Serum inhibition of antibiotic-dependent L-form growth in Staphylococcus aureus: likely involvement of serum high-density lipoprotein.

The L-form growth of Staphylococcus aureus induced by ampicillin or fosfomycin was found to be severely depressed by horse or bovine serum included in the culture medium. Fractionation of horse serum revealed that the inhibitory activity was copurified with high-density lipoprotein through floating ultracentrifugation, gel filtration, and isoelectric focusing. It was also shown that the inhibition was reversible: ampicillin-induced L-form cells lost their ability to form L-form colonies when mixed with the lipoprotein fraction or serum before plating, but the colony-forming ability was restored to the original level by treatment of the mixture with a protease, while simple dilution was ineffective. A mechanism for the growth inhibition remains to be clarified.