Streptococcus pneumoniae proteins released into medium upon inhibition of cell wall biosynthesis

Abstract

Inhibition of murein biosynthesis in Streptococcus pneumoniae by either penicillin or bacitracin leads to an increase in the amount of protein secreted into the medium. This process was studied in wild-type cells grown under lysis-permissive conditions as well as in an autolysin-deficient mutant. The time course of secretion did not follow cellular lysis but commenced immediately after the addition of the cell wall inhibitor in a manner similar to that described recently for cell wall and membrane components in various tolerant streptococci. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that this increase was not due to the stimulation of release of three protein components which are secreted under normal growth conditions; rather, a complex set of cellular proteins escaped from the antibiotic-treated pneumococci. The proteins released during bacitracin treatment was slightly different from those observed when penicillin was used. Analysis on sucrose gradients indicated that the secreted proteins were membrane bound rather than soluble. Membrane vesicles could indeed be detected by electron microscopy of negative-stained secreted material.