Cell wall deficiency and its effect on methicillin heteroresistance in Staphylococcus aureus

Clinical strains of Staphylococcus aureus with different phenotypic methicillin susceptibility characteristics, bearing or lacking the mecA gene, were tested for their ability to transform into a cell wall-deficient state under special conditions of cultivation. Conversion to L-form growth with formation of typical L-form ‘fried egg’ colonies and expression of oxacillin resistance was observed in sensitive (mecA-negative) and heteroresistant (mecA-positive) strains. Transmission electron microscopy observation of these strains revealed pleomorphic populations of cell wall-deficient cells with ultrastructure morphology similar to that of a control stable L-form strain of S. aureus. The results demonstrate that expression of phenotypic methicillin resistance could be associated with cell wall deficiency in S. aureus strains and could underlie the phenomenon of heteroresistance.