Human respiratory tract infections with mycoplasmas and their in vitro susceptibility to tetracyclines and some other antibiotics.

The role played by Mycoplasma pneumoniae and possibly by other species of mycoplasma in respiratory tract infections in man is considered. The in vitro sensitivity of human mycoplasmas, that is, M. hominis, M. fermentans, M. orale type 1, M. pneumoniae, M. salivarium and T-mycoplasmas to different tetracycline analogues and some other antibiotics was determined. The strains tested were often somewhat more susceptible to doxycycline than to methacycline, minocycline and tetracycline, while oxytetracycline, per unit of weight, was in most instances 8-16 times less effective than doxycycline. The difference between the minimum concentrations of the above-mentioned tetracyclines, that inhibited metabolism of M. pneumoniae in liquid medium, and the minimum lethal concentrations of these antibiotics was small. Variation of the inoculum size of this organism had comparatively little effect on the result of the susceptibility tests with doxycycline. The study indicates a cidal rather than static action of doxycycline on M. pneumoniae. This organism as well as T-mycoplasmas were sensitive to low concentrations of erythromycin, while the strains of the other species studied were resistant to even 100 mug/ml of this antibiotic. M. pneumoniae and T-mycoplasmas were resistant to 20-80 mg/ml lincomycin, while the other human mycoplasmas were moderately sensitive. Chloramphenicol and gentamycin were generally less effective, per unit of weight, than the tetracyclines against most of the strains of mycoplasma tested. Some known effects of antibiotic treatment of mycoplasma infections in man are also discussed.