Ce. Marx et al., LABORATORY AND CLINICAL-EVALUATION OF MYCOBACTERIUM-XENOPI ISOLATES, Diagnostic microbiology and infectious disease, 21(4), 1995, pp. 195-202
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.