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I would like to comment on a topic that has not been specifically discussed in any of the papers reviewed by Dr. Deresinski.
It concerns Mycobacterium xenopi and Mycobacterium celatum colonial morphology and how to tentatively distinguish between them. I agree with Wolinsky when he says that M. xenopi is not difficult to indentify in the laboratory (Infectious Disease Alert 1997;16:124-125). In fact, this species, when grown on L-J or other egg-based media, produces small, pigmented (photochromogenic), punctate colonies protruding from the medium surface. M. celatum, on the contrary, produces larger, usually non-pigmented (or weakly pigmented), flat colonies. On the basis of the above reported features, confusion would be more likely with M. avium complex rather than with M. xenopi.
In conclusion, the combination of colonial morphology, growth temperatures, biochemical tests, and DNA-probe results should lead the microbiologist to reasonably correct identification in most cases. For M. celatum, definitive identification relies upon HPLC analysis of mycolic acids.
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