Gen-Probe Rapid Diagnostic System for distinguishing between Mycobacterium avium and Mycobacterium paratuberculosis.

The article by Thoresen and Saxegaard (7) concerning the relationship between Mycobacterium avium and Mycobacterium paratuberculosis as determined by using the GenProbe Rapid Diagnostic System contains interesting observations, some of which are not unexpected. Because M. paratuberculosis is rather inactive biochemically and because of variability between strains, biochemical reactions appear to be an unreliable basis for characterization of this organism. Thus, according to Chiodini and van Kruningen (2), identification of a mycobacterial isolate as M. paratuberculosis relies on mycobactin-dependent culture conditions requiring at least 8 weeks of cultivation for visible growth. More recently, a close genetic relationship between M. avium and M. paratuberculosis has been demonstrated (3, 4). Using the Gen-Probe Inc. commercial gene probe kit, which is directed against 16S rRNA, Thoresen and Saxegaard could not distinguish between M. avium and M. paratuberculosis. However, some field strains of M. paratiuberculosis gave negative hybridization results with the M. avium probe, thus indicating some genetic variation between different strains of M. paratuberculosis. Since a species is defined genetically as a homogenous taxon, these results cast some doubt on the traditional criteria of defining an isolate as M. paratuberculosis. Given the suitability of 16S rRNA as a phylogenetic and taxonomic indicator (8), Thoresen and Saxegaard conclude that "further investigation of the 16S rRNA gene of M. paratuberculosis and M. avium could be an additional approach to clarifying the relationship between these two organisms." By using comparative sequencing of complete 16S rRNA genes, the relationships within the M. avium-M. intracellulare-M. paratuberculosis complex have recently been elucidated and M. paratuberculosis has been shown to possess a 16S rRNA sequence that is indistinguishable from that of certain M. avium-M. intracellulare serovars (1, 5). Thus, approaches other than 16S rRNA comparative sequence analysis are necessary to provide discerning sequence attributes for these two closely related organisms. Although numerical taxonomy has its own limits and, e.g., using characteristics currently considered numerical taxonomy cannot differentiate between established species such as M. avium and M. intracellulare (6), a recently performed numerical taxonomy analysis suggested that M. avium and M. paratuberculosis should be separated at a subspecies level (6). The available 16S rRNA sequencing (1, 5) and nucleic acid hybridization data (4) support this suggestion.