Fosfomycin and plasmidic resistance.

60 fosfomycin-resistant strains of gram-negative bacilli are submitted to conjugation experiments using as recipient cell E. coli K12 which is nalidixic acid resistant. After mating, the number of fosfomycin-resistant E. coli K2 colonies growing on selection plates containing nalidixic acid and fosfomycin never surpassed the normal rate of mutation for fosfomycin-resistance of the recipient strain. In 65% of the experiments, plasmidic resistance to other antimicrobials was transferred to E. coli K12, but was never accompanied by demonstrable fosfomycin resistance. High rate of normal mutation of recipient strain is signaled as the main problem for detecting the plasmidic nature of fosfomycin resistance. With our criteria regarding this fact we have been unable to confirm the plasmidic nature of fosfomycin resistance.