An insertion sequence (IS901), found in pathogenic strains of Mycobacterium avium, but absent in M. avium complex isolates from patients with acquired immune deficiency syndrome (AIDS), has been isolated and sequenced. This insertion element has a nucleotide sequence of 1472 bp, with one open reading frame (ORF1), which codes for a protein of 401 amino acids. The amino acid sequence, terminal ends and target site of IS901 are similar to those of IS900, present in Mycobacterium paratuberculosis. However, the DNA sequences of these two IS elements exhibit only 60% homology, compared to a DNA homology of 98% between their respective hosts. IS901, like IS900, appears to belong to a family of related insertion elements present in actinomycetes and other bacteria. M. avium strains containing IS901 were found to be more virulent in mice than closely related strains lacking IS901. IS901 may be a useful tool for the study of the genetics of virulence in the M. avium complex and for obtaining stable integration of foreign genes into mycobacteria.

Mycobacterium avium causes disease, principally tuberculosis in immunocompromised individuals. It is the most frequent cause of disseminated infections in AIDS patients in the West. The pathogen is also associated with disease in animals, chiefly birds and livestock, and may be isolated from environmental samples such as soil and water. Analysis of strains ofM. avium isolated from clinical, veterinary, and environmental sources for the presence of the mycobacterial insertion sequences IS900 and IS901 demonstrates the specific association of IS901 to animal pathogenic M. avium strains. In contrast, most clinical M. avium strains and all AIDS-derived strains examined so far lacked IS901. Significant differences in the plasmid contents and serotypes of strains with and without IS901 were also found. We therefore suggest that the presence of IS901 divides M. avium into two clearly distinct subtypes with differing host range, virulence, plasmid possession, and serotyping antigens. By using DNA sequence data from IS901 and M. avium DNA, a set of polymerase chain reactions were developed for the specific detection and differentiation of these subtypes. The Mycobacterium avium-M. intracellulare complex, or M. avium complex, is a group of related mycobacteria that cause widespread disease in animals (7, 8, 33). M. avium is generally considered to be an opportunistic pathogen in humans, causing tuberculosis, lymphadenitis, and disseminated infections in immunocompromised patients, particularly AIDS patients (5, 6, 11, 15). M. avium is now recognized as the cause of the most common disseminated bacterial infections in Western AIDS patients. The pathogen is thought to be never or only very rarely transmitted from person to person. The source of M. avium infection in humans is unknown, but its presence in animals, birds, and environmental samples suggests a number of possible reservoirs. The M. avium complex was initially classified into seroagglutination types: serotypes 1 to 3 were designated M. avium, and serotypes 4 to 21 were designated M. intracellulare. The close phenotypic similarity of the two species led to their being grouped in a single (M. avium) complex (26). DNA homology studies, however, indicated that although many M. intracellulare serotypes were genetically distinct from those of M. avium (sharing less than 56% DNA homology), serotypes 4, 5, 6, and 8 were in fact indistinguishable from those of M. avium and are now considered to be M. avium complex serotypes (1). These higher M. avium complex serotypes are those that are most commonly associated with AIDS (15), whereas the classical M. avium serotypes (1, 2, and 3) are those most commonly associated with infections in birds and other animals (7, 8, 25, 33). The reasons for these associations are presently unknown. Two specific mammalian pathogens are also closely related to M. avium (>95% DNA homology): M. paratuberculosis, the causative agent of Johne's disease (chronic regional enteritis) in ruminants (3, 27), but implicated in Crohn's disease in humans (...

SUMMARYThe virulence of different isolates of MAC was studied in naturally susceptible BALB/c mice. In preliminary experiments, MAC bacteria forming smooth transparent colonies on solid media (SmT variants) were found to be virulent for BALB/c mice, causing progressive infection; smooth opaque (SmOp) were generally avirulent, being slowly eliminated from the infeeted organs; and rough (Rg) variants were either avirulent or as virulent as SmT variants. We chose to compare the virulence of different isolates of MAC of different origins, studying only the SmT morphotype. Strains of MAC isolated from naturally infected animals were those that most consistently caused progressive infections. AIDS patients-derived isolates were of intermediate virulence or devoid of virulcnec in mice. The environmental strains were ehminated from miee or did not proliferate. Strains of MAC i.solated from individuals who were not infected by HIV varied in virulence from completely avirulent to highly virulent. There was no elose eorrelation between virulenee and restrietion fragment length polymorphism (RELP) type, although all highly virulent strains were of the A.i type. There was also no eorrelation between virulence analysed in vivo and the ability to grow in cultured macrophages.

Mycobacterium paratuberculosis has been isolated from tissue taken from patients with Crohn's disease and has been implicated in the etiology of this disease. On culture, the organisms appear initially as cell wall-deficient, spheroplast-like forms that are difficult to identify by conventional techniques. Here we examine 30 unidentified cultures by the polymerase chain reaction using primers specific for M. paratuberculosis, Mycobacterium tuberculosis, and Mycobacterium avium restriction fragment length polymorphism type A/I and also by a non-species-specific mycobacterial polymerase chain reaction. Six of these cultures, all from Crohn's disease, were shown to contain DNA from M. paratuberculosis. Cultures from both Crohn's disease and controls were found to contain mycobacterial DNA of unknown specific origin.

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