The possibility that the strains included within the Mycobacterium avium complex (MAC), but not belonging either to M. avium or to Mycobacterium intracellulare, may be members of undescribed taxa, has already been questioned by several taxonomists. A very homogeneous cluster of 12 strains characterized by identical nucleotide sequences both in the 16S rDNA and in the 16S–23S internal transcribed spacer was investigated. Similar strains, previously reported in the literature, had been assigned either to the species M. intracellulare on the basis of the 16S rDNA similarity or to the group of MAC intermediates. However, several phenotypical and epidemiological characteristics seem to distinguish these strains from all other MAC organisms. The unique mycolic acid pattern obtained by HPLC is striking as it is characterized by two clusters of peaks, instead of the three presented by all other MAC organisms. All of the strains have been isolated from humans and all but one came from the respiratory tract of elderly people. The clinical significance of these strains, ascertained for seven patients, seems to suggest an unusually high virulence. The characteristics of all the strains reported in the literature, genotypically identical to the ones described here, seem to confirm our data, without reports of isolations from animals or the environment or, among humans, from AIDS patients. Therefore, an elevation of the MAC variant was proposed and characterized here, with the name Mycobacterium chimaera sp. nov.; this increases the number of species included in the M. avium complex. The type strain is FI-01069T (=CIP 107892T=DSM 44623T).

INNO-LiPA MYCOBACTERIA (Innogenetics, Ghent, Belgium) is a reverse hybridization DNA probe assay that has been recently improved by increasing the number of identifiable mycobacterial species to 16. Our assessment, performed with 197 mycobacteria belonging to 81 taxa, revealed 100% specificity and sensitivity for 20 out of 23 probes. The probes specific for Mycobacterium fortuitum complex, for the Mycobacterium aviumintracellulare-scrofulaceum group, and for Mycobacterium intracellulare type 2 cross-reacted with several mycobacteria rarely isolated from clinical specimens. The overall sensitivity was 100%, and the overall specificity was 94.4%.

Four bacterial strains were isolated from independent clinical specimens in different countries and their genotypic and phenotypic characters support their classification in a novel species within the genus Mycobacterium. One strain was clearly responsible for a severe, post-traumatic wound infection in a healthy boy. The novel species, for which the name Mycobacterium monacense sp. nov. is proposed, is yellow-pigmented, non-photochromogenic and grows in less than a week on solid medium. Based on phenotypic investigations alone, distinction of these four strains from known scotochromogenic rapidly growing strains is problematic. However, the novel strains differ from any other mycobacterium in each of the molecular species markers investigated: the 16S rRNA gene, the 16S-23S rRNA gene internal transcribed spacer and the hsp65 gene. Of the strains investigated, two different sequevars were detected for the hsp65 region. Phylogenetic analysis revealed that these four strains were most closely related to Mycobacterium doricum. The type strain of Mycobacterium monacense sp. nov. is B9-21-178 T (=DSM 44395 T =CIP 109237 T ).

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