L-form generation in MT01 and its cls mutants. MT01: open squares; cls1 mutant: open triangles; cls2 mutant: filled squares; cls1/cls2 double mutant: filled triangles. L-forms are 'fried-egg-shaped' colonies that appear after prolonged incubation with cell-wall perturbing antimicrobials. The L-form has no cell wall, which we confirmed by disruption at low osmotic pressure. The means of at least two independent determinations are shown.