Fosfomycin trometamol: activity in vitro against urinary tract pathogens.

The spectrum of activity of fosfomycin embraces all the common causes of uncomplicated urinary tract infection. The activity is greatly affected by the conditions of the test. Glucose, phosphates and NaCl all interfere with the activity of the drug, whereas glucose-6-phosphate has a marked potentiating effect against many strains. The activity of fosfomycin is greater at acid than at alkaline pH; inoculum density also has an effect, but this is less marked at acid pH values. Fosfomycin is rapidly bactericidal to susceptible bacteria, causing lysis within 30 min. In contrast, fosmidomycin, which also has a narrower spectrum of activity than fosfomycin, is much more slowly bactericidal. In the form of its trometamol salt, fosfomycin is well absorbed after oral administration, and is excreted in high concentration in the urine. Experiments in an in-vitro model of the treatment of bacterial cystitis suggest that concentrations of fosfomycin achievable in urine after oral administration of high doses of the trometamol salt have a marked suppressive effect on bacterial growth without favouring the emergence of resistant mutants.