Tolerance in Staphylococcus aureus: Evidence for bacteriophage role

The conversion of nontolerant Staphyloccus aureus to a tolerant organism is reported. It had been shown previously that a nontolerant strain produces tolerant progeny when it is incubated in media that contain a bacteria-free filtrate derived from cultures of each of the three tolerant strains of S. aureus. The tolerant progeny retain the characteristic tolerance upon serial subculture, and cell-free filtrates prepared from such subcultures are capable of converting nontolerant organisms to tolerant bacteria. A study of filtrates made from one of the tolerant strains revealed that all of the converting activity was sedimented by ultracentrifugation. DNase I and RNase A did not reduce the activity of the sediment, but Proteinase K completely diminished it. Equilibrium centrifugation of filtrates in cesium chloride showed that the conversion activity was maximal in the fraction with a density of approximately 1.445±0.015 g/ml. When a portion of this fraction was examined with the electron microscope, hexagonal bacteriophages (20 nm in diameter) were seen at this density. This observation seems to be evidence that a bacteriophage is involved in conversion of S. aureus from a nontolerant to a tolerant organism.