[Investigations on the antibacterial activity of fosfomycin on DST-agar (author's transl)].

Since in some European countries the activity of antimicrobial substances are assessed on Disc-Sensitivity-Test (DST)-agar, routinely, it has been investigated whether and under which conditions Fosfomycin might be tested on DST-agar with the agar-dilution and -diffusion technique. A total of 320 strains representing 7 species were used as teststrains, and Mueller-Hinton (MH)-agar was employed as reference medium. On DST-agar the MIC-values were increased for some genera like Klebsiella by a factor of 128, a fact that was explained by the sixfold concentration of antagonistically acting Phosphate in DST-agar. Addition of glucose-6-phosphate (G-6-P) enhanced the sensitivity towards Fosfomycin only with some species. As the endpoint of growth is difficult to determine on DST-agar its use for agar-dilution tests with Fosfomycin can not be recommended. For agar-diffusion tests, however, this medium may be used provided that a thin film of growth or single colonies within the inhibition zones are being disregarded and that the disc content is increased from 50 microgram as being usual at present to 100 or 200 microgram. Diameters of inhibition zones that were obtained with such discs on DST-agar correlated very well (r = --0,89 und --0,93, respectively) with MIC-values derived from agar-dilution tests on MH-agar (+ G-6-P). With discs containing only 50 microgram no inhibition zones are formed by strains that should still be classified as "sensitive" (MIC: 8--32 microgram/ml).