Structural changes induced by subinhibitory concentrations of fosfomycin on Staphylococcus aureus and Bacillus cereus.

Exposure of Staphylococcus aureus ATCC 6538 and Bacillus cereus ATCC 11778 to 1/2 MIC fosfomycin for 3 and 6 h was studied by transmission and scanning electron microscopy. After 3 h, a loss of density and irregularity of shape in the septum was noted, while after 6 h a delayed separation of newly formed cells was observed in both species. In Staphylococcus aureus this resulted in the formation of chains of intact cells and cells in lysis. In Bacillus cereus, it produced filamentous forms with multiple septa. the differences between the effects observed at 3 h and 6 h may be due to time-related variations in cell permeability to fosfomycin, or to the existence of indirect mechanisms enabling fosfomycin to interfere with the enzymatic activity regulating the normal separation of cells, or to a combination of these factors.