Serological cross-reactions between Mycoplasma genitalium and Mycoplasma pneumoniae

The recently discovered mycoplasma species Mycoplasma genitalium was isolated from urethral specimens from men with nongonococcal urethritis (Tully et al., Lancet i:1288-1291, 1981). In a previous report (K. Lind, Lancet ii:1158-1159, 1982), prominent serological cross-reactions were demonstrated between this mycoplasma and M. pneumoniae. In the present study, the two mycoplasma species were compared more extensively. In classical mycoplasma medium without thallium acetate, M. genitalium grew more slowly than M. pneumoniae did but finally formed similar amounts of acetic acid and lactic acid from glucose. Although their colonies on solid medium were indistinguishable, transmission electron microscopy showed that the flask-formed cells of M. genitalium (especially their necks) were shorter than those of M. pneumoniae. The two species were distinct since DNA hybridization showed only 1.8% homology in base sequences, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed significantly different profiles of the two strains. However, considerable similarities were found in their antigenic reactions in various serological tests. The presence of common or closely related antigens was demonstrated in the two species with rabbit immune sera in complement fixation test with chloroform-methanol-extracted antigens by an indirect immunofluorescence test on microcolonies, and by metabolism inhibition and growth inhibition tests. Cross-reactions were also demonstrated by crossed immunoelectrophoresis. The role of M. genitalium as a human pathogen in the genital tract has not been assessed. If serological tests are to be used in this assessment, caution must be exercised due to the extensive cross-reactions demonstrated. Some of the species-specific antigens which we have demonstrated would be appropriate for use in such tests and would help to circumvent problems caused by cross-reactions.