Effects of Ca2+ ions on survival, growth, and cell size in Serratia marcescens SMG40 exposed to mecillinam.

Serratia marcescens strain SMG40 is sensitive to mecillinam killing at concentrations approaching those required for rod-to-sphere conversion. After a 1 h exposure to mecillinam, the type of response was found to depend on the conditions of post-incubation. Whereas they lysed soon after transfer to a tenfold-diluted broth, treated cells stopped dividing and enlarged impressively without immediate lysis when they were transferred to a complete low-agar medium and incubated at 30 degrees C. More remarkably, the presence of Ca2+ cations in any post-incubation medium definitely protected treated cells from lysis, enabled them to divide as spheres, and prevented enlargement. These survival-promoting and size-limiting effects were reversible upon elimination of Ca2+ from the medium. They were observable at far lower ionic concentrations than those required for osmotic stabilization. The growth rate depended on the Ca2+ concentration (from 10 microM) and on the presence of other supplements. Na+ cations also exerted a protective effect but acted differently from Ca2+ ions; the effects of Na+ and Ca2+ ions appeared as synergistic. A physiological activity of calcium is suggested for this bacterial strain, resulting in a RodA-like mecillinam resistant phenotype.