The mechanism(s) by which Mycobacterium tuberculosis crosses the alveolar wall to establish infection in the lung is not well known. In an attempt to better understand the mechanism of translocation and create a model to study the different stages of bacterial crossing through the alveolar wall, we established a two-layer transwell system. M. tuberculosis H37Rv was evaluated regarding the ability to cross and disrupt the membrane. M. tuberculosis invaded A549 type II alveolar cells with an efficiency of 2 to 3% of the initial inoculum, although it was not efficient in invading endothelial cells. However, bacteria that invaded A549 cells were subsequently able to be taken up by endothelial cells with an efficiency of 5 to 6% of the inoculum. When incubated with a bicellular transwell monolayer (epithelial and endothelial cells), M. tuberculosis translocated into the lower chamber with efficiency (3 to 4%). M. tuberculosis was also able to efficiently translocate across the bicellular layer when inside monocytes. Infected monocytes crossed the barrier with greater efficiency when A549 alveolar cells were infected with M. tuberculosis than when A549 cells were not infected. We identified two potential mechanisms by which M. tuberculosis gains access to deeper tissues, by translocating across epithelial cells and by traveling into the blood vessels within monocytes.Infection caused by M. tuberculosis represents one of the great tragedies in world history. Approximately 3 million people die annually of the disease (7), despite the availability of cheap, efficacious, and curative therapy for tuberculosis.Once more, it seems clear that the improvement of the knowledge about the mechanisms employed by M. tuberculosis to infect the host will certainly offer new opportunities for the development of both effective therapy and vaccine.M. tuberculosis is inhaled into the respiratory tract, eventually reaching the alveolar space. It has been assumed that the bacterium is ingested by alveolar macrophages and subsequently gains access to the bloodstream by being transported by the alveolar macrophages and blood monocytes across the alveolar wall (10). Recently, however, it was demonstrated by several groups that M. tuberculosis invades and survives within human type II alveolar epithelial cells in vitro (3, 14, 17), and a possible role for alveolar epithelial cells in vivo has been postulated. In fact, the chance that M. tuberculosis would encounter an alveolar epithelial cell (the average human male has 1,500 type II and 28,000 type I alveolar epithelial cells [22]) is significantly greater than encountering an alveolar macrophage (50 macrophages per alveolus [8]). Therefore, the participation of type II alveolar epithelial cells, alveolar macrophages, and blood monocytes in the translocation of M. tuberculosis across the alveolar wall is currently poorly understood. Previous work has established the use of an in vitro model with a bilayer with alveolar epithelial cells and human lung endothelial cells (6). Using this model, i...
SummaryPPE and PE gene families, which encode numerous proteins of unknown function, account for 10% of Mycobacterium tuberculosis genome. Mycobacterium avium genome has similar PPE and PE gene families. Using a temperature-sensitive phage phAE94 transposon mutagenesis system, a M. avium transposon library was created in the strain MAC109. Screening of individual mutants in human U937 macrophages for the ability to replicate intracellularly, we identified several attenuated clones. One of them, the 2D6 mutant, has a transposon interrupting a PPE gene (52% homologous to Rv 1787 in M. tuberculosis ) was identified. The mutant and the wild-type strain had comparable ability to enter macrophages. Challenge of mice with the 2D6 mutant resulted in approximately 1 log and 2 log fewer bacteria in the spleen, at 1 and 3 weeks after infection, compared with the wild-type bacterium. The 2D6 mutant grows like the wild-type bacterium in vitro . Vacuoles containing the 2D6 mutant acidified to pH 4.8; whereas, vacuoles containing wild-type bacterium were only slightly acidic. It was also observed that, in contrast to the wild-type bacterium, the 2D6 mutant did not prevent phagosome-lysosome fusion, and it is only expressed within macrophage but not in 7H9 broth. These results revealed a role for this PPE gene in the growth of M. avium in macrophages and in virulence in mice.
SummaryOrganisms of the Mycobacterium avium complex (MAC) are widely distributed in the environment, form biofilms in water pipes and potable water tanks, and cause chronic lung infections in patients with chronic obstructive pulmonary disease and cystic fibrosis. Pathological studies in patients with pulmonary MAC infection revealed granulomatous inflammation around bronchi and bronchioles. BEAS-2B human bronchial epithelial cell line was used to study MAC invasion. MAC strain A5 entered polarized BEAS-2B cells with an efficiency of 0.1 ± ± ± ± 0.03% in 2 h and 11.3 ± ± ± ± 4.0% in 24 h. In contrast, biofilm-deficient transposon mutants 5G4, 6H9 and 9B5 showed impaired invasion. Bacteria exposed to BEAS-2B cells for 24 h had greater ability to invade BEAS-2B cells compared with bacteria incubated in broth. M. avium had no impact on the monolayer transmembrane resistance. Scanning electron microscopy showed that MAC A5 forms aggregates on the surface of BEAS-2B cell monolayers, and transmission electron microscopy evidenced MAC within vacuoles in BEAS-2B cells. Cells infected with the 5G4 mutant, however, showed significantly fewer bacteria and no aggregates on the cell surface. Mutants had impaired ability to cause infection in mice, as well. The ability to form biofilm appeared to be associated with the invasiveness of MAC A5.
Mycobacterium avium complex infections occur in 30%-80% of patients with AIDS. Recent evidence supports the gastrointestinal tract as the source of M. avium. Although a reproducible animal model exists, a model more closely resembling the infection in AIDS patients is needed to answer pertinent questions regarding response to therapy and prophylaxis. Beige mice were infected orally (1 x 10(8) or 1 x 10(4) cfu, five doses), and consistent, reproducible disseminated infections after 4 and 8 weeks, respectively, were obtained. Bacteremia was observed in none to 70% of the animals depending on the strain used, and mortality ranged from none to 33%, also depending on the strain used. Concomitant ingestion of ethanol (4% of daily dietary calories) was associated with a significant increase in the number of viable bacteria recovered from liver, spleen, and appendix compared with animals not receiving ethanol. The orally infected animal model closely resembles M. avium infection in humans and may be important in investigating prophylaxis and therapy of this infection.
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