VIRULENCE AND MORPHOLOGICAL CHARACTERISTICS OF MAMMALIAN TUBERCLE BACILLI

Demonstration of cord formation by rough Mycobacterium abscessus variants: implications for the clinical microbiology laboratory

In low-income countries some infections caused by nontuberculous mycobacteria are misdiagnosed as multidrug-resistant tuberculosis. In most of these settings the observation of microscopic cords is the only technique used to identify Mycobacterium tuberculosis in the laboratory. In this article we definitively demonstrate that Mycobacterium abscessus, an emerging pulmonary pathogen, also forms microscopic cords.

Biosynthesis and Virulent Behavior of Lipids Produced by Mycobacterium tuberculosis: LAM and Cord Factor: An Overview

Mycobacterium tuberculosis is the causative agent of tuberculosis disease, which has developed a myriad of exceptional features contributing to its survival within the hostile environment of host cell. Unique cell wall structure with high lipid content plays an imperative role in the pathogenicity of mycobacteria. Cell wall components of MTB such as lipoarabinomannan and Trehalose dimycolate (cord factor) are virulent in nature apart from its virulence genes. Virulent effect of these factors on host cells reduces host cell immunity. LAM has been known to inhibit phagosome maturation by inhibiting the Ca²⁺/calmodulin phosphatidyl inositol-3-kinase hvps34 pathways. Moreover, TDM (Trehalose dimycolate) also inhibits fusion between phospholipid vesicles and migration of polymorphonuclear neutrophils. The objective of this paper is to understand the virulence of LAM and cord factor on host cell which might be helpful to design an effective drug against tuberculosis.

PRODUCTION OF BCG VACCINE IN A LIQUID MEDIUM CONTAINING TWEEN 80 AND A SOLUBLE FRACTION OF HEATED HUMAN SERUM : I. PRODUCTION AND VIABILITY OF THE CULTURE

Diffuse, submerged growth of BCG bacilli has been obtained in liquid media containing 0.02 per cent Tween 80 and the soluble fraction of human serum heated under acid conditions (pH 2.5) at 65°C. In the absence of glucose or glycerine,—which had a detrimental effect on viability—these cultures consisted predominantly of cells that were living and that remained viable during prolonged storage at temperatures ranging from 4 to 37°C.

INFECTION OF MICE WITH MAMMALIAN TUBERCLE BACILLI GROWN IN TWEEN-ALBUMIN LIQUID MEDIUM

Introduction of the bacilli by the mtravenous route or by feeding gives rise to a disease predominantly localized in the lungs. Following intracerebral infection, the bacilli first multiply rapidly in the brain tissue, and then invade other organs, producing lesions especially in the lungs. Injection of the bacilli by the intraperitoneal route is less effective than by either the intravenous or intracerebral routes; however, admixture of the bacilli with some of the components of egg yolk increases both the infectivity and the pulmonary localization. Different strains of mice differ markedly in their susceptibility to experimental tuberculous infection; the highest susceptibility was observed among the pigmented strains (line 1 dba and C57 black). Greater resistance does not appear to depend on the ability to prevent the establishment of infection, but rather corresponds to a slower rate of progression of the infectious process. It is possible to produce in mice tuberculosis presenting any desired degree of acuteness or chronicity by controlling certain factors which condition the initiation and the progression of the infection.

Enzymatic hydrolysis of trehalose dimycolate releases free mycolic acids during mycobacterial growth in biofilms

Mycobacterial species, like other microbes, spontaneously form multicellular drug-tolerant biofilms when grown in vitro in detergent-free liquid media. The structure of Mycobacterium tuberculosis biofilms is formed through genetically programmed pathways and is built upon a large abundance of novel extracellular free mycolic acids (FM), although the mechanism of FM synthesis remained unclear. Here we show that the FM in Mycobacterium smegmatis biofilms is produced through the enzymatic release from constitutively present mycolyl derivatives. One of the precursors for FM is newly synthesized trehalose dimycolate (TDM), which is cleaved by a novel TDM-specific serine esterase, Msmeg_1529. Disruption of Msmeg_1529 leads to undetectable hydrolytic activity, reduced levels of FM in the mutant, and retarded biofilm growth. Furthermore, enzymatic hydrolysis of TDM remains conserved in M. tuberculosis, suggesting the presence of a TDM-specific esterase in this pathogen. Overall, this study provides the first evidence for an enzymatic release of free mycolic acids from cell envelope mycolates during mycobacterial growth.

Redundant function of cmaA2 and mmaA2 in Mycobacterium tuberculosis cis cyclopropanation of oxygenated mycolates

The Mycobacterium tuberculosis cell envelope contains a wide variety of lipids and glycolipids, including mycolic acids, long-chain branched fatty acids that are decorated by cyclopropane rings. Genetic analysis of the mycolate methyltransferase family has been a powerful approach to assign functions to each of these enzymes but has failed to reveal the origin of cis cyclopropanation of the oxygenated mycolates. Here we examine potential redundancy between mycolic acid methyltransferases by generating and analyzing M. tuberculosis strains lacking mmaA2 and cmaA2, mmaA2 and cmaA1, or mmaA1 alone. M. tuberculosis lacking both cmaA2 and mmaA2 cannot cis cyclopropanate methoxymycolates or ketomycolates, phenotypes not shared by the mmaA2 and cmaA2 single mutants. In contrast, a combined loss of cmaA1 and mmaA2 had no effect on mycolic acid modification compared to results with a loss of mmaA2 alone. Deletion of mmaA1 from M. tuberculosis abolishes trans cyclopropanation without accumulation of trans-unsaturated oxygenated mycolates, placing MmaA1 in the biosynthetic pathway for trans-cyclopropanated oxygenated mycolates before CmaA2. These results define new functions for the mycolic acid methyltransferases of M. tuberculosis and indicate a substantial redundancy of function for MmaA2 and CmaA2, the latter of which can function as both a cis and trans cyclopropane synthase for the oxygenated mycolates.

The lta4h locus modulates susceptibility to mycobacterial infection in zebrafish and humans

Exposure to Mycobacterium tuberculosis produces varied early outcomes, ranging from resistance to infection to progressive disease. Here we report results from a forward genetic screen in zebrafish larvae that identify multiple mutant classes with distinct patterns of innate susceptibility to Mycobacterium marinum. A hypersusceptible mutant maps to the lta4h locus encoding leukotriene A(4) hydrolase, which catalyzes the final step in the synthesis of leukotriene B(4) (LTB(4)), a potent chemoattractant and proinflammatory eicosanoid. lta4h mutations confer hypersusceptibility independent of LTB(4) reduction, by redirecting eicosanoid substrates to anti-inflammatory lipoxins. The resultant anti-inflammatory state permits increased mycobacterial proliferation by limiting production of tumor necrosis factor. In humans, we find that protection from both tuberculosis and multibacillary leprosy is associated with heterozygosity for LTA4H polymorphisms that have previously been correlated with differential LTB(4) production. Our results suggest conserved roles for balanced eicosanoid production in vertebrate resistance to mycobacterial infection.

Microscopic cords, a virulence-related characteristic of Mycobacterium tuberculosis, are also present in nonpathogenic mycobacteria

The aggregation of mycobacterial cells in a definite order, forming microscopic structures that resemble cords, is known as cord formation, or cording, and is considered a virulence factor in the Mycobacterium tuberculosis complex and the species Mycobacterium marinum. In the 1950s, cording was related to a trehalose dimycolate lipid that, consequently, was named the cord factor. However, modern techniques of microbial genetics have revealed that cording can be affected by mutations in genes not directly involved in trehalose dimycolate biosynthesis. Therefore, questions such as "How does mycobacterial cord formation occur?" and "Which molecular factors play a role in cord formation?" remain unanswered. At present, one of the problems in cording studies is the correct interpretation of cording morphology. Using optical microscopy, it is sometimes difficult to distinguish between cording and clumping, which is a general property of mycobacteria due to their hydrophobic surfaces. In this work, we provide a new way to visualize cords in great detail using scanning electron microscopy, and we show the first scanning electron microscopy images of the ultrastructure of mycobacterial cords, making this technique the ideal tool for cording studies. This technique has enabled us to affirm that nonpathogenic mycobacteria also form microscopic cords. Finally, we demonstrate that a strong correlation exists between microscopic cords, rough colonial morphology, and increased persistence of mycobacteria inside macrophages.

TB research at UT-Houston--a review of cord factor: new approaches to drugs, vaccines and the pathogenesis of tuberculosis

Tuberculosis remains a major threat as drug resistance continues to increase. Pulmonary tuberculosis in adults is responsible for 80% of clinical cases and nearly 100% of transmission of infection. Unfortunately, since we have no animal models of adult type pulmonary tuberculosis, the most important type of disease remains largely out of reach of modern science and many fundamental questions remain unanswered. This paper reviews research dating back to the 1950's providing compelling evidence that cord factor (trehalose 6,6 dimycolate [TDM]) is essential for understanding tuberculosis. However, the original papers by Bloch and Noll were too far ahead of their time to have immediate impact. We can now recognize that the physical and biologic properties of cord factor are unprecedented in science, especially its ability to switch between two sets of biologic activities with changes in conformation. While TDM remains on organisms, it protects them from killing within macrophages, reduces antibiotic effectiveness and inhibits the stimulation of protective immune responses. If it comes off organisms and associates with lipid, TDM becomes a driver of tissue damage and necrosis. Studies emanating from cord factor research have produced (1) a rationale for improving vaccines, (2) an approach to new drugs that overcome natural resistance to antibiotics, (3) models of caseating granulomas that reproduce multiple manifestations of human tuberculosis. (4) evidence that TDM is a key T cell antigen in destructive lesions of tuberculosis, and (5) a new understanding of the pathology and pathogenesis of postprimary tuberculosis that can guide more informative studies of long standing mysteries of tuberculosis.

Specific serum agglutination of erythrocytes sensitized with extracts of tubercle bacilli

A hemagglutination reaction has been described between sheep erythrocytes treated with a component of a polysaccharide fraction of mammalian tubercle bacilli and the sera of experimental animals or of tuberculous patients. Evidence has been presented for the specificity of this reaction. A modification of the test, involving an inhibition reaction, has been developed for the detection and quantitation of minute amounts of the material responsible for the hemagglutination reaction.

The effect of chick embryo extract on the growth and morphology of tubercle bacilli

Aqueous extracts of 11-day-old chick embryos enhance the growth of tubercle bacilli in oleic acid-albumin media. These extracts also increase the tendency of virulent strains to exhibit the serpentine pattern of growth and confer this property on avirulent variants which normally grow unoriented in clumps. Growth in the presence of chick embryo extract slightly increases the virulence of the virulent strains but does not confer virulence on the avirulent variants.

The effect of wetting agents on the growth of tubercle bacilli

Tween 80 and Triton A20 are two water-dispersible, non-ionic, surface-active agents which favor dispersed growth of tubercle bacilli in aqueous media probably by wetting the bacterial surface. Tween 80 is a polyoxyethylene ester of sorbitan monooleate and is liable to enzymatic hydrolysis by lipases. Triton A20 in an arylalkyl polyether of phenol which appears resistant to the known enzymes of animal tissues. Tween 80 loses its ability to disperse cultures of tubercle bacilli in media containing serum; Triton A20 does not. Tween 80 increases the yield of growth, probably by supplying oleic acid to the bacilli; Triton A20 does not. In concentrations sufficient to cause dispersed browth, Tween 80 (purified by removal of unesterified fatty acid) and Triton A20 are completely innocuous for virulent tubercle bacilli. However, Triton A20 exhibits a marked toxic effect on the avirulent variants of mammalian strains; Tween 80 does not. The two wetting agents also differ in their effects on the morphological aspects of the bacterial cultures. Whereas Triton A20 prevents the formation of large amorphous bacillary clumps, it is less effective in preventing the orientation of the virulent bacilli resulting in the formation of long bacillary strands. Tween 80 on the contrary prevents also the formation of these cords of bacilli and exerts therefore a more effective dispersing effect on cultures of virulent tubercle bacilli.

The Mycobacterium tuberculosis phoPR operon is positively autoregulated in the virulent strain H37Rv

The attenuated Mycobacterium tuberculosis H37Ra strain is an isogenic counterpart of the virulent paradigm strain H37Rv. Recently, a link between a point mutation in the PhoP transcriptional regulator and avirulence of H37Ra was established. Remarkably, a previous study demonstrated negative autoregulation of the phoP gene in H37Ra. These findings led us to study the transcriptional autoregulation of PhoP in the virulent H37Rv strain. In contrast to the negative autoregulation of PhoP previously published for H37Ra, our experiments using a phoP promoter-lacZ fusion showed that PhoP is positively autoregulated in both H37Rv and H37Ra compared with an H37Rv phoP deletion mutant constructed in this study. Using quantitative reverse transcription-PCR (RT-PCR) analysis, we showed that the phoP gene is transcribed at similar levels in H37Rv and H37Ra. Gel mobility shift and DNase I footprinting assays allowed us to identify the precise binding region of PhoP from H37Rv to the phoP promoter. We also carried out RT-PCR studies to demonstrate that phoP is transcribed together with the adjacent gene phoR, which codes for the cognate histidine kinase of the phoPR two-component system. In addition, quantitative RT-PCR studies showed that phoR is independently transcribed from a promoter possibly regulated by PhoP. Finally, we discuss the possible role in virulence of a single point mutation found in the phoP gene from the attenuated H37Ra strain but not in virulent members of the M. tuberculosis complex.

Tumor necrosis factor signaling mediates resistance to mycobacteria by inhibiting bacterial growth and macrophage death

Tumor necrosis factor (TNF), a key effector in controlling tuberculosis, is thought to exert protection by directing formation of granulomas, organized aggregates of macrophages and other immune cells. Loss of TNF signaling causes progression of tuberculosis in humans, and the increased mortality of Mycobacterium tuberculosis-infected mice is associated with disorganized necrotic granulomas, although the precise roles of TNF signaling preceding this endpoint remain undefined. We monitored transparent Mycobacterium marinum-infected zebrafish live to conduct a stepwise dissection of how TNF signaling operates in mycobacterial pathogenesis. We found that loss of TNF signaling caused increased mortality even when only innate immunity was operant. In the absence of TNF, intracellular bacterial growth and granuloma formation were accelerated and was followed by necrotic death of overladen macrophages and granuloma breakdown. Thus, TNF is not required for tuberculous granuloma formation, but maintains granuloma integrity indirectly by restricting mycobacterial growth within macrophages and preventing their necrosis.

Control of M. tuberculosis ESAT-6 secretion and specific T cell recognition by PhoP

Analysis of mycobacterial strains that have lost their ability to cause disease is a powerful approach to identify yet unknown virulence determinants and pathways involved in tuberculosis pathogenesis. Two of the most widely used attenuated strains in the history of tuberculosis research are Mycobacterium bovis BCG (BCG) and Mycobacterium tuberculosis H37Ra (H37Ra), which both lost their virulence during in vitro serial passage. Whereas the attenuation of BCG is due mainly to loss of the ESAT-6 secretion system, ESX-1, the reason why H37Ra is attenuated remained unknown. However, here we show that a point mutation (S219L) in the predicted DNA binding region of the regulator PhoP is involved in the attenuation of H37Ra via a mechanism that impacts on the secretion of the major T cell antigen ESAT-6. Only H37Ra “knock-ins” that carried an integrated cosmid with the wild-type phoP gene from M. tuberculosis H37Rv showed changes in colony morphology, increased virulence, ESAT-6 secretion, and induction of specific T cell responses, whereas other H37Ra constructs did not. This finding established a link between the PhoP regulator and ESAT-6 secretion that opens exciting new perspectives for elucidating virulence regulation in M. tuberculosis.

Mycobacterium tuberculosis, the causative agent of human tuberculosis is an extremely successful human pathogen in spite of its lack of classical virulence factors, such as toxins. The pathogenesis of this bacterium is closely linked with its ability to circumvent destruction by the host cell, which depends on a large variety of mycobacterial lipids and secreted proteins. Genome comparison of fully virulent strains with closely related, but attenuated strains that have lost their ability to cause disease is a powerful approach to identify factors contributing to mycobacterial virulence. In this study we have compared the virulent paradigm strain of tuberculosis research M. tuberculosis H37Rv, with the widely used, attenuated laboratory variant, H37Ra. This analysis, combined with gene complementation studies, has identified a mutation in the two-component system regulator PhoP that impacts on the secretion and T cell recognition of the 6-kD early-secreted antigenic target ESAT-6. In previous studies, both PhoP and the ESAT-6 secretion system ESX-1 have been identified independently as major virulence determinants of M. tuberculosis, so the finding that their action is interconnected opens exciting new insights and perspectives into the virulence regulation of tubercle bacilli.

Genetic basis of virulence attenuation revealed by comparative genomic analysis of Mycobacterium tuberculosis strain H37Ra versus H37Rv

Tuberculosis, caused by Mycobacterium tuberculosis, remains a leading infectious disease despite the availability of chemotherapy and BCG vaccine. The commonly used avirulent M. tuberculosis strain H37Ra was derived from virulent strain H37 in 1935 but the basis of virulence attenuation has remained obscure despite numerous studies. We determined the complete genomic sequence of H37Ra ATCC25177 and compared that with its virulent counterpart H37Rv and a clinical isolate CDC1551. The H37Ra genome is highly similar to that of H37Rv with respect to gene content and order but is 8,445 bp larger as a result of 53 insertions and 21 deletions in H37Ra relative to H37Rv. Variations in repetitive sequences such as IS6110 and PE/PPE/PE-PGRS family genes are responsible for most of the gross genetic changes. A total of 198 single nucleotide variations (SNVs) that are different between H37Ra and H37Rv were identified, yet 119 of them are identical between H37Ra and CDC1551 and 3 are due to H37Rv strain variation, leaving only 76 H37Ra-specific SNVs that affect only 32 genes. The biological impact of missense mutations in protein coding sequences was analyzed in silico while nucleotide variations in potential promoter regions of several important genes were verified by quantitative RT-PCR. Mutations affecting transcription factors and/or global metabolic regulations related to in vitro survival under aging stress, and mutations affecting cell envelope, primary metabolism, in vivo growth as well as variations in the PE/PPE/PE-PGRS family genes, may underlie the basis of virulence attenuation. These findings have implications not only for improved understanding of pathogenesis of M. tuberculosis but also for development of new vaccines and new therapeutic agents.

Cell population heterogeneity in Mycobacterium tuberculosis H37Rv

The laboratory strain H37Rv represents one of the most commonly used strains in the study of Mycobacterium tuberculosis. Despite the apparent stability of the strain, the absence of a selective pressure for virulence factors could lead to the in vitro accumulation of attenuated mutants. To assess this hypothesis, we performed a systematic analysis of individual clones isolated from subcultured M. tuberculosis H37Rv and from a non-subcultured frozen stock. First, we studied two virulence indicators: neutral red staining and content of phthiocerol dimycocerosates (PDIMs). We found that H37Rv formed a mixed population containing wild-type cells, as well as neutral red and PDIM mutants. Then, we compared the global gene expression of 3 isolated clones (which displayed various phenotypes) and the non-subcultured stock, by microarray analysis. This transcriptional profiling confirmed that a significant heterogeneity existed despite, and in addition to, the neutral red and PDIM phenotypes. These results strongly suggest that great caution must be taken in extrapolating data obtained with M. tuberculosis H37Rv grown in vitro, and it would be prudent to study several independent clones to obtain valid conclusions. For this purpose, the neutral red and PDIM phenotypes might be useful indicators of undesired heterogeneity.

Experimental disseminated encephalomyelitis in white mice

Disseminated encephalomyelitis was readily induced in mice of the Swiss strain by means of repeated intramuscular and subcutaneous injections of apparently normal mouse brain mixed with an adjuvant. The latter consisted of autoclaved virulent tubercle bacilli and heavy liquid petrolatum, a modification of the Freund adjuvant. The syndrome and the histopathological picture of the induced malady were essentially similar to those in monkeys, rabbits, and guinea pigs, previously reported by others. Certain exceptional characteristics of the affection, as occurring in mice, suggest that they may be the animals of choice for its study as well as for that of other encephalitides. Not only were the signs indicative of marked involvement of the central nervous system but also of the respiratory mechanism, and only a few injections of mouse brain-adjuvant mixture were required to evoke the neurological symptom complex in almost every animal.

A point mutation in the two-component regulator PhoP-PhoR accounts for the absence of polyketide-derived acyltrehaloses but not that of phthiocerol dimycocerosates in Mycobacterium tuberculosis H37Ra

Similarities between Mycobacterium tuberculosis phoP-phoR mutants and the attenuated laboratory strain M. tuberculosis H37Ra in terms of morphological and cytochemical properties, lipid content, gene expression and virulence attenuation prompted us to analyze the functionality of this two-component regulator in the latter strain. Sequence analysis revealed a base substitution resulting in a one-amino-acid change in the likely DNA-binding region of PhoP in H37Ra relative to H37Rv. Using gel-shift assays, we show that this mutation abrogates the ability of the H37Ra PhoP protein to bind to a 40-bp segment of its own promoter. Consistent with this result, the phoP gene from H37Rv but not that from H37Ra was able to restore the synthesis of sulfolipids, diacyltrehaloses and polyacyltrehaloses in an isogenic phoP-phoR knock-out mutant of M. tuberculosis Moreover, complementation of H37Ra with phoP from H37Rv fully restored sulfolipid, diacyltrehalose and polyacyltrehalose synthesis, clearly indicating that the lack of production of these lipids in H37Ra is solely due to the point mutation in phoP. Using a pks2-3/4 knock-out mutant of M. tuberculosis H37Rv, evidence is further provided that the above-mentioned polyketide-derived acyltrehaloses do not significantly contribute to the virulence of the tubercle bacillus in a mouse model of infection. Reasons for the attenuation of H37Ra thus most likely stand in other virulence factors, many of which are expected to belong to the PhoP regulon and another of which, unrelated to PhoP, appears to be the lack of production of phthiocerol dimycocerosates in this strain.

Molecular and supra-molecular structure related differences in toxicity and granulomatogenic activity of mycobacterial cord factor in mice

To establish the structure biological activity relationship of cord factor (trehalose 6,6'-dimycolate, TDM), we compared the molecular or supra-molecular structure of TDM micelles with toxicity, thymic atrophy and granulomatogenicity in lungs and spleen of BALB/c mice. According to the difference in the mycolyl subclass composition, TDM was divided into two groups, one possessing alpha-, methoxy- and keto-mycolates in M. tuberculosis H37Rv, M. bovis BCG and M. kansasii (group A) and the other having alpha-, keto- and wax ester-mycolates in M. avium serotype 4, M. phlei and M. flavescens (group B), although mycolic acid molecular species composition differed in each group considerably. Supra-molecular structure of TDM micelle differed species to species substantially and the micelle size of TDM from M. bovis BCG Connaught was the largest. The highest toxicity was shown with TDM from M. tuberculosis H37Rv which possessed the highest amount of alpha- (47.3%) and methoxy-mycolates (40.8%), while TDM from M. phlei having the low amount of alpha-mycolate (11.6%) showed almost no toxicity with the given doses. The thymic atrophy was observed with TDM from group A, but not with TDM from group B. On the other hand, TDM from group B showed massive lung granulomatogenic activity based on the histological observations and organ indices. Taken together, group A TDM showed a wide variety of micelle sizes and specific surface areas, high to low toxicity and marked to moderate granulomatogenicity, while group B TDM showed smaller sizes of micelles and larger specific surface areas, lower toxicity but higher granulomatogenicity in lungs. Existence of higher amount of longer chain alpha-mycolates in TDM appeared to be essential for high toxicity and thymic apoptotic activity, whereas TDM possessing wax ester-mycolate with smaller sized micelles seemed to be less toxic, but more granulomatogenic in lungs in mice. Thus, the mycolic acid subclass and molecular species composition of TDM affect critically the micelle forms, toxicity and granulomatogenicity in mice, while the relative abundances and carbon chain length of alpha-mycolate affected the toxicity in mice.

Why is long-term therapy required to cure tuberculosis?

The authors argue that understanding and countering general bacterial mechanisms of phenotypic antibiotic resistance may hold the key to reducing the duration of treatment of all recalcitrant bacterial infections, including tuberculosis.

Versatile polyketide enzymatic machinery for the biosynthesis of complex mycobacterial lipids

The cell envelope of Mycobacterium tuberculosis (Mtb) is a treasure house of a variety of biologically active molecules with fascinating architectures. The decoding of the genetic blueprint of Mtb in recent years has provided the impetus for dissecting the metabolic pathways involved in the biosynthesis of lipidic metabolites. The focus of the Highlight is to emphasize the functional role of polyketide synthase (PKS) proteins in the biosynthesis of complex mycobacterial lipids. The catalytic as well as mechanistic versatility of PKS. in generating metabolic diversity and the significance of recently discovered fatty acyl-AMP ligases in establishing "biochemical crosstalk" between fatty acid synthases (FASs) and PKSs is described. The phenotypic heterogeneity and remodeling of the mycobacterial cell wall in its aetiopathogenesis is discussed.

Long-chain multiple methyl-branched fatty acid-containing lipids of Mycobacterium tuberculosis: biosynthesis, transport, regulation and biological activities

The cell envelope of pathogenic mycobacteria is highly distinctive in that it contains a number of lipids esterified with structurally related long-chain multi-methyl-branched fatty acids. These lipids have long been thought to play important roles in the cell envelope structure as well as in the pathogenicity of the tubercle bacillus. This review summarizes what is known about the biosynthesis of long-chain multiple methyl-branched fatty acid-containing lipids in Mycobacterium tuberculosis and describes the most recent findings about their regulation, transport across the different layers of the cell envelope and their biological functions.

Spontaneous reversion of Mycobacterium abscessus from a smooth to a rough morphotype is associated with reduced expression of glycopeptidolipid and reacquisition of an invasive phenotype

Mycobacterium abscessus is an increasingly important cause of human disease; however, virulence determinants are largely uncharacterized. Previously, it was demonstrated that a rough, wild-type human clinical isolate (390R) causes persistent, invasive infection, while a smooth isogenic mutant (390S) has lost this capability. During serial passage of 390S, a spontaneous rough revertant was obtained, which was named 390V. This revertant regained the ability to cause persistent, invasive infection in human monocytes and the lungs of mice. Glycopeptidolipid (GPL), which plays a role in environmental colonization, was present in abundance in the cell wall of 390S, and was associated with sliding motility and biofilm formation. In contrast, a marked reduction in the amount of GPL in the cell wall of 390R and 390V was correlated with cord formation, a property associated with mycobacterial virulence. These results indicate that the ability to switch between smooth and rough morphologies may allow M. abscessus to transition between a colonizing phenotype and a more virulent, invasive form.

Of mice, men, and elephants: Mycobacterium tuberculosis cell envelope lipids and pathogenesis

Mycolic acids and structures attached to them constitute a major part of the protective envelope of Mycobacterium tuberculosis, and for this reason, their role in tuberculosis pathogenesis has been extensively studied. In this issue of the JCI, Rao et al. examine the effect of trans-cyclopropanation of oxygenated mycolic acids attached to trehalose dimycolate (TDM) on the murine immune response to infection (see the related article beginning on page 1660). Surprisingly, they found that an M. tuberculosis mutant lacking trans-cyclopropane rings was hypervirulent in mice. The recent recognition of a hypervirulence phenotype in mice associated with laboratory and clinical M. tuberculosis strains with altered cell wall components has provided new insights into how M. tuberculosis may establish persistent infection. However, to date, characterization of these bioactive products in pathogenesis has been largely reductionistic; the relationship of their effects observed in mice to the persistent infection and tuberculosis caused by M. tuberculosis observed in humans remains obscure.

Mycobacterial sulfolipid shows a virulence by inhibiting cord factor induced granuloma formation and TNF-alpha release

Virulence mechanism of infection with Mycobacterium tuberculosis is currently focused to be clarified in the context of cell surface lipid molecule. Comparing two mycobacterial glycolipids, we observed toxicity and prominent granulomatogenic activity of trehalose 6,6'-dimycolate (TDM) injection in mice, evident by delayed body weight gain and histological observations, whereas 2,3,6,6'-tetraacyl trehalose 2'-sulfate (SL) was non-toxic and non-granulomatogenic. Likewise, TDM but not SL caused temporarily, but marked increase of lung indices, indicative of massive granuloma formation. Interestingly, co-administration of TDM and SL prevented these symptoms distinctively and SL inhibited TDM-induced release of tumor necrosis factor alpha (TNF-alpha) in a dose-dependent manner. Histological findings and organ index changes also showed marked inhibition of TDM induced granuloma formation by co-administration of SL. Simultaneous injection of SL together with TDM was highly effective for this protection, as neither injection 1h before nor after TDM injection showed highly inhibitory. In parallel studies on a cellular level, TDM elicited strong TNF-alpha release from alveolar but not from peritoneal macrophages in vitro. This effect was blocked when alveolar macrophages were incubated in wells simultaneously coated with TDM and SL, indicating that SL suppresses TDM-induced TNF-alpha release from macrophages. Our results suggest a novel mechanism by which SL could contribute to virulence at early stage of mycobacterial infection or stimulation with the glycolipids by counteracting the immunopotentiating effect of TDM.

Mycobacterium tuberculosisH37Ra and H37Rv Differential Growth and Cytokine/Chemokine Induction in Murine MacrophagesIn Vitro

The role of tumor necrosis factor-alpha (TNF-alpha) in controlling growth of Mycobacterium tuberculosis in murine peritoneal macrophages infected in vitro was studied. TNF-alpha was shown to be required but not sufficient, and the amount of TNF-alpha produced by the infected cells did not correlate with the extent of growth control. In this system, TNF-alpha-dependent control of growth of the avirulent strain H37Ra was independent of inducible nitric oxide synthase (iNOS) and interferon-gamma (IFN-gamma), as shown by the infection of macrophages from selected gene-disrupted mice. TNF-alpha-mediated bacteriostasis of H37Ra in the infected macrophages was associated with increased expression of selected Th1-type cytokines and chemokines. In contrast, growth of the virulent strain H37Rv in macrophages involved upregulation by infected cells of Th2-type cytokines, including interleukin-5 (IL-5), IL-10, and IL-13. Taken together, these results suggest that the particular nature of macrophage activation and the cytokine and chemokine response to infection with different M. tuberculosis strains determine the ability of the cells to control the growth of the intracellular bacilli.

The Virulence-associated Two-component PhoP-PhoR System Controls the Biosynthesis of Polyketide-derived Lipids in Mycobacterium tuberculosis

Two-component regulatory signal transduction systems are important elements of the adaptative response of prokaryotes to a variety of environmental stimuli. Disruption of PhoP-PhoR in Mycobacterium tuberculosis dramatically attenuates virulence, implying that this system directly and/or indirectly coordinates the expression of important virulence factors whose identity remains to be established. Interestingly, in knockingout the PhoP-PhoR two-component system in M. tuberculosis Mt103, dramatic changes in the colonial morphology, cording properties, and reactivity of the mutant strain to the basic dye neutral red, all intrinsic properties of tubercle bacilli known to correlate with virulence, were noted. Because deficiencies in the ability of the mutant to form serpentine cords and stain with the dye are likely the results of alterations of its cell envelope composition, we undertook to analyze the lipid content of phoP and phoP-phoR mutants constructed in two different strains of M. tuberculosis. Our results indicate that PhoP coordinately and positively regulates the synthesis of methyl-branched fatty acid-containing acyltrehaloses known to be restricted to pathogenic species of the M. tuberculosis complex, namely diacyltrehaloses, polyacyltrehaloses, and sulfolipids. Evidence is also provided that PhoP but not PhoR is required for the production of these lipids. This work represents an important step toward the functional characterization of PhoP-PhoR and the understanding of complex lipid synthesis in M. tuberculosis.

The role of trehalose dimycolate (cord factor) on morphology of virulent M. tuberculosis in vitro

SETTING

M. tuberculosis (MTB) lose virulence during prolonged culture on artificial media. This loss of virulence is associated with a change in colony morphology. Several studies suggested that trehalose 6,6' dimycolate (TDM or cord factor), contributes to colony morphology.

OBJECTIVE

To investigate the role of TDM in colony morphology of MTB using clinical isolates selected to have colony morphology typical of virulent or attenuated organisms.

DESIGN

Use immunohistochemical and physical chemical methods to assess the presence and distribution of TDM in rapidly growing pellicles of MTB.

RESULTS

TDM forms an insoluble crystalline monolayer at the air-water interfaces that is more rigid than that formed by any other biologic amphiphile and is strong enough to support a spreading pellicle of MTB. The surface of young pellicles of the isolate with virulent morphology displayed the regular linear pattern characteristic of monolayers of TDM. TDM was also identified in the open spaces of pellicles of MTB by immunohistochemistry. MTB with morphology of attenuated organisms had neither of these properties.

CONCLUSION

These data suggest that the characteristic morphology of colonies of virulent MTB is due to TDM released from the surface of the organisms.

Animal models of tuberculosis

It was Robert Koch who recognized the spectrum of pathology of tuberculosis (TB) in different animal species. The examination of clinical specimens from infected humans and animals confirmed the variable patterns of pathological reactions in different species. Guinea pigs are innately susceptible while humans, mice and rabbits show different level of resistance depending upon their genotype. The studies of TB in laboratory animals such as mice, rabbits and guinea pigs have significantly increased our understanding of the aetiology, virulence and pathogenesis of the disease. The introduction of less than five virulent organisms into guinea pigs by the respiratory route can produce lung lesions, bacteraemia and fatal diseases, which helped the extrapolation of results of such experiments to humans. The similarities in the course of clinical infection between guinea pigs and humans allow us to model different forms of TB and to evaluate the protective efficacy of candidate vaccines in such systems. The only limitation of this model, however, is a dearth of immunological reagents that are required for the qualitative and quantitative evaluation of the immune responses, with special reference to cytokines and cell phenotypes. Another limitation is the higher cost of guinea pigs compared with mice. The rabbit is relatively resistant to Mycobacterium tuberculosis, however following infection with virulent Mycobacterium bovis, the rabbit produces pulmonary cavities like humans. The rabbit model, however, is also limited by the lack of the immunological reagents. Mice are the animal of choice for studying the immunology of mycobacterial infections and have contributed much to our current understanding of the roles of various immunological mechanisms of resistance. The resistance of mice to the development of classic TB disease, however, represents a significant disadvantage of the mouse model. Although non-human primates are closely related to humans, owing to high cost and handing difficulties they have not been exploited to a large extent. As all existing animal models fail to mimic the human disease perfectly, efforts should be focused on the development of the non-human primate(s) as the alternative animal model for TB.

Studies on the virulence of tubercle bacilli; isolation and biological properties of a constituent of virulent organisms

The bacillary cords characteristic for virulent tubercle bacilli are readily disrupted when wet bacilli are suspended in hydrocarbons such as paraffin oil or petroleum ether. The disruption of cords is due to the removal of a material coating the surface of the bacilli and causing them to adhere to each other. This material can be obtained from virulent bacilli by extracting them with petroleum ether. It is a lipid. Avirulent variants of tubercle bacilli do not yield a similar material after extraction in the same manner; only little of it is obtained from BCG bacilli. The following properties of the fraction obtained by petroleum ether extraction are described: (a) It inhibits the migration of leukocytes in vitro. (b) If repeatedly injected in small doses into mice, it is toxic, whereas a single high dose does not give rise to toxic manifestations. (c) The susceptibility of mice to the toxic action of repeated injections parallels to some extent their degree of susceptibility to infection with the strain of tubercle bacilli from which the fraction was obtained. (d) The injection of the extracted material into guinea pigs does not induce a state of allergic reactivity toward tuberculin. Likewise, tuberculin-positive guinea pigs do not show hypersensitivity against injections of the extracted substance. Bacilli extracted with petroleum ether do not lose their viability. They grow out normally in vitro, and they are still pathogenic. However, the removal of the petroleum ether-soluble lipid from the bacilli results in a loss of the ability of the organisms to inhibit the migration of polymorphonuclear leukocytes. Moreover, mice and guinea pigs infected with extracted bacilli may develop tuberculosis considerably slower than animals injected with comparable amounts of unextracted organisms. The significance of these findings is discussed in relation to the problem of the virulence of tubercle bacilli.

Comparative cell wall core biosynthesis in the mycolated pathogens, Mycobacterium tuberculosis and Corynebacterium diphtheriae

The recent determination of the complete genome sequence of Corynebacterium diphtheriae, the aetiological agent of diphtheria, has allowed a detailed comparison of its physiology with that of its closest sequenced pathogenic relative Mycobacterium tuberculosis. Of major importance to the pathogenicity and resilience of the latter is its particularly complex cell envelope. The corynebacteria share many of the features of this extraordinary structure although to a lesser level of complexity. The cell envelope of M. tuberculosis has provided the molecular targets for several of the major anti-tubercular drugs. Given a backdrop of emerging multi-drug resistant strains of the organism (MDR-TB) and its continuing global threat to human health, the search for novel anti-tubercular agents is of paramount importance. The unique structure of this cell wall and the importance of its integrity to the viability of the organism suggest that the search for novel drug targets within the array of enzymes responsible for its construction may prove fruitful. Although the application of modern bioinformatics techniques to the 'mining' of the M. tuberculosis genome has already increased our knowledge of the biosynthesis and assembly of the mycobacterial cell wall, several issues remain uncertain. Further analysis by comparison with its relatives may bring clarity and aid the early identification of novel cellular targets for new anti-tuberculosis drugs. In order to facilitate this aim, this review intends to illustrate the broad similarities and highlight the structural differences between the two bacterial envelopes and discuss the genetics of their biosynthesis.

Variability of BCG strains (Bacillus Calmette-Guérin)

Three different cultures of BCG propagated for over 2 years in a liquid medium containing Tween 80 and albumin were found to differ in several of their intrinsic properties. Cultures of the three strains were found to consist of morphologically heterogeneous populations—each culture being made up of three main colonial types—spreading, intermediate, and non-spreading. The percentage distribution of colonial types was characteristic for each culture and remained constant during cultivation in liquid media. Injection of the various cultures into mice and guinea pigs resulted in a self-limited disease. The distribution, extent, and duration of the lesions were also characteristic for each culture. Both the spreading and non-spreading substrains derived from the various cultures exhibited the degree of attenuation of virulence characteristic of the parent strain.

Studies on the virulence of tubercle bacilli; the relationship of the physiological state of the organisms to their pathogenicity

On the basis of earlier observations dealing with the relation of a petroleum ether-soluble material (cord factor) obtained from young cultures of virulent tubercle bacilli to the pathogenicity of these organisms, it was expected that young cultures yielding more cord factor than older ones of the same strain would also be more virulent for susceptible animals. By infecting mice with equal numbers of bacteria from 3 day and 3 week old cultures, significant differences in the character of disease produced were observed. The mice infected with the younger cultures died of a rapid, septicemic infection with tuberculous lesions in many organs including the heart. A tuberculous myocarditis was probably the immediate cause of death. Mice infected with the older bacteria died of a chronic disease corresponding to the well known mouse tuberculosis. In these cases, the heart was completely free of lesions. No histologic tissue reactions typical of tuberculosis were seen in the animals dying from the acute type of the disease. A similar rapidly progressing infection was observed in rabbits infected with bacteria from young cultures. The symptoms corresponded to the ones seen in the disease known as the Yersin type of tuberculosis. It seems that the pathology of this latter can be produced with every type of pathogenic mycobacteria, human as well as bovine and avian, provided the cultures used are young. Thus it may be inferred that the acute type of tuberculosis is more frequent than commonly accepted both in experimental infection and in the naturally occurring disease. It is proposed to explain the mechanism of this acute infection within the framework of the cord factor hypothesis.

On the response of genetically resistant and susceptible rabbits to the quantitative inhalation of human type tubercle bacilli and the nature of resistance to tuberculosis

If genetically resistant and susceptible rabbits inhale a certain number of human type tubercle bacilli, no tuberculosis in the lungs of the resistant animals is seen, as a rule several months after infection, while there is a variable and often extensive disease in the susceptible rabbits. The analogy to the presence or absence of active tuberculosis in man infected with the tubercle bacillus is evident. The inhaled tubercle bacilli multiply for but a short time in the resistant rabbits and are usually rapidly and completely destroyed. In the susceptible rabbits, the bacilli multiply profusely for a much longer time and persist in large numbers even months after inhalation. Whatever be the cause of the more rapid destruction of tubercle bacilli in the resistant animal, the resulting more rapid release of the contained antigens enhances the development of allergic sensitivity and antibodies in these animals. The development of an acquired resistance against tubercle bacilli of the human type is sufficiently rapid to affect the genesis of the initial gross primary pulmonary foci that result from the inhalation of a given number of bacilli. The greater the genetic resistance, the fewer the initial primary foci. Variations in genetic resistance are essentially variations in the rate of development of acquired resistance. It is suggested that variations in genetic resistance to inhaled human type tubercle bacilli may affect the prevalence of alveolar phagocytes capable of acquiring adequate resistance to the growth of the bacilli in their cytoplasm. The prevalence of such cells is subject to hormonal and immunological influences.

Comparative expression studies of a complex phenotype: cord formation in Mycobacterium tuberculosis

The aggregation of mycobacteria into structures known as cords is an intrinsic property of the human tubercle bacillus. This property is thought to be determined by the lipid composition of the bacterial cell surface and may contribute to the virulence of the organism. Using microarray technology, we compared the pattern of gene expression of H37Rv, a virulent, cording strain of Mycobacterium tuberculosis, with H37Ra, an avirulent, non-cording strain derived from the same original patient isolate, under five different nutrient combinations and growth conditions. Under all of these conditions, H37Rv formed cords and H37Ra did not. By focusing our analysis only on genes that were differentially expressed under all conditions, we hoped to enrich the resulting gene list for genes associated with cording. We identified 22 genes that were consistently expressed at higher levels in H37Rv than in H37Ra under all conditions tested. Genes involved in lipid metabolism and the cell membrane were significantly enriched in our gene list, indicating that the cell wall and the cell membrane may be the major sites of difference between these two strains. This work represents a new strategy for enriching gene lists for relevant genes, which may also be applicable for other types of problems.

Requirement for kasB in Mycobacterium mycolic acid biosynthesis, cell wall impermeability and intracellular survival: implications for therapy

Mycobacterium tuberculosis infects one-third of the world's population and causes two million deaths annually. The unusually low permeability of its cell wall contributes to the ability of M. tuberculosis to grow within host macrophages, a property required for pathogenesis of infection. Mycobacterium marinum is an established model for discovering genes involved in mycobacterial infection. Mycobacterium marinum mutants with transposon insertions in the beta-ketoacyl-acyl carrier protein synthase B gene (kasB) grew poorly in macrophages, although growth in vitro was unaffected. Detailed analyses by thin-layer chromatography, nuclear magnetic resonance (NMR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, infrared spectroscopy, and chemical degradations showed that the kasB mutants synthesize mycolic acids that are 2-4 carbons shorter than wild type; the defect was localized to the proximal portion of the meromycolate chain. In addition, these mutants showed a significant (approximately 30%) reduction in the abundance of keto-mycolates, with a slight compensatory increase of both alpha- and methoxy-mycolates. Despite these small changes in mycolate length and composition, the kasB mutants exhibited strikingly altered cell wall permeability, leading to a marked increase in susceptibility to lipophilic antibiotics and the host antimicrobial molecules defensin and lysozyme. The abnormalities of the kasB mutants were fully complemented by expressing M. tuberculosis kasB, but not by the closely related gene kasA. These studies identify kasB as a novel target for therapeutic intervention in mycobacterial diseases.

Sulfolipid deficiency does not affect the virulence of Mycobacterium tuberculosis H37Rv in mice and guinea pigs

Lipids that are found only in the cell envelope of pathogenic mycobacteria, such as those containing multiple methyl-branched fatty acids, have long been thought to play a role in pathogenesis. Among these complex lipids, sulfolipids have been the most extensively studied over the last 50 years. The numerous biological effects exhibited by purified sulfolipids on phagocytic cells led to the idea that these molecules are probably important virulence factors facilitating the intracellular survival of Mycobacterium tuberculosis. However, definitive evidence to support this concept has been lacking. The recent construction of an isogenic sulfolipid-deficient mutant of M. tuberculosis H37Rv (Sirakova et al., J. Biol. Chem. 276:16833-16839, 2001) has for the first time provided the opportunity to directly assess the contribution of these complex lipids to pathogenesis. In the present study, we show that against all expectations, sulfolipid deficiency does not significantly affect the replication, persistence, and pathogenicity of M. tuberculosis H37Rv in mice and guinea pigs or in cultured macrophages.

Influence of trehalose 6,6'-dimycolate (TDM) during mycobacterial infection of bone marrow macrophages

The relative role of surface lipids in the innate macrophage response to infection with mycobacteria remains unknown. Trehalose 6,6'-dimycolate (TDM), a major component of the mycobacterial cell wall, can elicit hypersensitive as well as T-cell-independent foreign body responses. The T-cell-independent contribution of TDM to the primary macrophage response to mycobacterial infection was investigated. Bone-marrow-derived macrophages isolated from C57BL/6 mice were infected with native Mycobacterium tuberculosis (MTB) or with MTB delipidated using petroleum ether extraction methods. The removal of surface lipids caused decreased bacterial survival in macrophages, but there was no loss of bacterial growth in broth culture. Bacterial survival within macrophages was restored upon reconstitution of the bacteria with purified TDM. The cytokine and chemokine parameters of the macrophage responses were also investigated. The amounts of IL-1beta, TNF-alpha, IL-6 and MIP-1alpha produced were significantly reduced following delipidation, but were restored upon reconstitution with TDM. The amount of IL-12 produced, but not the amount of IL-10 produced, was also significantly reduced upon macrophage infection with delipidated MTB. Furthermore, nitric oxide responses were not impaired upon infection with delipidated MTB, suggesting that intracellular survival and macrophage secretion of cytokines and chemokines are differentially controlled. These studies indicate that TDM is a major component contributing to the innate macrophage responses to MTB infection.

Dysregulated response to mycobacterial cord factor trehalose-6,6'-dimycolate in CD1D-/- mice

The biologic effects of the mycobacterial glycolipid trehalose-6,6'-dimycolate (TDM) include granuloma formation and macrophage activation and are dependent on physical conformation. In mice, the group II CD1 surface molecule CD1d has been implicated in glycolipid presentation. The importance of CD1d interactions in pathology has yet to be established. We hypothesized that mice lacking CD1d (CD1D(-/-)) would demonstrate dysregulated granulomatous response to TDM, compared with CD1D(+/-) heterozygous controls. Mice were intravenously injected with TDM-coated polystyrene-divinylbenzene beads and examined for histologic response and for changes in inflammatory cytokine and chemokine mRNA. Control CD1D heterozygous mice demonstrated a granulomatous response, which peaked at day 5. Increased mRNA for tumor necrosis factor-alpha (TNF-alpha) and macrophage inflammatory protein-1alpha (MIP-1alpha) correlated with development of granulomas, with very little change in interleukin-1beta (IL-1beta) and monocyte chemoattractant protein-1 (MCP-1). In contrast, the CD1D(-/-) mice revealed markedly different responses. Five days after administration, severe pulmonary hemorrhage was induced. The relative size of inflammation surrounding coated bead in the CD1D(-/-) mice was nearly double that induced in the CD1D(+/-) mice. CD1D(-/-) mice also demonstrated elevated mRNA for both inflammatory cytokines and chemokines by day 1 after administration, significantly earlier than responses seen in the heterozygous controls.