Abstract
Serratia marcescens was easily induced to form spheroplasts by beta-lactam antibiotics in the presence of Ca2+ or Mg2+ without an osmotic stabilizer such as sucrose. The spheroplasts grew in volume, although they could not divide. They were stable for more than 10 h at 37 degrees C in a medium containing a high concentration of antibiotic, and they had the ability to revert to the original bacillary form. Ca2+ was more effective in spheroplast induction than Mg2+. The effect was proportional to the concentration of cations. In 40% of 180 clinical isolates of S. marcescens, more than 40% of the original bacterial cells were induced to form spheroplasts by ceftizoxime in a medium supplemented with 40 mM Ca2+. A high spheroplast induction rate was observed even in medium with 10 mM Ca2+. Few isolates that were supersusceptible to ceftizoxime (MIC, less than 0.2 microgram/ml) were induced to form spheroplasts at a high rate. No difference in spheroplast induction rate or extent between antibiotic-resistant strains and relatively susceptible strains (MIC, greater than 0.2 microgram/ml) was found. The serotype of S. marcescens had no effect on the spheroplast induction rate. Monocations (Na+ and K+) had little effect on spheroplast induction.