SUMMARY: The growth of some strains of on solid medium was inhibited by rabbit antisera incorporated in filter-paper discs. Other strains were resistant to the same antisera. Rings of precipitate were observed in the agar around the discs. The largest number of precipitation rings and the most intense ones occurred with antiserum-sensitive strains. The antiserum-sensitive strains were also more sensitive than the resistant strains to colistin and polymyxin B when these antibiotics were incorporated in paper discs. Colonies of antiserum-resistant strains developed on solid medium containing low concentrations of horse serum, while colonies of sensitive strains developed only on solid medium containing higher concentrations of serum. The growth of antiserum-sensitive strains, but not resistant ones, was suppressed on solid medium at pH 6·5. All these differences between sensitive and resistant strains were only expressed on solid medium; the phenomena were not observed when the organisms were grown in liquid medium. It is suggested that the differences between sensitive and resistant strains are due to changes in the composition of the mycoplasma membranes.


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