Factors in the Fermentation‐Inhibition Test for Measurement of Growth‐Inhibiting Antibody to Mycoplasma pneumoniae

Factors in the fermentation-inhibition test for the measurement of growth-inhibiting antibody in serum to Mycoplasma pneumoniae were studied. The fermentation-inhibiting antibody titer, as read on the day when the color of the control cup without serum had just changed to yellow, was constant among inoculum dilutions of 10–2 to 10–5 (106 to 103 CFU/ml) of a M. pneumoniae broth culture. The use of 10–3 to 10–5 dilutions (105 to 103 CFU/ml) was adequate for inoculation, inasmuch as one day delay in reading did not result in a significant decrease in the test. Heat inactivation of the serum gave no significant effect on the titer. The test was simple and reliable. The growth-inhibiting antibody was shown to be detectable in the test, when the growth of M. pneumoniae was suppressed at least to 1/100 of the growth of the control without serum. The growth-inhibiting antibody titer rose later than the complement-fixing antibody titer in some cases after M. pneumoniae infection. It is suggested that, when an erythromycin-sensitive strain of M. pneumoniae is used, the titer transiently rises and does not show a real growth-inhibiting antibody titer in sera from patients under erythromycin administration.