaluated the effect of re-adding mecillinam or adding piperacillin, a specific inhibitor of septum formation, at various stages of the reshaping process. It was found that mecillinam was active only when added within the first 30 min of the reshaping process, while piperacillin was active only after 30 min when the cells were close to starting to divide again. These findings provide further support for our previous proposal that, in bacterial rods, elongation and septation are two alternating and competing events of the cell cycle, and are linked to each other in such a way as to force bacterial rods to grow to a given length.