The degree of cross-linking of the peotidoglycan of Staphylococcus aureus H and mutants lacking penicillin-binding proteins 1 and 4 was studies. No major changes were ovserved in organisms lacking protein 1 whereas oloss of protein 4 was accompanied by a marked reduction in the degree of cross-linked and the absence of a membrane-bound ‘model’ transpeptidase activity. A smimlar effect was achieved when cultures of the staphylococci were treated with the β-lactam antibiotic cefoxitin. At low concentrations (0.05μg ml-1) cefoxition shows highest affinity for protein 4 to which it appears to bind irreversibly. Treatment of the mutant lacking protein 4 with this concentration of the antibiotic did not affect the degree of cross-linkage.
The possibility that the decrease in cross-linkage was a consequence of DD-carboxypeptidase activity on peptidoglycan precursors was investigated. Although both S. aureus H and the mutants possessed such activity it was insensitive to benzylpenicillin and cefoxition and the role of this enzyme(s) in peptidoglycan biosynthesis remains unknown.
We conclude that in vivo protein 4 acts as a transpeptidase involved in the secondary cross-linking of peptidoglycan and this activity is necessary to achieve the high degree of cross-linkage observed in the peptidoglycan of staphylococci.