In vitro development of resistance to fosfomycin.

Resistant colonies develop with an apparent high frequency in zones of inhibition around fosfomycin filter discs on nutrient agar. The resistance is due, primarily, to loss of the fosfomycin transport system. The resistant colony type usually observed in the inhibition zones seldom arise directly by mutation from a cell sown in the area of the zone of inhibition. Instead, small translucent colonies develop first in the inhibition zone. Within these small translucent colonies, mutational events occur which give rise to the normal resistant type colonies.