Abstract
SUMMARY: Alteration of the tonicity of a modified Edward medium by means of NaCl or other solutes resulted in very marked effects on the growth of several Mycoplasma strains. With both fluid and solid media, all grew best at about 10 atmospheres osmotic pressure (water activity, aw, 0.9925). The most exacting species, Mycoplasma gallisepticum, failed to grow outside the range 6.8-14 atmospheres (aw 0.995-0.990), while the least exacting, M. laidlawii, multiplied at up to 27 atmospheres (aw 0.980. Osmotic requirements were not appreciably altered by serial growth in hypertonic or hypotonic media but were, to some extent, conditioned by the serum content of the medium.