If you have a subject with hypogamma, either due to CVID or other "constitutional" causes, such XLA, or acquired, due to immunosuppression, usually with Rituximab, who present with a picture of inflammatory arthritis, either monoarthritis or oligoarthritis, suspect infection by either Mycoplasma hominis or Ureaplasma urealyticum, particularly if usual cultures are negative and the usual antibiotics do not work.
Both organisms are overrepresented in subjects with septic arthritis developing against a background of hypogamma, particularly on Rituximab. The most commonly affected joints are hip, knee, shoulder, elbow, ankle and PIP. Prosthetic joint infections, particularly affecting the hip have been reported several times.
These fastidious organisms will not grow on routine cultures. M.hominis may show up on anaerobic culture plates after 5 days, but is best cultured on PPLO (pleuropneumonia like organism) broth or looked for by 16S RNA. Similarly, U.urealyticum is best cultured on Ureaplasma differential agar, Ureaplasma broth, or A7B agar. Fortunately, the last 3 will also grow M.hominis. U.urealyticum can also be identified on 16S RNA analysis.
If a person with hypogamma and putative inflammatory arthritis has proven culture negative on synovial fluid and blood culture, do ask for these special media or 16S RNA to isolate these 2 organisms.
Ureaplasma requires urea in its culture medium for growth. It produces urease with breaks down urea into ammonia, and can thus change the colour of phenol red to pink. M.hominis metabolises arginine, but not glucose. Ureaplasma ferments neither, but produces a "managanese reaction" in culture.
If the usual antibiotics have not worked, there would be a natural tendency to attribute the inflamed joint(s) to the underlying disorder such as RA or Lupus, or make a new diagnosis of one of the above two, and treat with steroids. This may lead to a worsening of such arthritis, so special caution is warranted in subjects with hypogamma.
A course of doxycycline may be indicated if the usual antibiotics which target cell walls (which these organisms lack) have not worked. Resolution of fever/arthritis with doxycycline supports the likelihood of infection by one of the above two.
