LABORATORY AND CLINICAL-EVALUATION OF MYCOBACTERIUM-XENOPI ISOLATES

Mycobacterium xenopi and Mycobacterium avium complex (MAC) are biochem ically similar. To define the laboratory characteristics of M. xenopi that distinguish it from MAC, 53 M. xenopi isolates from different are as in the United Stales and 47 isolates recovered at one hospital were evaluated by 13 biochemical tests, AccuProbe MAC (Gen-Probe, Inc., Sa n Diego, CA, LISA), colony morphology, formation of X-colonies, pigmen tation in response to light, growth on MacConkey agar without crystal violet, and relative growth rates at 25 degrees C, 36 degrees C, and 4 5 degrees C on solid media. Relative growth rates of 10 M. xenopi and II MAC isolates were measured at 25 degrees C, 36 degrees C, and 42 de grees C in Middlebrook broth processed using the BACTEC TB System. Ten M. xenopi were tested for -nitro-alpha-acetylamino-beta-hydroxypropio phenone inhibition at 36 degrees C and 42 degrees C. Reevaluation of 8 1 isolates previously identified as MAC by biochemical tests alone rev ealed that two were M. xenopi. The most reliable characteristics disti nguishing M. xenopi from MAC were the presence of X-colonies (M. xenop i 97% vs MAC 1%), positive 3-day arylsulfatase (M. xenopi 88% vs MAC 1 %), growth at 25 degrees C (M. xenopi 0% vs MAC 100%), and AccuProbe M AC test results (M. xenopi 0% hybridized). Retrospective chart review of 37 patients lasing American Thoracic Society criteria revealed that six (16%) patients had clinically important isolates. Ar one of our h ospitals M. xenopi was the second most common mycobacterial species is olated for 1990-1992, accounting for 27% of all isolates, whereas at o ur other hospital it accounted for 1% of isolates. We conclude that M. xenopi and MAC can be readily differentiated using the AccuProbe, tem perature-dependent growth rates, and a few biochemical tests. Finally, few M. xenopi isolates are clinically important, even in HIV-infected patients.