cell shape is regulated in salmonella enterica serovar typhimurium by complex mechanisms which coordinate growth with cell division and maintain bacillar cell shape through the whole cell cycle by controlling synthesis and degradation of cell envelope macromolecules. by using mecillinam, a β-lactam antibiotic that kills sensitive bacteria by interfering with the mechanisms regulating cell shape, it was demonstrated that genes controlling cell shape have an essential role since their inactivation was found to be lethal. moreover, this lethality was conditional because it appeared only when growth proceeded on solid medium whereas growth in liquid medium was not affected. it was also found that many mutations which conferred resistance to mecillinam behaved as suppressors of that lethality. among the genes concerned with mecillinam action, several related with global regulatory systems (cysb/cyse, cya/crp, spot, lps, mucm) were identifed. it was also demonstrated that some of those genes not only modifed the response to the antibiotic but also shown complex interactions with functions (slt, opggh) related to the cell envelope.