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Introduction. The incidence of Mycobacterium avium complex (MAC) pulmonary disease (MAC PD), a refractory chronic respiratory tract infection, is increasing worldwide. MAC has three predominant colony morphotypes: smooth opaque (SmO), smooth transparent (SmT) and rough (Rg). Aim. To determine whether colony morphotypes can predict the prognosis of MAC PD, we evaluated the virulence of SmO, SmT and Rg in mice and in human macrophages. Methodology. We compared the characteristics of mice and human macrophages infected with the SmO, SmT, or Rg morphotypes of M. avium subsp. hominissuis 104. C57BL/6 mice and human macrophages derived from peripheral mononuclear cells were used in these experiments. Results. In comparison to SmO- or SmT-infected mice, Rg-infected mice revealed severe pathologically confirmed pneumonia, increased lung weight and increased lung bacterial burden. Rg-infected macrophages revealed significant cytotoxicity, increased bacterial burden, secretion of proinflammatory cytokines (TNF-α and IL-6) and chemokines (CCL5 and CCL3), and formation of cell clusters. Rg formed larger bacterial aggregates than SmO and SmT. Cytotoxicity, bacterial burden and secretion of IL-6, CCL5 and CCL3 were induced strongly by Rg infection, and were decreased by disaggregation of the bacteria. Conclusion. M. avium Rg, which is associated with bacterial aggregation, has the highest virulence among the predominant colony morphotypes.

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.

Mycobacterium avium is a common pathogen in AIDS patients and, in a large percentage of those patients, M. avium infection appears to be acquired via the gastrointestinal tract. M. avium is able to bind to and enter human and murine intestinal epithelial cells in vitro and in vivo. The invasion by and intracellular fate of M. avium in the HT-29 intestinal epithelial cell line was examined in an ultrastructural study. Bacterial contact with polarised cells was observed 10-15 min after monolayer infection and in polarised monolayers this always occurred in areas lacking microvilli. Contact with HT-29 cells did not appear to take place in a preferential area on the bacterial cell. Following invasion, M. avium was encountered within vacuoles containing either single or multiple bacteria; the latter evolved to contain only an individual bacterium. Vacuoles containing more than one bacterium were seen early in the infection and eventually underwent segmentation, with each bacterium occupying a vacuole. No bacteria were observed outside vacuoles up to 5 days after infection.

BALB/c and severe combined immunodeficient (SCID) mice were inoculated intraperitoneally with Mycobacterium avium and the numbers of cfu were monitored for 70 days in spleen, liver, lung, kidney, brain and peritoneum. While BALB/c mice formed typical granulomas and controlled bacterial growth in organs, a delay in development of lesions and a modest containment of infection were observed in SCID mice. In the spleen of BALB/c mice, in which bacterial growth was contained, macrophages (Mo) and natural killer (NK) cell numbers increased > or = 4.2 times and T- and B-cell numbers increased > or = 1.8 times after 42 days of infection; conversely, a low recruitment of mononuclear cells was observed in the spleen of SCID mice, where M. avium proliferated efficiently. Unlike visceral organs, a pronounced decrease in the number of cfu was observed in the peritoneum of BALB/c mice, concomitantly with a > or = 31.7-fold increase in Mo and NK cells and a > or = 9.1-fold increase in T and B cells. In the peritoneum of SCID mice only a bacteriostatic effect was observed despite a > or = 56.7-fold increase in Mo and NK cells and a > or = 22.3-fold increase in T and B cells. These results suggest that while an intact immune response can efficiently control M. avium infection in the spleen and peritoneum of BALB/c mice, cells of the innate immune system such as Mo and NK cells play a role in the containment of bacterial growth in the peritoneum, but not spleen, of SCID mice.

Mycobacterium avium is an intracellular pathogen that is associated with disseminated infection in acquired immunodeficiency syndrome (AIDS). Patients with AIDS appear to acquire M. avium mainly through the gastrointestinal tract. Previous studies have shown that healthy mice given M. avium orally develop disseminated infection after 2-4 weeks. The chief site of M. avium invasion of the intestinal mucosa is the terminal ileum. To learn more about the pathophysiology of M. avium infection of the intestinal mucosa, C57BL/6 bg+ bg+ mice were infected orally with M. avium strain 101 and groups of six mice were killed each week for 8 weeks. The terminal ileum was then prepared for histopathological studies and electron microscopy. A delayed inflammatory response was observed and influx of neutrophils in the Peyer's patches was the only abnormality seen at 1 week. A severe inflammatory response was seen from week 2 to week 5 and necrosis of intestinal villi was observed 6 weeks after infection. These results indicate that invasion and infection of the normal intestine by M. avium results in a severe inflammatory response with segmental necrosis of the intestinal mucosa.