Diphtheroid-like properties of Mycoplasma arthritidis

Mycoplasma arthritidis is a pleuropneumonia-like micro-organism able to incite extensive joint inflammation in rodents and associated with anterior uveitis in mice and rats. Diphtheroids have been reported in apparently pure cultures of M. arthritidis and serological cross-reactions demonstrated between the two. Cultures of M. arthritidis propagated in this laboratory have also appeared to be contaminated with diphtheroids, and repeated attempts have been made to clone the mycoplasma free from the putative contaminant. Failure to separate the two has led us to inquire into the possibility of the two organisms being the same agent, capable of exhibiting different morphology under varying cultural conditions. Since the pathogenicity of M. arthritidis for rodents is easily demonstrated through a single tail-vein inoculation we sought any comparable effects the 'diphtheroid' may have when concentrated from conventional bacteriological growth medium. The possibility of confusing mycoplasma with diphtheroids is of clinical interest because of the high incidence of ostensibly harmless diphtheroids isolated from patients with conjunctivitis and (occasionally) rheumatoidal joints.