Differential growth characteristics and streptomycin susceptibility of virulent and avirulent Mycobacterium tuberculosis strains in a novel fibroblast-mycobacterium microcolony assay

Updated Review on the Mechanisms of Pathogenicity in Mycobacterium abscessus, a Rapidly Growing Emerging Pathogen

In recent years, Mycobacterium abscessus has appeared as an emerging pathogen, with an increasing number of disease cases reported worldwide that mainly occur among patients with chronic lung diseases or impaired immune systems. The treatment of this pathogen represents a challenge due to the multi-drug-resistant nature of this species and its ability to evade most therapeutic approaches. However, although predisposing host factors for disease are well known, intrinsic pathogenicity mechanisms of this mycobacterium are still not elucidated. Like other mycobacteria, intracellular invasiveness and survival inside different cell lines are pathogenic factors related to the ability of M. abscessus to establish infection. Some of the molecular factors involved in this process are well-known and are present in the mycobacterial cell wall, such as trehalose-dimycolate and glycopeptidolipids. The ability to form biofilms is another pathogenic factor that is essential for the development of chronic disease and for promoting mycobacterial survival against the host immune system or different antibacterial treatments. This capability also seems to be related to glycopeptidolipids and other lipid molecules, and some studies have shown an intrinsic relationship between both pathogenic mechanisms. Antimicrobial resistance is also considered a mechanism of pathogenicity because it allows the mycobacterium to resist antimicrobial therapies and represents an advantage in polymicrobial biofilms. The recent description of hyperpathogenic strains with the potential interhuman transmission makes it necessary to increase our knowledge of pathogenic mechanisms of this species to design better therapeutic approaches to the management of these infections.

Changing paradigms in the treatment of tuberculosis

Tuberculosis, caused by Mycobacterium tuberculosis, is a disease long dealt with, but still remains the second leading cause of death world-wide. The current anti-tubercular chemotherapy primarily targets the microbial pathogenesis, which however, is failing due to the development of drug resistance. Moreover, with fewer new drugs reaching the market, there is a need to focus on alternate treatment approaches that could be used as stand-alone or adjunct therapy and the existing drugs, referred to as Track II chemotherapy. This article is an attempt to review the changing global patterns of tuberculosis and its treatment. Further, newer drug delivery approaches like multi-particulate drug carriers which increase the therapeutic efficacy and bring down the systemic toxicity associated with drugs have also been discussed. There is also a need to use interventions which can be used as Track II therapy. Host-directed therapeutics (HDT) is an emerging area concept in which host cell functions and hence the response to pathogens can be modulated, which can help manage TB. HDT decreases damage induced due to inflammation and necrosis in the lungs and other parts of the body due to the disease. Various immuno-modulatory pathways have been discussed in this review which could be explored further to treat TB. An in-depth understanding of multi-particulate drug carriers and HDT could help in dealing with tuberculosis; however, there is still a long way to go.

Bacterial Strain-Dependent Dissociation of Cell Recruitment and Cell-to-Cell Spread in Early M. tuberculosis Infection

In the initial stage of respiratory infection, Mycobacterium tuberculosis traverses from alveolar macrophages to phenotypically diverse monocyte-derived phagocytes and neutrophils in the lung parenchyma. Here, we compare the in vivo kinetics of early bacterial growth and cell-to-cell spread of two strains of M. tuberculosis: a lineage 2 strain, 4334, and the widely studied lineage 4 strain H37Rv. Using flow cytometry, live cell sorting of phenotypic subsets, and quantitation of bacteria in cells of the distinct subsets, we found that 4334 induces less leukocyte influx into the lungs but demonstrates earlier population expansion and cell-to-cell spread. The earlier spread of 4334 to recruited cells, including monocyte-derived dendritic cells, is accompanied by earlier and greater magnitude of CD4+ T cell activation. The results provide evidence that strain-specific differences in interactions with lung leukocytes can shape adaptive immune responses in vivo. IMPORTANCE Tuberculosis is a leading infectious disease killer worldwide and is caused by Mycobacterium tuberculosis. After exposure to M. tuberculosis, outcomes range from apparent elimination to active disease. Early innate immune responses may contribute to differences in outcomes, yet it is not known how bacterial strains alter the early dynamics of innate immune and T cell responses. We infected mice with distinct strains of M. tuberculosis and discovered striking differences in innate cellular recruitment, cell-to-cell spread of bacteria in the lungs, and kinetics of initiation of antigen-specific CD4 T cell responses. We also found that M. tuberculosis can spread beyond alveolar macrophages even before a large influx of inflammatory cells. These results provide evidence that distinct strains of M. tuberculosis can exhibit differential kinetics in cell-to-cell spread which is not directly linked to early recruitment of phagocytes but is subsequently linked to adaptive immune responses.

Mycobacterium abscessus: Shapeshifter of the Mycobacterial World

In this review we will focus on unique aspects of Mycobacterium abscessus (MABS) which we feel earn it the designation of "shapeshifter of the mycobacterial world." We will review its emergence as a distinct species, the recognition and description of MABS subspecies which are only now being clearly defined in terms of pathogenicity, its ability to exist in different forms favoring a saprophytic lifestyle or one more suitable to invasion of mammalian hosts, as well as current challenges in terms of antimicrobial therapy and future directions for research. One can see in the various phases of MABS, a species transitioning from a free living saprophyte to a host-adapted pathogen.

Prison break: pathogens' strategies to egress from host cells

A wide spectrum of pathogenic bacteria and protozoa has adapted to an intracellular life-style, which presents several advantages, including accessibility to host cell metabolites and protection from the host immune system. Intracellular pathogens have developed strategies to enter and exit their host cells while optimizing survival and replication, progression through the life cycle, and transmission. Over the last decades, research has focused primarily on entry, while the exit process has suffered from neglect. However, pathogen exit is of fundamental importance because of its intimate association with dissemination, transmission, and inflammation. Hence, to fully understand virulence mechanisms of intracellular pathogens at cellular and systemic levels, it is essential to consider exit mechanisms to be a key step in infection. Exit from the host cell was initially viewed as a passive process, driven mainly by physical stress as a consequence of the explosive replication of the pathogen. It is now recognized as a complex, strategic process termed "egress," which is just as well orchestrated and temporally defined as entry into the host and relies on a dynamic interplay between host and pathogen factors. This review compares egress strategies of bacteria, pathogenic yeast, and kinetoplastid and apicomplexan parasites. Emphasis is given to recent advances in the biology of egress in mycobacteria and apicomplexans.

Evolution and role of corded cell aggregation in Mycobacterium tuberculosis cultures

The aim of this study was to evaluate the evolution and role of corded cell aggregation in Mycobacterium tuberculosis cultures according to growth time and conditions. Thus, in standard culture using aerated 7H9 Middlebrook broth supplemented with 0.05% Tween 80, a dramatic CFU decrease was observed at the end of the exponential phase. This phase was followed by a stable stationary phase that led to dissociation between the optical density (O.D.) and CFU values, together with the formation of opaque colonies in solid culture. Further analysis revealed that this was due to cording. Scanning electron microscopy showed that cording led to the formation of very stable coiled structures and corded cell aggregations which proved impossible to disrupt by any of the physical means tested. Modulation of cording with a high but non-toxic concentration of Tween 80 led to a slower growth rate, avoidance of a sudden drop-off to the stationary phase, the formation of weaker cording structures and the absence of opaque colonies, together with a lower survival at later time-points. An innovative automated image analysis technique has been devised to characterize the cording process. This analysis has led to important practical consequences for the elaboration of M. tuberculosis inocula and suggests the importance of biofilm formation in survival of the bacilli in the extracellular milieu.

Mycobacterium tuberculosis may escape helper T cell recognition by infecting human fibroblasts

The host immune response can limit Mycobacterium tuberculosis (Mtb) spreading in primary tuberculosis (TB) without eradicating all bacilli, which can persist causing latent TB infection and are responsible for reactivation TB. Persistent Mtb is confined to granulomas within phagocytes, but it is also found in other non-immune cells. We focused on fibroblasts since these cells participate to the granuloma formation and were shown to be infected in latent TB infections. We show that in vitro both Mtb and Bacille Calmette-Guérin actively replicate in human fibroblasts. Mycobacterial infection of fibroblasts causes a significant inhibition of interferon (IFN)-γ induced membrane expression of major histocompatibility complex class II molecules in these cells. The functional consequence of in vitro infection is a significant reduction of the fibroblast capacity to present peptides and soluble proteins to autologous specific CD4(+) T cell clones. Moreover, fibroblasts are capable of presenting antigen derived from the processing of heat-killed Mtb, but not from viable Mtb. Data indicate that IFN-γ treated fibroblasts are capable of presenting antigens derived from the processing of whole bacteria in addition to the capacity to present peptides and isolated proteins. Interestingly, Mtb infected fibroblasts lose this capacity, suggesting that Mtb may evade T helper immune surveillance by infecting fibroblasts.

Cord-forming mycobacteria induce DNA meshwork formation by human peripheral blood mononuclear cells

Mononuclear phagocytes, that is, monocytes and macrophages, are central in the defense against mycobacteria. Mycobacterium abscessus is an opportunistic mycobacterial species able to cause chronic pulmonary infections in patients with cystic fibrosis but also soft tissue infections in immunocompetent individuals. Pathogenic isolates of M. abscessus with rough colony morphology form cord-like aggregates. In this study, we investigated the in vitro response of human peripheral blood mononuclear cells (PBMCs) from healthy blood donors to cord-forming M. abscessus strains from cystic fibrosis patients with clinical lung infection. Microscopic examination revealed that the PBMCs produced a coarse fibrous meshwork containing DNA and histones, which surrounded the mycobacterial cords. Thus, the bacterial cord formations were entrapped by monocytes and lymphocytes aggregated onto the DNA-rich meshwork fibers. Mycobacterium abscessus strains with smooth colony morphology, which do not form cords and are readily phagocytosed, did not induce any meshwork formation in PBMCs. The chromatin meshwork may represent a defense mechanism against nondigestible invaders.

Microcolonies in fluoroquinolone agar proportion susceptibility testing of Mycobacterium tuberculosis: an indicator of drug resistance

Microcolony growth of Mycobacterium tuberculosis on agar proportion susceptibility testing is neither well-defined nor previously reported with fluoroquinolone susceptibility testing. We describe here M. tuberculosis microcolony growth with fluoroquinolones, and assess its clinical significance. We screened 797 M. tuberculosis isolates for ofloxacin resistance (2.0 μg/mL) by agar proportion; 19 ofloxacin-resistant and 38 ofloxacin-susceptible isolates were selected for more detailed susceptibility testing with ofloxacin, ciprofloxacin, levofloxacin (all at 2.0 μg/mL) and moxifloxacin (0.5 μg/mL). The 57 isolates were also tested at two concentrations both above and below the critical concentrations. Microcolonies were defined as colonies 0.2-0.4 mm in diameter; confirmed microcolonies were present on repeat testing. Of the 57 isolates tested in detail, 7 grew microcolonies, of which 2 (0.3% of all isolates tested) had confirmed microcolonies on repeat testing (6 tests performed, and microcolonies were present on at least 4). Both M. tuberculosis isolates were ofloxacin-resistant on screening, and had ofloxacin minimum inhibitory concentration (MIC) >8 μg/mL. The five other isolates were ofloxacin-susceptible on screening, but had regular colony growth (i.e., resistance) at the drug concentration that initially resulted in microcolonies (ofloxacin 0.5 or 1.0 μg/mL). Microcolonies were observed infrequently with fluoroquinolone susceptibility testing, but when confirmed, they were associated with drug resistance.

Galectin-3, a marker for vacuole lysis by invasive pathogens

Shigella bacteria invade macrophages and epithelial cells and following internalization lyse the phagosome and escape to the cytoplasm. Galectin-3, an abundant protein in macrophages and epithelial cells, belongs to a family of beta-galactoside-binding proteins, the galectins, with many proposed functions in immune response, development, differentiation, cancer and infection. Galectins are synthesized as cytosolic proteins and following non-classical secretion bind extracellular beta-galactosides. Here we analysed the localization of galectin-3 following entry of Shigella into the cytosol and detected a striking phenomenon. Very shortly after bacterial invasion, intracellular galectin-3 accumulated in structures in vicinity to internalized bacteria. By using immuno-electron microscopy analysis we identified galectin-3 in membranes localized in the phagosome and in tubules and vesicles that derive from the endocytic pathway. We also demonstrated that the binding of galectin-3 to host N-acetyllactosamine-containing glycans, was required for forming the structures. Accumulation of the structures was a type three secretion system-dependent process. More specifically, existence of structures was strictly dependent upon lysis of the phagocytic vacuole and could be shown also by Gram-positive Listeria and Salmonella sifA mutant. We suggest that galectin-3-containing structures may serve as a potential novel tool to spot vacuole lysis.

Infection by tubercular mycobacteria is spread by nonlytic ejection from their amoeba hosts

To generate efficient vaccines and cures for Mycobacterium tuberculosis, we need a far better understanding of its modes of infection, persistence, and spreading. Host cell entry and the establishment of a replication niche are well understood, but little is known about how tubercular mycobacteria exit host cells and disseminate the infection. Using the social amoeba Dictyostelium as a genetically tractable host for pathogenic mycobacteria, we discovered that M. tuberculosis and M. marinum, but not M. avium, are ejected from the cell through an actin-based structure, the ejectosome. This conserved nonlytic spreading mechanism requires a cytoskeleton regulator from the host and an intact mycobacterial ESX-1 secretion system. This insight offers new directions for research into the spreading of tubercular mycobacteria infections in mammalian cells.

Vaccination with Bacille-Calmette Guérin promotes mycobacterial control in guinea pig macrophages infected in vivo

Tuberculosis pleurisy was induced by inoculation of virulent (H37Rv strain or Erdman strain) or attenuated (H37Ra strain) green-fluorescent protein-expressing Mycobacterium tuberculosis into guinea pigs that had or had not been vaccinated with Bacille-Calmette Guérin (BCG). Pleural fluid and cells were analyzed for phagosome-lysosome (P-L) fusion, on the basis of confocal microscopy, intracellular and extracellular bacterial survival, and production of cytokine mRNA. BCG vaccination increased fluid volume and cellular accumulation, significantly enhanced P-L fusion, and significantly decreased intracellular bacterial survival in pleural-effusion macrophages of the guinea pigs infected with the 2 virulent strains. Furthermore, significant increases in interferon-gamma, transforming growth factor-beta, tumor necrosis factor-alpha, and interleukin-12p40 cytokine mRNA were seen in the pleural cells of the BCG-vaccinated guinea pigs.

Flotillin and RacH modulate the intracellular immunity of Dictyostelium to Mycobacterium marinum infection

Mycobacterium marinum, a close relative of Mycobacterium tuberculosis, provides a useful model to study the pathogenesis of tuberculosis in genetically tractable model organisms. Using the amoeba Dictyostelium discoideum as a host, we show that expression of the M. marinum protein MAG24-1 is crucial to interfere with phagosome maturation. We find that two host proteins - the flotillin homologue vacuolin and p80, a predicted copper transporter - accumulate at the vacuole during pathogen replication until it finally ruptures and the bacteria are released into the host cytosol. Flotillin-1 accumulation at the replication niche and its rupture were also observed in human peripheral blood monocytes. By infecting various Dictyostelium mutants, we show that the absence of one of the two Dictyostelium vacuolin isoforms renders the host more immune to M. marinum. Conversely, the absence of the small GTPase RacH renders the host more susceptible to M. marinum proliferation but inhibits its cell-to-cell spreading.

Is adipose tissue a place for Mycobacterium tuberculosis persistence?

Background

Mycobacterium tuberculosis, the etiological agent of tuberculosis (TB), has the ability to persist in its human host for exceptionally long periods of time. However, little is known about the location of the bacilli in latently infected individuals. Long-term mycobacterial persistence in the lungs has been reported, but this may not sufficiently account for strictly extra-pulmonary TB, which represents 10–15% of the reactivation cases.

Methodology/Principal Findings

We applied in situ and conventional PCR to sections of adipose tissue samples of various anatomical origins from 19 individuals from Mexico and 20 from France who had died from causes other than TB. M. tuberculosis DNA could be detected by either or both techniques in fat tissue surrounding the kidneys, the stomach, the lymph nodes, the heart and the skin in 9/57 Mexican samples (6/19 individuals), and in 8/26 French samples (6/20 individuals). In addition, mycobacteria could be immuno-detected in perinodal adipose tissue of 1 out of 3 biopsy samples from individuals with active TB. In vitro, using a combination of adipose cell models, including the widely used murine adipose cell line 3T3-L1, as well as primary human adipocytes, we show that after binding to scavenger receptors, M. tuberculosis can enter within adipocytes, where it accumulates intracytoplasmic lipid inclusions and survives in a non-replicating state that is insensitive to the major anti-mycobacterial drug isoniazid.

Conclusions/Significance

Given the abundance and the wide distribution of the adipose tissue throughout the body, our results suggest that this tissue, among others, might constitute a vast reservoir where the tubercle bacillus could persist for long periods of time, and avoid both killing by antimicrobials and recognition by the host immune system. In addition, M. tuberculosis-infected adipocytes might provide a new model to investigate dormancy and to evaluate new drugs for the treatment of persistent infection.

Evaluación de la capacidad invasiva de las micobacterias no pigmentadas de crecimiento rápido mediante el estudio de la morfología de las microcolonias en fibroblastos

OBJECTIVE

The invasive capacity of rapidly-growing nonpigmented mycobacteria strains was evaluated by means of a fibroblast microcolony assay and related to the colony phenotype on Middlebrook 7H11 and to the clinical significance of the isolates.

METHODS

Twenty-nine strains [Mycobacterium chelonae (8), M. fortuitum (6), M. peregrinum (5), M. abscessus (5), M. mucogenicum (4) and M. septicum (1)], proceeding from a bacterial collection and clinical isolates, were evaluated. The smooth or rough phenotype of the colonies was assessed in Middlebrook 7H11 medium. Intracellular invasiveness was determined by the fibroblast-microcolony assay described by Shepard. Quantitative culture characteristics were compared with Student's t-test, and qualitative characteristics with Fisher's exact test.

RESULTS

No significant differences were found between colonies with different phenotypes or strains having a different clinical significance, except for two strains of Mycobacterium chelonae isolated from cases of catheter-related bacteremia, which showed elongated microcolonies. M. fortuitum and M. peregrinum strains showed larger microcolonies than M. chelonae, M. abscessus and M. mucogenicum, and displayed a fluffy appearance, while the latter two strains showed rounded colonies.

CONCLUSION

Very few strains of mycobacteria had invasive capacity and the majority of strains isolated from human infections do not show this characteristic; hence this trait is not essential for mycobacteria to cause infection in humans.

A mycobacterial virulence gene cluster extending RD1 is required for cytolysis, bacterial spreading and ESAT-6 secretion

Initiation and maintenance of infection by mycobacteria in susceptible hosts are not well understood. A screen of Mycobacterium marinum transposon mutant library led to isolation of eight mutants that failed to cause haemolysis, all of which had transposon insertions in genes homologous to a region between Rv3866 and Rv3881c in Mycobacterium tuberculosis, which encompasses RD1 (Rv3871-Rv3879c), a known virulence gene cluster. The M. marinum mutants showed decreased virulence in vivo and failed to secrete ESAT-6, like M. tuberculosis RD1 mutants. M. marinum mutants in genes homologous to Rv3866-Rv3868 also failed to accumulate intracellular ESAT-6, suggesting a possible role for those genes in synthesis or stability of the protein. These transposon mutants and an ESAT-6/CFP-10 deletion mutant all showed reduced cytolysis and cytotoxicity to macrophages and significantly decreased intracellular growth at late stages of the infection only when the cells were infected at low multiplicity of infection, suggesting a defect in spreading. Direct evidence for cell-to-cell spread by wild-type M. marinum was obtained by microscopic detection in macrophage and epithelial monolayers, but the mutants all were defective in this assay. Expression of M. tuberculosis homologues complemented the corresponding M. marinum mutants, emphasizing the functional similarities between M. tuberculosis and M. marinum genes in this region that we designate extRD1 (extended RD1). We suggest that diminished membranolytic activity and defective spreading is a mechanism for the attenuation of the extRD1 mutants. These results extend recent findings on the genomic boundaries and functions of M. tuberculosis RD1 and establish a molecular cellular basis for the role that extRD1 plays in mycobacterial virulence. Disruption of the M. marinum homologue of Rv3881c, not previously implicated in virulence, led to a much more attenuated phenotype in macrophages and in vivo, suggesting that this gene plays additional roles in M. marinum survival in the host.

Haemophilus ducreyi requires an intact flp gene cluster for virulence in humans

An intact Haemophilus ducreyi flp operon is essential for microcolony formation in vitro. tadA is the 9th of 15 genes in the operon and has homology to NTPases of type IV secretion systems. Fifteen human volunteers were experimentally infected with both H. ducreyi 35000HP and the tadA mutant, 35000HP.400. Papules developed at similar rates at sites inoculated with the mutant and parent, while pustules formed at 36.4% of parent sites and at 0% of mutant sites (P = 0.001). Compared to 35000HP, 35000HP.400 had only a modest but significant reduction in lesion scores in the temperature-dependent rabbit model of chancroid. These data suggest that proteins secreted by the flp locus are required for full expression of virulence by H. ducreyi in humans but have less of a role in virulence in an animal model of infection.

Mycobacterium marinum escapes from phagosomes and is propelled by actin-based motility

Mycobacteria are responsible for a number of human and animal diseases and are classical intracellular pathogens, living inside macrophages rather than as free-living organisms during infection. Numerous intracellular pathogens, including Listeria monocytogenes, Shigella flexneri, and Rickettsia rickettsii, exploit the host cytoskeleton by using actin-based motility for cell to cell spread during infection. Here we show that Mycobacterium marinum, a natural pathogen of fish and frogs and an occasional pathogen of humans, is capable of actively inducing actin polymerization within macrophages. M. marinum that polymerized actin were free in the cytoplasm and propelled by actin-based motility into adjacent cells. Immunofluorescence demonstrated the presence of host cytoskeletal proteins, including the Arp2/3 complex and vasodilator-stimulated phosphoprotein, throughout the actin tails. In contrast, Wiskott-Aldrich syndrome protein localized exclusively at the actin-polymerizing pole of M. marinum. These findings show that M. marinum can escape into the cytoplasm of infected macrophages, where it can recruit host cell cytoskeletal factors to induce actin polymerization leading to direct cell to cell spread.

Demonstration of spread by Mycobacterium tuberculosis bacilli in A549 epithelial cell monolayers

We developed an in vitro tissue-culture model to analyze the process involved in mycobacterial spread through lung epithelial cell monolayers. A549 cells were infected with low numbers of viable Mycobacterium tuberculosis bacilli expressing the gfp gene. Subsequent addition of a soft agarose overlay prevented the dispersal of the bacilli from the initial points of attachment. By fluorescence microscopy the bacteria were observed to infect and grow within the primary target cells; this was followed by lysis of the infected cells and subsequent infection of adjacent cells. This process repeated itself until an area of clearing (plaque formation) was observed. The addition of amikacin after initial infection did not prevent intracellular growth; however, subsequent plaque formation was not observed. Plaque formation was also observed after infection with Mycobacterium bovis BCG bacilli, but the plaques were smaller than those formed after infection with M. tuberculosis. These observations reinforce the possibility that cell-to-cell spreading of M. tuberculosis bacilli, particularly early in the course of infection within lung macrophages, pneumocytes, and other cells, may be an important component in the infectious process.

Interleukin-1 or tumor necrosis factor-alpha augmented the cytotoxic effect of mycobacteria on human fibroblasts: application to evaluation of pathogenesis of clinical isolates of Mycobacterium tuberculosis and M. avium complex

Mycobacteria-induced in vitro events reflecting human tuberculosis can contribute to the evaluation of the pathogenesis of Mycobacterium tuberculosis (MTB). In this study, we propose such an in vitro method based on live mycobacteria-induced cytotoxicity to human cell lines. When human lung-derived normal fibroblast cell line MRC-5 was infected with various strains of mycobacteria (M. tuberculosis H(37)Rv and H(37) Ra, Mycobacterium avium 427S and 2151SmO, and Mycobacterium bovis BCG Pasteur and Tokyo), the fibroblasts were killed by mycobacteria according to the degree of virulence. Other human originated macrophage (U-937, THP-1), myeloid (HL-60), and epithelial carcinoma (A549) cell lines exhibited a similar cytotoxic response to virulent mycobacteria. MRC-5 was most susceptible to virulent mycobacteria among various human cell lines examined. The cytotoxicity was enhanced by the proinflammatory cytokines, interleukin-1 (IL-1) and tumor necrosis factor-a (TNF-alpha), which in the absence of mycobacteria stimulate the growth of normal human fibroblasts. This in vitro evaluation system was applied to clinical isolates of drug-sensitive MTB (DS-MTB), drug-resistant MTB (DR-MTB) including multidrug-resistant (MDR-MTB), and M. avium complex (MAC). MTB strains (n = 24) exhibited strong cytotoxic activity, but MAC strains (n = 5) had only weak activity. Furthermore, there was no significant difference in cytotoxicity between DS-MTB (n = 11) and DR-MTB (n = 13). Collectively, these results suggest that this new in vitro system is useful for evaluating the pathogenesis of mycobacteria and that there was no difference in the pathogenesis between drug-susceptible and drug-resistant clinical isolates.

Necrosis of lung epithelial cells during infection with Mycobacterium tuberculosis is preceded by cell permeation

Mycobacterium tuberculosis establishes infection, progresses towards disease, and is transmitted from the alveolus of the lung. However, the role of the alveolar epithelium in any of these pathogenic processes of tuberculosis is unclear. In this study, lung epithelial cells (A549) were used as a model in which to examine cytotoxicity during infection with either virulent or avirulent mycobacteria in order to further establish the role of the lung epithelium during tuberculosis. Infection of A549 cells with M. tuberculosis strains Erdman and CDC1551 demonstrated significant cell monolayer clearing, whereas infection with either Mycobacterium bovis BCG or Mycobacterium smegmatis LR222 did not. Clearing of M. tuberculosis-infected A549 cells correlated to necrosis, not apoptosis. Treatment of M. tuberculosis-infected A549 cells with streptomycin, but not cycloheximide, demonstrated a significant reduction in the necrosis of A549 cell monolayers. This mycobacterium-induced A549 necrosis did not correlate to higher levels of intracellular or extracellular growth by the mycobacteria during infection. Staining of infected cells with propidium iodide demonstrated that M. tuberculosis induced increased permeation of A549 cell membranes within 24 h postinfection. Quantitation of lactate dehydrogenase (LDH) release from infected cells further demonstrated that cell permeation was specific to M. tuberculosis infection and correlated to A549 cellular necrosis. Inactivated M. tuberculosis or its subcellular fractions did not result in A549 necrosis or LDH release. These studies demonstrate that lung epithelial cell cytotoxicity is specific to infection by virulent mycobacteria and is caused by cellular necrosis. This necrosis is not a direct correlate of mycobacterial growth or of the expression of host cell factors, but is preceded by permeation of the A549 cell membrane and requires infection with live bacilli.

Virulent Mycobacterium tuberculosis strains evade apoptosis of infected alveolar macrophages

Human alveolar macrophages (AMphi) undergo apoptosis following infection with Mycobacterium tuberculosis in vitro. Apoptosis of cells infected with intracellular pathogens may benefit the host by eliminating a supportive environment for bacterial growth. The present study compared AMphi apoptosis following infection by M. tuberculosis complex strains of differing virulence and by Mycobacterium kansasii. Avirulent or attenuated bacilli (M. tuberculosis H37Ra, Mycobacterium bovis bacillus Calmette-Guérin, and M. kansasii) induced significantly more AMphi apoptosis than virulent strains (M. tuberculosis H37Rv, Erdman, M. tuberculosis clinical isolate BMC 96.1, and M. bovis wild type). Increased apoptosis was not due to greater intracellular bacterial replication because virulent strains grew more rapidly in AMphi than attenuated strains despite causing less apoptosis. These findings suggest the existence of mycobacterial virulence determinants that modulate the apoptotic response of AMphi to intracellular infection and support the hypothesis that macrophage apoptosis contributes to innate host defense in tuberculosis.

Preliminary characterization of a Mycobacterium abscessus mutant in human and murine models of infection

The ability to persist in the host after the establishment of infection is an important virulence determinant for mycobacteria. Mycobacterium abscessus is a rapidly growing mycobacterial species which causes a variety of clinical syndromes in humans. We have obtained a rough, wild-type human clinical isolate of M. abscessus (M. abscessus-R) and a smooth, attenuated mutant (M. abscessus-S) which spontaneously dissociated from the clinical isolate. We have found that M. abscessus-R is able to persist and multiply in a murine pulmonary infection model in contrast to M. abscessus-S, which is rapidly cleared. To understand the basis for this difference, we characterized the behavior of these variants in human tissue culture models of infection. M. abscessus-R is able to persist and multiply in human monocytes, while M. abscessus-S is deficient in this ability. Both of these variants are phagocytized by human monocytes. M. abscessus-R resides in a phagosome typical for pathogenic mycobacteria with a tightly adherent phagosomal membrane. In contrast, M. abscessus-S resides in a "loose" phagosome with the phagosomal membrane separated from the bacterial cell wall. Both M. abscessus variants also have distinctive growth patterns in a recently described fibroblast-mycobacterium microcolony assay, with M. abscessus-R exhibiting growth characteristics similar to those previously reported for virulent M. tuberculosis and M. abscessus-S exhibiting growth characteristics similar to those previously reported for avirulent M. tuberculosis. In both the monocyte infection assay and the murine pulmonary infection model, numerous infected mononuclear phagocyte aggregates develop at sites of M. abscessus-R infection, but are absent with M. abscessus-S infection. We conclude that a mutation has occurred in the M. abscessus-S variant which has altered the ability of this organism to persist and multiply in host cells and that this may be related to the phenotypic changes we have observed in our tissue culture models of infection.