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Possession of the macrophage-induced gene by isolates of the Mycobacterium avium complex is not associated with significant clinical disease

¹ Department of Laboratory Medicine, Seoul National University Boramae Hospital, Seoul, Republic of Korea

² Department of Laboratory Medicine, Seoul National University Hospital, Seoul, Republic of Korea

³ Department of Laboratory Medicine, Kangdong Sacred Heart Hospital, Seoul, Republic of Korea

⁴ Aerospace Medical Center, ROKAF, Chungcheongbuk-do, Republic of Korea

Correspondence
Sue Shin
jeannie{at}snu.ac.kr

Received March 26, 2008
Accepted October 19, 2008

The Mycobacterium avium complex (MAC) is the most frequently isolated species among non-tuberculous mycobacteria (NTM) clinical isolates. Physicians pay attention to the differential diagnosis of the disease caused by MAC from tuberculosis because of their similar clinical presentations. Expression of the macrophage-induced gene (mig) is one of the virulence phenotypes in MAC, but it has not been determined whether the presence of the mig gene itself has any relationship with clinical disease or whether it is merely a marker for MAC. To uncover the significance of the mig gene among MAC clinical isolates, positive cultures from respiratory specimens from patients in a tertiary referral centre were identified by sequencing the 16S rRNA gene. The mig gene was also evaluated using PCR and sequence analysis. The medical records from the patients were reviewed retrospectively. The diagnostic criteria from the American Thoracic Association were adopted for the diagnosis of NTM lung disease. A total of 45 MAC clinical isolates were identified over a period of 1 year. Following 16S rRNA sequencing, all of the 23 M. avium isolates were categorized as sequevar I. Among the 22 Mycobacterium intracellulare isolates, 18 strains were identified as M. intracellulare sequevar I and the remaining four consisted of one each of sequevars II, III, IV and V. The proportion of cases that fitted the diagnostic criteria of NTM lung disease was 26.7 % (12/45). The mig PCR results were 100 % positive for the MAC isolates studied, irrespective of their species, sequevar or disease-causing properties. However, following bootstrap analysis of the mig sequences, we observed definite grouping between M. avium and M. intracellulare. Thus the mig gene is a species-specific marker with distinct sequence diversity between the two species M. avium and M. intracellulare, but there is poor correlation between disease-causing properties and specific mig sequences.

Abbreviations: AFB, acid-fast bacilli; MAC, Mycobacterium avium complex; NTM, non-tuberculous mycobacteria.