The purpose of this study was to determine the susceptibility of various strains of Mycoplasma bovis, Mycoplasma dispar, and Ureaplasma diversum, which are prevalent causes of
pneumonia in calves, to 16
antimicrobial agents in vitro. The MICs of the
antimicrobial agents were determined by a serial broth dilution method for 16 field strains and the type strain of M. bovis, for 19 field strains and the type strain of M. dispar, and for 17 field strains of U. diversum. Final MICs for M. bovis and M. dispar were read after 7 days and final MICs for U. diversum after 1 to 2 days. All strains tested were susceptible to
tylosin,
kitasamycin, and
tiamulin but were resistant to nifuroquine and
streptomycin. Most strains of U. diversum were intermediately susceptible to
oxytetracycline but fully susceptible to
chlortetracycline; most strains of M. bovis and M. dispar, however, were resistant to both agents. Strains of M. dispar and U. diversum were susceptible to
doxycycline and
minocycline, but strains of M. bovis were only intermediately susceptible. Susceptibility or resistance to
chloramphenicol,
spiramycin,
spectinomycin,
lincomycin, or
enrofloxacin depended on the species but was not equal for the three species. The type strains of M. bovis and M. dispar were more susceptible to various
antimicrobial agents, including
tetracyclines, than the field strains. This finding might indicate that M. bovis and M. dispar strains are becoming resistant to these agents.
Antimicrobial agents that are effective in vitro against all three mycoplasma species can be considered for treating
mycoplasma infections in pneumonic calves. Therefore,
tylosin,
kitasamycin, and
tiamulin may be preferred over
oxytetracycline and
chlortetracycline.