Circulating tuberculostearic acid in tuberculosis patients

Tuberculostearic Acid Controls Mycobacterial Membrane Compartmentalization

The intracellular membrane domain (IMD) is a laterally discrete region of the mycobacterial plasma membrane, enriched in the subpolar region of the rod-shaped cell. Here, we report genome-wide transposon sequencing to discover the controllers of membrane compartmentalization in Mycobacterium smegmatis. The putative gene cfa showed the most significant effect on recovery from membrane compartment disruption by dibucaine. Enzymatic analysis of Cfa and lipidomic analysis of a cfa deletion mutant (Δcfa) demonstrated that Cfa is an essential methyltransferase for the synthesis of major membrane phospholipids containing a C19:0 monomethyl-branched stearic acid, also known as tuberculostearic acid (TBSA). TBSA has been intensively studied due to its abundant and genus-specific production in mycobacteria, but its biosynthetic enzymes had remained elusive. Cfa catalyzed the S-adenosyl-l-methionine-dependent methyltransferase reaction using oleic acid-containing lipid as a substrate, and Δcfa accumulated C18:1 oleic acid, suggesting that Cfa commits oleic acid to TBSA biosynthesis, likely contributing directly to lateral membrane partitioning. Consistent with this model, Δcfa displayed delayed restoration of subpolar IMD and delayed outgrowth after bacteriostatic dibucaine treatment. These results reveal the physiological significance of TBSA in controlling lateral membrane partitioning in mycobacteria. IMPORTANCE As its common name implies, tuberculostearic acid is an abundant and genus-specific branched-chain fatty acid in mycobacterial membranes. This fatty acid, 10-methyl octadecanoic acid, has been an intense focus of research, particularly as a diagnostic marker for tuberculosis. It was discovered in 1934, and yet the enzymes that mediate the biosynthesis of this fatty acid and the functions of this unusual fatty acid in cells have remained elusive. Through a genome-wide transposon sequencing screen, enzyme assay, and global lipidomic analysis, we show that Cfa is the long-sought enzyme that is specifically involved in the first step of generating tuberculostearic acid. By characterizing a cfa deletion mutant, we further demonstrate that tuberculostearic acid actively regulates lateral membrane heterogeneity in mycobacteria. These findings indicate the role of branched fatty acids in controlling the functions of the plasma membrane, a critical barrier for the pathogen to survive in its human host.

Tuberculostearic Acid-Containing Phosphatidylinositols as Markers of Bacterial Burden in Tuberculosis

One-fourth of the global human population is estimated to be infected with strains of the Mycobacterium tuberculosis complex (MTBC), the causative agent of tuberculosis (TB). Using lipidomic approaches, we show that tuberculostearic acid (TSA)-containing phosphatidylinositols (PIs) are molecular markers for infection with clinically relevant MTBC strains and signify bacterial burden. For the most abundant lipid marker, detection limits of ∼10² colony forming units (CFUs) and ∼10³ CFUs for bacterial and cell culture systems were determined, respectively. We developed a targeted lipid assay, which can be performed within a day including sample preparation—roughly 30-fold faster than in conventional methods based on bacterial culture. This indirect and culture-free detection approach allowed us to determine pathogen loads in infected murine macrophages, human neutrophils, and murine lung tissue. These marker lipids inferred from mycobacterial PIs were found in higher levels in peripheral blood mononuclear cells of TB patients compared to healthy individuals. Moreover, in a small cohort of drug-susceptible TB patients, elevated levels of these molecular markers were detected at the start of therapy and declined upon successful anti-TB treatment. Thus, the concentration of TSA-containing PIs can be used as a correlate for the mycobacterial burden in experimental models and in vitro systems and may prospectively also provide a clinically relevant tool to monitor TB severity.

Tuberculostearic acid, a potential parameter for scoring system construction for tuberculous meningitis diagnosis

Background

This study aimed to validate the value of tuberculostearic acid (TBSA) whether it could implicate the existence of Mycobacterium tuberculosis (Mtb) and assist for the clinical diagnosis of tuberculous meningitis (TBM).

Methods

The patient's cerebrospinal fluid (CSF) specimen was collected through the lumbar puncture and detected for TBSA with gas chromatography/mass spectrometry. At the same time, gold standard tests, i.e., CSF direct culture, CSF smear microscopy, or nucleic acid amplification tests, for Mtb were routinely performed. Furthermore, we evaluated all patients by the Lancet consensus scoring system, which classifies suspected patients to "Definite (depend on gold standard results only)," "Probable (>10 pts without imaging or >12 pts with imaging information)," "Possible (6-9 pts without imaging or 6-11 pts with imaging)," and "Not (<6 pts or with alternative diagnoses)" TBM.

Results

In total, 140 patients were admitted for our study included 27 confirmed TBM patients and 50 TBSA-positive patients. Sensitivity (0.7407, confidence interval [CI] 95%: 0.5372-0.8889) and specificity (0.7345, CI 95%: 0.6432-0.8132) were calculated. The Lancet consensus scoring system was also applied to evaluate the possibility of TBM in suspected patients, finding that TBSA-positive patients showed a similar grouping distribution as the definite TBM patients.

Conclusions

Our study implicates that the prospective utilization of TBSA is worth combining into a scoring system for characterizing the features of Mtb, showing a great potential of TBM diagnosis by TBSA in future.

Newer Diagnostic Tests and their Application in Pediatric TB

The diagnosis of childhood tuberculosis is a challenge due to the pauci-bacillary nature of infection and the difficulty in obtaining appropriate sample. In the past 2-3 decades, many new tests were introduced for the diagnosis of tuberculosis (TB) and some of them have been evaluated for their application in pediatric tuberculosis as well. There is an attempt to improve smear microscopy by introducing light-emitting diode (LED) fluorescence microscopy and there are also some automated digital microscopy platforms under evaluation. Introduction of automated liquid culture platform along with rapid molecular based identification methods have considerably reduced the time delay in mycobacterial culture. Recent addition of many nucleic acid amplification platforms like Amplicor PCR, Genprobe, Xpert MTB/Rif, line probe assays, loop mediated isothermal amplification etc are also been found to be useful. Latest techniques like microarray and gene sequencing are also being used in clinical laboratories with variable results. Indirect methods of TB diagnosis like T cell based assays including tuberculin skin test and interferon-gamma release assays have their role primarily in the diagnosis of latent TB. Biomarkers are the latest addition in the battery of TB diagnostic tests facilitating diagnosis using easily accessible samples like urine, blood and breath of patients. Many biomarkers are still under evaluation and some of them are found to have a potential role as promising diagnostic tests of future.

Lazy, dynamic or minimally recrudescent? On the elusive nature and location of the mycobacterium responsible for latent tuberculosis

In the absence of symptoms characteristic of tuberculosis (TB), a condition termed clinical latency, diagnosis is currently impossible by detection of the microorganism itself and resorts to the demonstration of an immunological memory response to antigens of Mycobacterium tuberculosis (Mtb). Whether latency is synonymous to chronic persistent infection with viable Mtb in all instances has been difficult to establish. The physical and physiological state of Mtb during latency is much disputed: are organisms mostly dormant, in a nonreplicating state of persistence, and characterized by lipid inclusions and metabolic adaptation to hypoxia, or do they continue to replicate and sometimes even escape from the fringes of granulomatous lesions or alveolar epithelial cells into adjacent airways, thereby inducing recurring immune responses? The physical nature of Mtb during latency is important as it determines which antimicrobial agents may be used to kill it, which immunomodulating strategies (including post-exposure vaccines) may be appropriate to contain it, and which diagnostic measures may be most useful to discriminate latent from reactivating infection. Two major viewpoints exist: one argues that Mtb persists mostly in a lazy state within granulomatous lesions, but periodically recrudesces, and that there is considerable heterogeneity for different sites within the lesion and within the infected lung. Throughout latency, there is a dynamic immunological interplay between Mtb and the host, necessitating continuous recruitment of cells into the granuloma, and reactivation occurs when this dynamic cellular exchange becomes dysregulated. Another view holds that dormant Mtb reside within alveolar epithelial cells in the lung apices and in adipocytes, with reactivation being associated with the upregulation of resuscitation promoting factors within Mtb and the escape of newly dividing microorganisms into alveoli and bronchi in the form of lipid pneumonia. These views need not be mutually exclusive. However, if minimal intermittent recrudescence were to take place within the alveolar space, this would contradict the very definition of latency, which implies that no access of Mtb to the airways exists during latency.

Total synthesis of phosphatidylinositol mannosides of Mycobacterium tuberculosis

The total synthesis of phosphatidylinositol mannosides (PIMs), a key class of antigenic glycolipids found on the cell wall of Mycobacterium tuberculosis, is described. The synthetic strategy relied on a [4 + 3] glycosylation of tetramannoside 1 and pseudotrisaccharide 2, which allowed for convergent access to the glycan backbone of the phosphatidylinositol dimannoside (PIM2) and hexamannoside (PIM6). A short practical synthesis of tuberculostearic acid was achieved based on a copper-catalyzed cross-coupling reaction. Union of the glycan and lipid parts resulted in the first total synthesis of native PIM2 and PIM6.