Physiological significance of the peptidoglycan hydrolase, LytM, in Staphylococcus aureus.

Autolysins in bacteria are peptidoglycan hydrolases with roles in growth, turnover and cell lysis. LytM was identified as the only autolysin in a previously reported autolysis-deficient (lyt(-) ) strain of Staphylococcus aureus. Purified LytM has been studied in great detail for its lytic properties and its production is elevated in vancomycin-resistant S. aureus. However, the postulated roles of LytM in S. aureus are largely speculative. Studies utilizing a reporter strain where the lytM promoter was cloned in front of a promoterless lacZ gene and fused in S. aureus strain SH1000 suggest that the expression of lytM is the highest during the early exponential phase. Additionally, lytM expression was downregulated in agr(-) mutants. The expression of lytM was not affected by the presence of cell wall inhibitors in the growth medium. To further determine the significance of LytM in staphylococcal autolysis, the gene encoding LytM was deleted by site-directed mutagenesis. The deletion of lytM, however, did not alter the rate of staphylococcal cell autolysis. Surprisingly, when the lytM mutation was combined with the lyt(-) mutant, the lytic activity band of the lyt(-) strain was still apparent in the lytM:lyt(-) double mutant. Purified full-length His-tagged LytM did not demonstrate any lytic activity against S. aureus cells. Surprisingly, cultures of S. aureus lytM deletion mutant lysed at a significantly faster rate compared with the wild-type S. aureus in the presence of oxacillin. The findings of this study raise questions about LytM as an autolysin and the significance of this protein should thus be investigated beyond its role as an autolysin.