Factors influencing the activity of the trometamol salt of fosfomycin.

The antibacterial activity of trometamol fosfomycin and its potentiation by glucose-6-phosphate were found to vary considerably in different culture media, although not all bacteria tested were equally affected. In general, the lowest MICs were obtained in the presence of glucose-6-phosphate in nutrient broth and Eugonbroth. The influence of inoculum size, pH and potentiation by glucose-6-phosphate were investigated by agar incorporation MIC titrations on nutrient agar. The activity of trometamol fosfomycin against many strains increased as the pH was lowered from 7.9 to 5.5. Only 9% of the strains showed an inoculum effect when tested at pH 5.5 in the presence of glucose-6-phosphate compared with 37% of the strains tested at pH 7.1 and 22% of the strains tested at pH 7.9. In the absence of glucose-6-phosphate, about 10% of the strains showed an inoculum effect at all three pH levels. The addition of glucose-6-phosphate to culture media seems reasonable for the testing of fosfomycin susceptibility.