A study was made in liquid medium over 7 days at 37° C. of the inhibitory action of nineteen antibacterial, antifungal and antiprotozoal drugs on twenty strains ofM. gallisepticum, eight other avian mycoplasmata, six mammalian mycoplasmata, two saprophytic mycoplasmata and the L-form ofStreptobacillus moniliformis(L–1).The twenty strains ofM. gallisepticumfrom Britain and other countries showed a similar range of drug sensitivity except where resistant strains were included. Tylosin and demethylchlortetracycline appeared to have the highest inhibitory action, followed by erythromycin, spiramycin, tetracycline, chlortetracycline, oxytetracycline and ethidium bromide. A ‘Zone Phenomenon’ frequently occurred with sodium aurothiomalate, inhibition often being observed between 0·1 and 2·0 μg./ml. Polymixin and nystatin had no inhibitory effect on the growth of any mycoplasmata tested. With the exception of erythromycin and streptomycin in some cases, the pattern of sensitivity observed with the mycoplasmata of diverse origin was similar to that ofM. gallisepticum, most strains, however, being somewhat more resistant thanM. gallisepticumto many of the drugs.

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