Abstract
An insertion sequence - IS901 - present in pathogenic strains of M. avium (RFLP type A/I) was sequenced. The nucleotide and amino acid sequences revealed that IS901 belongs to a family of widespread atypical insertion sequences present in Actinomycetes and other bacteria. It is almost exclusively found in isolates from birds and animals, including the wood pigeon bacillus, being invariably associated with M. avium serotypes 1, 2 and 3 - the ’classical’ M. avium strains. IS901 is absent in M. avium isolates from AIDS patients. IS901 appears to be involved in pathogenicity of M. avium. Type A/I strains evolved from an M. avium parent strain (RFLP type A) by acquisition of multiple copies of IS901. Primers derived from the nucleotide sequence of this insertion sequence and from IS900 (present in M. paratuberculosis) have been used to differentially amplify DNA from various members of the M. avium complex. MAI infections in AIDS patients from the developed world are caused predominantly by RFLP type A strains. In African AIDS patients with MAI, however, RFLP type H was the most common strain identified; the exact identity of type H remains unclear. Both type A and type H were found in their corresponding environment, clearly identifying the environment as the likely source of MAI infections in AIDS patients. Infection may occur through direct inoculation via the respiratory and digestive tracts or by reactivation of an endogenous colonisation. Unlike type A strains, type H strains do not possess plasmids commonly associated with AIDS isolates. However, no significant difference in plasmid content was found between MAI isolates from AIDS and non-AIDS patients, suggesting that plasmids play no role in pathogenicity.