Synthesis and release of sulfolipid by Mycobacterium avium during growth andcell division

New targets and inhibitors of mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress made in the development of inhibitors of sulfur metabolism enzymes.

Sulfate metabolism in mycobacteria

Pathogenic bacteria have developed numerous mechanisms to survive inside a hostile host environment. The human pathogen Mycobacterium tuberculosis (M. tb) is thought to control the human immune response with diverse biomolecules, including a variety of exotic lipids. One prevalent M. tb-specific sulfated metabolite, termed sulfolipid-1 (SL-1), has been correlated with virulence though its specific biological function is not known. Recent advances in our understanding of SL-1 biosynthesis will help elucidate the role of this curious metabolite in M. tb infection. Furthermore, the study of SL-1 has led to questions regarding the significance of sulfation in mycobacteria. Examples of sulfated metabolites as mediators of interactions between bacteria and plants suggest that sulfation is a key modulator of extracellular signaling between prokaryotes and eukaryotes. The discovery of novel sulfated metabolites in M. tb and related mycobacteria strengthens this hypothesis. Finally, mechanistic and structural data from sulfate-assimilation enzymes have revealed how M. tb controls the flux of sulfate in the cell. Mutants with defects in sulfate assimilation indicate that the fate of sulfur in M. tb is a critical survival determinant for the bacteria during infection and suggest novel targets for tuberculosis drug therapy.

Drug targets in mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress in the development of inhibitors of sulfur metabolism enzymes.

Identification, function and structure of the mycobacterial sulfotransferase that initiates sulfolipid-1 biosynthesis

Sulfolipid-1 (SL-1) is an abundant sulfated glycolipid and potential virulence factor found in Mycobacterium tuberculosis. SL-1 consists of a trehalose-2-sulfate (T2S) disaccharide elaborated with four lipids. We identified and characterized a conserved mycobacterial sulfotransferase, Stf0, which generates the T2S moiety of SL-1. Biochemical studies demonstrated that the enzyme requires unmodified trehalose as substrate and is sensitive to small structural perturbations of the disaccharide. Disruption of stf0 in Mycobacterium smegmatis and M. tuberculosis resulted in the loss of T2S and SL-1 formation, respectively. The structure of Stf0 at a resolution of 2.6 A reveals the molecular basis of trehalose recognition and a unique dimer configuration that encloses the substrate into a bipartite active site. These data provide strong evidence that Stf0 carries out the first committed step in the biosynthesis of SL-1 and establish a system for probing the role of SL-1 in M. tuberculosis infection.

Discovery of sulfated metabolites in mycobacteria with a genetic and mass spectrometric approach

The study of the metabolome presents numerous challenges, first among them being the cataloging of its constituents. A step in this direction will be the development of tools to identify metabolites that share common structural features. The importance of sulfated molecules in cell-cell communication motivated us to develop a rapid two-step method for identifying these metabolites in microorganisms, particularly in pathogenic mycobacteria. Sulfurcontaining molecules were initially identified by mass spectral analysis of cell extracts from bacteria labeled metabolically with a stable sulfur isotope (34SO 4 2-). To differentiate sulfated from reduced-sulfur-containing molecules, we employed a mutant lacking the reductive branch of the sulfate assimilation pathway. In these sulfur auxotrophs, heavy sulfate is channeled exclusively into sulfated metabolites. The method was applied to the discovery of several new sulfated molecules in Mycobacterium tuberculosis and Mycobacterium smegmatis. Because a sulfur auxotrophic strain is the only requirement of the approach, many microorganisms can be studied in this manner. Such genetic engineering in combination with stable isotopic labeling can be applied to various metabolic pathways and their products.

Sulfotransferases and sulfatases in mycobacteria

Analysis of the genomes of M. tuberculosis, M. leprae, M. smegmatis, and M. avium has revealed a large family of genes homologous to known sulfotransferases. Despite reports detailing a suite of sulfated glycolipids in many mycobacteria, a corresponding family of sulfotransferase genes remains uncharacterized. Here, a sequence-based analysis of newly discovered mycobacterial sulfotransferase genes, named stf1-stf10, is presented. Interestingly, two sulfotransferase genes are highly similar to mammalian sulfotransferases, increasing the list of mycobacterial eukaryotic-like protein families. The sulfotransferases join an equally complex family of mycobacterial sulfatases: a large family of sulfatase genes has been found in all of the mycobacterial genomes examined. As sulfated molecules are common mediators of cell-cell interactions, the sulfotransferases and sulfatases may be involved in regulating host-pathogen interactions.

The Mycobacterium avium complex

Mycobacterium avium complex (MAC) disease emerged early in the epidemic of AIDS as one of the common opportunistic infections afflicting human immunodeficiency virus-infected patients. However, only over the past few years has a consensus developed about its significance to the morbidity and mortality of AIDS. M. avium was well known to mycobacteriologists decades before AIDS, and the MAC was known to cause disease, albeit uncommon, in humans and animals. The early interest in the MAC provided a basis for an explosion of studies over the past 10 years largely in response to the role of the MAC in AIDS opportunistic infection. Molecular techniques have been applied to the epidemiology of MAC disease as well as to a better understanding of the genetics of antimicrobial resistance. The interaction of the MAC with the immune system is complex, and putative MAC virulence factors appear to have a direct effect on the components of cellular immunity, including the regulation of cytokine expression and function. There now is compelling evidence that disseminated MAC disease in humans contributes to both a decrease in the quality of life and survival. Disseminated disease most commonly develops late in the course of AIDS as the CD4 cells are depleted below a critical threshold, but new therapies for prophylaxis and treatment offer considerable promise. These new therapeutic modalities are likely to be useful in the treatment of other forms of MAC disease in patients without AIDS. The laboratory diagnosis of MAC disease has focused on the detection of mycobacteria in the blood and tissues, and although the existing methods are largely adequate, there is need for improvement. Indeed, the successful treatment of MAC disease clearly will require an early and rapid detection of the MAC in clinical specimens long before the establishment of the characteristic overwhelming infection of bone marrow, liver, spleen, and other tissue. Also, a standard method of susceptibility testing is of increasing interest and importance as new effective antimicrobial agents are identified and evaluated. Antimicrobial resistance has already emerged as an important problem, and methods for circumventing resistance that use combination therapies are now being studied.

Gamma interferon reverses inhibition of leukocyte bactericidal activity by a 25-kilodalton fraction from Mycobacterium tuberculosis

In this study we examined the effects of Mycobacterium tuberculosis cell extracts on the phagocytic activity of polymorphonuclear leukocytes and cultured peripheral blood monocytes. M. tuberculosis cell extracts were fractionated on Sephacryl S-200 columns, and a 25-kilodalton glycolipoprotein was shown to inhibit the intracellular killing ability of these leukocytes but had no effect on their phagocytic potential. This same fraction inhibited fusion of phagosomes with lysosomes, as assessed by noting the transfer of acridine orange from lysosomes to phagosomes. This fraction was shown to have a maximal inhibitory effect when it was in the form of an intact carbohydrate-lipid-protein complex. Gamma interferon (IFN-gamma), but not IFN-alpha, reversed the inhibitory effect of the mycobacterial component on bactericidal activity and on fusion of phagosomes and lysosomes. Thus, this 25-kilodalton fraction of M. tuberculosis cell extract may be important in protecting organisms against phagocytic degradation, an effect which can be reversed by IFN-gamma.

Comparison of 15 laboratory and patient-derived strains of Mycobacterium avium for ability to infect and multiply in cultured human macrophages

Mycobacterium avium is a cause of nontuberculous chronic granulomatous infections which is attracting increased attention as a frequent opportunistic pathogen in acquired immunodeficiency syndrome. Some important aspects of its human pathogenicity were investigated by using cultured human macrophages infected with it. The uptake and replication of various strains of M. avium in the macrophages could be measured by CFU counts of the bacteria in samples of lysed, sonicated macrophages. Microscopic counts of acid-fast bacilli were not useful because the bacteria multiplying in the macrophages were usually not acid fast. Electron microscopy showed the intracellular bacilli to multiply by transverse fission, to be surrounded in individual vacuoles by a broad electronlucent zone, and to have thinner cell walls than extracellularly grown M. avium. Fifteen strains, including examples of serovars 1, 2, 4, 8, and 9, were studied for uptake and rate of replication in cultured macrophages from three normal subjects. The strains were isolates from patients with nontuberculous granulomatous infection, acquired immunodeficiency syndrome, or unrelated problems, or they were laboratory reference cultures. There were no differences among them in phagocytosis, but there were differences in intracellular replication. Laboratory strains tended to be avirulent, that is, they did not replicate in the macrophages. Patient isolates usually were virulent and could be compared for virulence by intracellular replication rates. Virulence correlated with flat, transparent bacterial colony morphology on nutrient agar but not with serovar or kind of patient from whom the bacteria were isolated. However, among strains of transparent colony morphology there were wide differences in virulence. A virulent bacilli generally produced domed, opalescent colonies on nutrient agar. A virulent bacilli predominated in populations of M. avium conditioned to growth in bacteriologic culture medium. Bacilli of virulent colony morphology predominated in populations passaged through cultured macrophages. The model described here presents a new approach to the investigation of the pathogenicity of M. avium for human subjects and may be more patient relevant than animal models.

Isolation of deoxyribonucleic acid from Mycobacterium avium by rapid nitrogen decompression

Deoxyribonucleic acid (DNA) of high molecular weight could be isolated from cells of Mycobacterium avium if the cells were exposed to nitrogen gas at 1,500 lb/in2 for 30 min and then brought to atmospheric pressure by rapid decompression. DNA isolated from the cells had a molecular weight of 4.8 x 10(6) to 17.4 x 10(6). DNA was also released into the fluid in which the cells were suspended during nitrogen decompression. One-half of this DNA, representing 3% of the total DNA phosphorus in the cells had a uniform molecular weight of 4.2 x 10(6). This DNA was linear in conformation, and removal of associated carbohydrates did not change its sedimentation rate. The biological function or significance of the 4-megadalton DNA was not determined.

Effect of cerulenin on Streptococcus faecalis macromolecular synthesis and cell division

The antibiotic cerulenin has been used to study macromolecular synthesis and cell division in Streptococcus faecalis. The data suggest that lipid and lipoteichoic acid synthesis as well as cell number increase are affected prior to any observable effects on overall mass increase or DNA, RNA, protein, or peptidoglycan synthesis. Treatment with cerulenin of cultures growing at various rates and analysis of the subsequent cell divisions indicate that the antibiotic may block a cell cycle event that precedes the completion of chromosome replication by about 10 min.

Ammonium ion requirement for the cell cycle of Mycobacterium avium

Mycobacterium avium has a defined cell cycle in which small cells elongate to about five times their original length and then divide by fragmentation. The nitrogen requirement for production of maximal number of colony-forming units was assessed by varying concentrations and kinds of nitrogen source in the medium. Ferric ammonium citrate at a concentration in 7H10 medium of 0.17 mumol/ml or ammonium chloride at 0.25 mumol/ml as the nitrogen source permitted the cells to elongate and to undergo limited division, with the final culture at 4 x 10(7) colony-forming units per ml. Ammonium chloride at 2.5 mumol/ml or glutamine at 1.37 mumol/ml supported completion of the cell cycle with final colony-forming units at about 5 x 10(8)/ml. Other amino acids, including glutamic acid, at 2.5 mumol/ml did not support completion of the cell cycle, although in most cases an intermediate number of colony-forming units per milliliter were formed. Limited uptake of [(14)C]glutamic acid and uptake of [(14)C]glutamine were not detectable until cell fission began. Cells not limited for nitrogen took up five times as much (35)S during fission as limited cells did during the same time. The nonlimited cells contained 10 times as much sulfolipid as the nitrogen-limited cells at the end of the cell cycle. These results demonstrate that rapidly dividing cells of M. avium utilize amino acids and sulfur and also synthesize sulfolipids in events that are apparently separable from metabolic functions of elongating cells. The results are contrasted with those found for other mycobacteria in which no cell cycle has been demonstrated.