Construction of a penicillin-tolerant laboratory mutant of Staphylococcus aureus

Numerous conflicting reports describing clinical penicillin tolerant Staphylococcus aureus isolates have raised questions concerning the reproducibility and genetic homogeneity of such isolates. The purpose of our study was to construct a tolerant mutant which would then serve as a reference strain and could also be used for testing the potential significance of antibiotic tolerance during chemotherapy of staphylococcal infections. A penicillin sensitive laboratory mutant strain was mutagenized and exposed to cycles of penicillin selection. A culture was then obtained which lost about 1 log of viability during exposure to up to 1000 times the MIC of penicillin. The same conditions caused a 3-4 log decline in the viability of the parent cell. The tolerance property was stable for at least 150 generations of nonselective growth and was reproducible in randomly picked colonies. The superior survival during treatment was also demonstrable in cultures grown in rabbit serum or cerebrospinal fluid. Penicillin tolerant cultures could also be obtained by selection without mutagenesis, suggesting that such a mutant may indeed be selected for in the clinical environment.