Optical configuration of staphylococcal cell wall serine.

Studies on the amino acid content of the cell wall of gram-positive bacteria have shown them to be composed of relatively few amino acids. One of the principal amino acids found in the cell wall of certain staphylococci is serine (1, 2, 5, 6). Although the optical configuration of staphylococcal cell wall serine has not been determined, Whitney and Grula (8) found that Micrococcus lysodeikticus grown in the presence of radioactive D-serine incorporated labeled serine in the peptidoglycan. Harney et al. (4), in studies on the cell wall of strains of Leuconostoc mesenteroides, have reported the presence of L-serine. Our interest in the nature of the configuration of the serine in staphylococcal cell wall developed as a result of the marked effect the serine content of the wall has on the lysis of strains of Staphylococcus epidermidis by lysostaphin (H. P. Browder et al., Bacteriol Proc. p. 14, 1966). Increasing amounts of cell wall serine resulted in a decrease in susceptibility to this lytic agent. The strains of S. epidermidis and the S. aureus 209P used in these studies were obtained from V. T. Schuhardt of the University of Texas. The Smith, K6, Weldwood, and Adams strains of S. aureus were from the collection of D. E. Rogers of Vanderbilt University, and the S. albus 12228 is an American Type Culture Collection strain. Staphylococcal cell walls were prepared from late log-phase cultures as previously described (2). Trypticase Soy Broth (Baltimore Biological Laboratory) was used as the source of nutrient for all of the staphylococci with the exception of S. epidermidis 115. Poor initial growth on Trypticase Soy Broth (TSB) prompted us to grow this organism on Micro Inoculum Broth (Difco). Following adaptation of the culture to TSB, cell wall from strain 115 was also prepared by using the TSB medium. Hydrolysates of the lyophilized cell walls were prepared by suspending the wall (ca. 10 mg/ml) in 6 N HCI, sealing the tubes evacuated to < 100 mm Hg, and heating for 20 hr in an oil bath at 110 C. Tube contents were filtered and taken to dryness over KOH in vacuo. The hydrolysates were dissolved in distilled water and assayed for serine microbiologically and on the Beckman Spinco amino acid analyzer. To assess the loss of serine under the conditions of hydrolysis, seryl-glycine and glycyl-serine were hydrolyzed with S. aureus Copenhagen cell wall (contains no serine) under the described conditions. The resulting hydrolysate was assayed for serine on the amino acid analyzer with a serine recovery of 86.2%. The reported results