Characterization of the presumed peptide cross-links in the soluble peptidoglycan fragments synthesized by protoplasts of Streptococcus faecalis.

Protoplasts of Streptococcus faecalis ATCC 9790 were produced with the aid of lysozyme, and the ability of these bodies to synthesize soluble, peptide cross-linked peptidoglycan (PG) fragments was examined. Lysozyme digests of PG isolated using gel filtration from the supernatant medium of protoplasts grown in the presence of [14C]acetate and L-[3H]lysine contained small amounts of PG having KD expected for peptide cross-linked dimers and trimers. Addition of benzyl penicillin (300 mug/ml) to growing protoplast cultures did not affect the net amount of PG fragments synthesized but resulted in inhibition of synthesis of dimer and trimer fractions by 27 and 59%, respectively. Failure of penicillin to completely inhibit the accumulation of the dimer fraction was attributed to the presence of atypical forms of dimer. In fact, the supernatant medium of penicillin-treated cultures did not contain detectable amounts of typical peptide cross-linked dimer. The degree of peptide cross-linkage of protoplast PG was at most only 13% of that found in walls isolated from intact streptococci. The relative amounts of monomers, dimers, and trimers synthesized during early and late stages of protoplast growth was approximately the same. Protoplasts synthesized soluble PG fragments in amounts which were of the same order of magnitude as that expected for insoluble PG produced by an equivalent amount of intact streptococci.