The projected long-term prevalence of multidrug-resistant (MDR) tuberculosis depends upon the relative fitness of MDR Mycobacterium tuberculosis strains, compared with non-MDR strains. While many experimental models have tested the in vitro or in vivo fitness costs of various drug resistance mutations, fewer epidemiologic studies have attempted to validate these experimental findings.
We performed a case-control study comparing drug resistance–associated mutations from MDR M. tuberculosis strains causing multiple cases in a household to matched MDR strains without evidence of secondary household cases.
Eighty-eight multiple-case and 88 single-case household MDR strains were analyzed for 10 specific drug resistance–associated polymorphisms previously associated with fitness effects. We found that the isoniazid-resistant katG Ser315Thr mutation occurred more than twice as frequently in multiple-case households than in single-case households (odds ratio [OR], 2.39; 95% confidence interval [CI], 1.21–4.70), corroborating previous experimental findings. However, strains carrying both the katG Ser315Thr mutation and the rpsL Lys43Arg mutation were less likely to be found in multiple-case households (OR, 0.09; 95% CI, .01–.73), suggesting a negative epistatic interaction which contrasts previous findings.
The case-control design presents a useful approach for assessing in vivo fitness effects of drug resistance mutations.
MDR M. tuberculosis; antibiotic resistance; fitness cost; transmission; case-control study; molecular epidemiology; katG; rpsL; epistasis; Lima, Peru
The Xpert® MTB/RIF (Xpert) assay is a rapid PCR-based assay for the detection of Mycobacterium tuberculosis complex DNA (MTBc) and mutations associated with rifampin resistance (RIF). An updated version introduced in 2011, the G4 Xpert, included modifications to probe B and updated analytic software.
An analytical study was performed to assess Xpert detection of mutations associated with rifampin resistance in rifampin-susceptible and -resistant isolates. A clinical study was performed in which specimens from US and non-US persons suspected of tuberculosis (TB) were tested to determine Xpert performance characteristics. All specimens underwent smear microscopy, mycobacterial culture, conventional drug-susceptibility testing and Xpert testing; DNA from isolates with discordant rifampin resistance results was sequenced.
Among 191 laboratory-prepared isolates in the analytical study, Xpert sensitivity for detection of rifampin resistance associated mutations was 97.7% and specificity was 90.8%, which increased to 99.0% after DNA sequencing analysis of the discordant samples. Of the 1,096 subjects in the four clinical studies, 49% were from the US. Overall, Xpert detected MTBc in 439 of 468 culture-positive specimens for a sensitivity of 93.8% (95% confidence interval [CI]: 91.2%–95.7%) and did not detect MTBc in 620 of 628 culture-negative specimens for a specificity of 98.7% (95% CI: 97.5%–99.4%). Sensitivity was 99.7% among smear-positive cases, and 76.1% among smear-negative cases. Non-determinate MTBc detection and false-positive RIF resistance results were low (1.2 and 0.9%, respectively).
The updated Xpert assay retained the high sensitivity and specificity of the previous assay versions and demonstrated low rates of non-determinate and RIF resistance false positive results.
Electronic supplementary material
The online version of this article (doi:10.1186/s12879-016-2039-4) contains supplementary material, which is available to authorized users.
Tuberculosis; Diagnostics; GeneXpert; Xpert; G4
Molecular diagnostics that rapidly and accurately predict resistance to fluoroquinolone drugs and especially later-generation agents promise to improve treatment outcomes for patients with multidrug-resistant tuberculosis and prevent the spread of disease. Mutations in the gyr genes are known to confer most fluoroquinolone resistance, but knowledge about the effects of gyr mutations on susceptibility to early- versus later-generation fluoroquinolones and about the role of mutation-mutation interactions is limited. Here, we sequenced the full gyrA and gyrB open reading frames in 240 multidrug-resistant and extensively drug-resistant tuberculosis strains and quantified their ofloxacin and moxifloxacin MIC by testing growth at six concentrations for each drug. We constructed a multivariate regression model to assess both the individual mutation effects and interactions on the drug MICs. We found that gyrB mutations contribute to fluoroquinolone resistance both individually and through interactions with gyrA mutations. These effects were statistically significant. In these clinical isolates, several gyrA and gyrB mutations conferred different levels of resistance to ofloxacin and moxifloxacin. Consideration of gyr mutation combinations during the interpretation of molecular test results may improve the accuracy of predicting the fluoroquinolone resistance phenotype. Further, the differential effects of gyr mutations on the activity of early- and later-generation fluoroquinolones requires further investigation and could inform the selection of a fluoroquinolone for treatment.
Fluoroquinolone susceptibility-testing is an important step in the design of effective treatment regimens for multidrug-resistant tuberculosis. Here we compare ciprofloxacin, ofloxacin and moxifloxacin resistance results from 226 multidrug-resistant samples. The low level of concordance observed supports that drug sensitivity tests should be performed for the specific FLQ planned for clinical use. The results also support the new WHO recommendation for testing moxifloxacin at a critical-concentration of 2.0μg/ml.
drug susceptibility test; multidrug resistant tuberculosis; critical concentration
Mycobacterium tuberculosis is successfully evolving antibiotic resistance, threatening attempts at tuberculosis epidemic control. Mechanisms of resistance, including the genetic changes favored by selection in resistant isolates, are incompletely understood. Using 116 newly and 7 previously sequenced M. tuberculosis genomes, we identified genomewide signatures of positive selection specific to the 47 resistant genomes. By searching for convergent evolution, the independent fixation of mutations at the same nucleotide site or gene, we recovered 100% of a set of known resistance markers. We also found evidence of positive selection in an additional 39 genomic regions in resistant isolates. These regions encode pathways of cell wall biosynthesis, transcriptional regulation and DNA repair. Mutations in these regions could directly confer resistance or compensate for fitness costs associated with resistance. Functional genetic analysis of mutations in one gene, ponA1, demonstrated an in vitro growth advantage in the presence of the drug rifampicin.
Small nucleolar RNAs are a highly conserved group of small RNAs found in eukaryotic cells. Genes encoding these RNAs are diversely located throughout the genome. They are functionally conserved, performing post transcriptional modification (methylation and pseudouridylation) of rRNA and other nuclear RNAs. They belong to two major categories: the C/D box and H/ACA box containing snoRNAs. U3 snoRNA is an exceptional member of C/D box snoRNAs and is involved in early processing of pre-rRNA. An antisense sequence is present in each snoRNA which guides the modification or processing of target RNA. However, some snoRNAs lack this sequence and often they are called orphan snoRNAs.
We have searched snoRNAs of Entamoeba histolytica from the genome sequence using computational programmes (snoscan and snoSeeker) and we obtained 99 snoRNAs (C/D and H/ACA box snoRNAs) along with 5 copies of Eh_U3 snoRNAs. These are located diversely in the genome, mostly in intergenic regions, while some are found in ORFs of protein coding genes, intron and UTRs. The computationally predicted snoRNAs were validated by RT-PCR and northern blotting. The expected sizes were in agreement with the observed sizes for all C/D box snoRNAs tested, while for some of the H/ACA box there was indication of processing to generate shorter products.
Our results showed the presence of snoRNAs in E. histolytica, an early branching eukaryote, and the structural features of E. histolytica snoRNAs were well conserved when compared with yeast and human snoRNAs. This study will help in understanding the evolution of these conserved RNAs in diverse phylogenetic groups.
U3 snoRNA; Guide/ orphan snoRNAs; Entamoeba histolytica
Accidental autoclaving of L-glutamine was found to facilitate the Agrobacterium infection of a non host plant like tea in an earlier study. In the present communication, we elucidate the structural changes in L-glutamine due to autoclaving and also confirm the role of heat transformed L-glutamine in Agrobacterium mediated genetic transformation of host/non host plants.
When autoclaved at 121°C and 15 psi for 20 or 40 min, L-glutamine was structurally modified into 5-oxo proline and 3-amino glutarimide (α-amino glutarimide), respectively. Of the two autoclaved products, only α-amino glutarimide facilitated Agrobacterium infection of a number of resistant to susceptible plants. However, the compound did not have any vir gene inducing property.
We report a one pot autoclave process for the synthesis of 5-oxo proline and α-amino glutarimide from L-glutamine. Xenobiotic detoxifying property of α-amino glutarimide is also proposed.
The reemergence of tuberculosis in its present-day manifectations – single, multiple and extensive drug resistant forms and as HIV-TB coinfections – has resulted in renewed research on fundamental questions such as the nature of the organism itself, Mycobacterium tuberculosis, the molecular basis of its pathogenesis, definition of the immunological response in animal models and humans, and development of new intervention strategies such as vaccines and drugs. Foremost among these developments has been the precise chemical definition of the complex and distinctive cell wall of M. tuberculosis, elucidation of the relevant pathways and underlying genetics responsible for the synthesis of the hallmark moities of the tubercle bacillus such as the mycolic acid-arabinogalactan-peptidoglycan complex, the phthiocerol- and trehalose-containing effector lipids, the phosphatidylinositol-containing mannosides, lipomannosides and lipoarabinomannosides, major immunomodulators, and others. In this review, the laboratory personnel that have been the focal point of some to these developments review recent progress towards a comprehensive understanding of the basic physiology and functions of the cell wall of M. tuberculosis.
Arabinogalactan (AG) and lipoarabinomannan (LAM) are the two major cell wall (lipo)polysaccharides of mycobacteria. They share arabinan chains made of linear segments of α-1,5-linked d-Araf residues with some α-1,3-branching, the biosynthesis of which offers opportunities for new chemotherapeutics. In search of the missing arabinofuranosyltransferases (AraTs) responsible for the formation of the arabinan domains of AG and LAM in Mycobacterium tuberculosis, we identified Rv0236c (AftD) as a putative membrane-associated polyprenyl-dependent glycosyltransferase. AftD is 1400 amino acid-long, making it the largest predicted glycosyltransferase of its class in the M. tuberculosis genome. Assays using cell-free extracts from recombinant Mycobacterium smegmatis and Corynebacterium glutamicum strains expressing different levels of aftD indicated that this gene encodes a functional AraT with α-1,3-branching activity on linear α-1,5-linked neoglycolipid acceptors in vitro. The disruption of aftD in M. smegmatis resulted in cell death and a decrease in its activity caused defects in cell division, reduced growth, alteration of colonial morphology, and accumulation of trehalose dimycolates in the cell envelope. Overexpression of aftD in M. smegmatis, in contrast, induced the accumulation of two arabinosylated compounds with carbohydrate backbones reminiscent of that of LAM and a degree of arabinosylation dependent on aftD expression levels. Altogether, our results thus indicate that AftD is an essential AraT involved in the synthesis of the arabinan domain of major mycobacterial cell envelope (lipo)polysaccharides.
arabinogalactan; arabinosyltransferase; lipoarabinomannan; Mycobacterium; tuberculosis
We report on the identification of a glycosyltransferase (GT) from Mycobacterium tuberculosis H37Rv, Rv3779, of the membranous GT-C superfamily responsible for the direct synthesis of polyprenyl-phospho-mannopyranose and thus indirectly for lipoarabinomannan, lipomannan, and the higher-order phosphatidyl-myo-inositol mannosides.
Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharide biosynthesis in Mycobacterium spp. Here, the crystallization and preliminary crystallographic analysis of GpgS from M. tuberculosis and of its complex with UDP are reported.
Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharide biosynthesis in mycobacteria. These important molecules are believed to be involved in the regulation of fatty-acid and mycolic acid synthesis. The enzyme belongs to the recently defined GT81 family of retaining glycosyltransferases (CAZy, Carbohydrate-Active Enzymes Database; see http://www.cazy.org). Here, the purification, crystallization and preliminary crystallographic analysis are reported of GpgS from Mycobacterium tuberculosis and of its complex with UDP. GpgS crystals belonged to space group I4, with unit-cell parameters a = 98.85, b = 98.85, c = 127.64 Å, and diffracted to 2.6 Å resolution. GpgS–UDP complex crystals belonged to space group I4, with unit-cell parameters a = 98.32, b = 98.32, c = 127.96 Å, and diffracted to 3.0 Å resolution.
glycosyltransferases; methylglucose lipopolysaccharides; Mycobacterium tuberculosis
Synopsis Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharides (MGLP) biosynthesis in Mycobacterium spp. Here we report the crystallization and preliminary crystallographic analysis of GpgS from Mycobacterium tuberculosis and its complex with UDP at 2.6 Å and 3.0 Å resolution, respectively.
Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharides (MGLP) biosynthesis in mycobacteria. These important molecules are believed to be involved in the regulation of fatty acid and mycolic acid synthesis. The enzyme belongs to the recently defined GT81 family of retaining glycosyltransferases (CAZy, Carbohydrate-Active enZymes data base; see www.cazy.org). Here we report the purification, crystallization and preliminary crystallographic analysis of GpgS from Mycobacterium tuberculosis and its complex with UDP. GpgS crystals belong to space group I4, with unit-cell parameters a = 98.85, b = 98.85, c= 127.64 Å, and diffract to 2.6 Å resolution. GpgS-UDP complex crystals belong to space group I4 with unit-cell parameters a= 98.32, b= 98.32, c= 127.96 Å, and diffract to 3.0 Å resolution.
glycosyltransferase; methylglucose lipopolysaccharides; Mycobacterium; X ray structure
In Mycobacterium tuberculosis two related Z-prenyl diphosphate synthases, E,Z-farnesyl diphosphate synthase (Rv1086) and decaprenyl diphosphate synthase (Rv2361c) work in series to synthesize decaprenyl phosphate (C50) from isopentenyl diphosphate and E-geranyl diphosphate. Decaprenyl phosphate plays a central role in the biosynthesis of essential mycobacterial cell wall components, such as the mycolyl-arabinogalactan-peptidoglycan complex and lipoarabinomannan; thus, its synthesis has attracted considerable interest as a potential therapeutic target. Rv1086 is a unique prenyl diphosphate synthase in that it adds only one isoprene unit to geranyl diphosphate generating the 15 carbon product (E,Z-farnesyl diphosphate). Rv2361c then adds a further 7 isoprene units to E,Z-farnesyl diphosphate in a processive manner to generate the 50 carbon prenyl diphosphate, which is then dephosphorylated to generate a carrier for activated sugars. The molecular basis for chain length discrimination by Rv1086 during synthesis is unknown. We also report the structure of apo Rv1086 with citronellyl diphosphate bound and with the product mimic E,E-farnesyl diphosphate bound. We report the structures of Rv2361c in the apo form, with isopentyl diphosphate bound and with a substrate analogue, citronellyl diphosphate. The structures confirm the enzymes are very closely related. Detailed comparison reveals structural differences that account for chain length control in Rv1086. We have tested this hypothesis and have identified a double mutant of Rv1086 which makes a range of longer lipid chains.
Drug design; enzyme mechanism; tuberculosis; x-ray crystallography; inhibitors
Mycobacteria produce two unique families of cytoplasmic polymethylated polysaccharides - the methylglucose lipopolysaccharides (MGLPs) and the methylmannose polysaccharides (MMPs) - the physiological functions of which are still poorly defined. Towards defining the roles of these polysaccharides in mycobacterial physiology, we generated knock-out mutations of genes in their putative biosynthetic pathways.
We report here on the characterization of the Rv1208 protein of Mycobacterium tuberculosis and its ortholog in Mycobacterium smegmatis (MSMEG_5084) as the enzymes responsible for the transfer of the first glucose residue of MGLPs. Disruption of MSMEG_5084 in M. smegmatis resulted in a dramatic decrease in MGLP synthesis directly attributable to the almost complete abolition of glucosyl-3-phosphoglycerate synthase activity in this strain. Synthesis of MGLPs in the mutant was restored upon complementation with wild-type copies of the Rv1208 gene from M. tuberculosis or MSMEG_5084 from M. smegmatis.
This is the first evidence linking Rv1208 to MGLP biosynthesis. Thus, the first step in the initiation of MGLP biosynthesis in mycobacteria has been defined, and subsequent steps can be inferred.
Mycobacterium tuberculosis utilizes the methylerythritol phosphate (MEP) pathway for biosynthesis of isopentenyl diphosphate and its isomer, dimethylallyl diphosphate, precursors of all isoprenoid compounds. This pathway is of interest as a source of new drug targets, as it is absent from humans and disruption of the responsible genes has shown a lethal phenotype for Escherichia coli. In the MEP pathway, 4-diphosphocytidyl-2-C-methyl-d-erythritol is formed from 2-C-methyl-d-erythritol 4-phosphate (MEP) and CTP in a reaction catalyzed by a 4-diphosphocytidyl-2-C-methyl-d-erythritol synthase (IspD). In the present work, we demonstrate that Rv3582c is essential for M. tuberculosis: Rv3582c has been cloned and expressed, and the encoded protein has been purified. The purified M. tuberculosis IspD protein was capable of catalyzing the formation of 4-diphosphocytidyl-2-C-methyl-d-erythritol in the presence of MEP and CTP. The enzyme was active over a broad pH range (pH 6.0 to 9.0), with peak activity at pH 8.0. The activity was absolutely dependent upon divalent cations, with 20 mM Mg2+ being optimal, and replacement of CTP with other nucleotide 5′-triphosphates did not support activity. Under the conditions tested, M. tuberculosis IspD had Km values of 58.5 μM for MEP and 53.2 μM for CTP. Calculated kcat and kcat/Km values were 0.72 min−1 and 12.3 mM−1 min−1 for MEP and 1.0 min−1 and 18.8 mM−1 min−1 for CTP, respectively.
Z-prenyl diphosphate synthases catalyze the sequential condensation of isopentenyl diphosphate with allylic diphosphates to synthesize polyprenyl diphosphates. In mycobacteria, these are precursors of decaprenyl phosphate, a molecule which plays a central role in the biosynthesis of essential mycobacterial cell wall components, such as the mycolyl-arabinogalactan-peptidoglycan complex and lipoarabinomannan. Recently, it was demonstrated that open reading frame Rv2361c of the Mycobacterium tuberculosis H37Rv genome encodes a unique prenyl diphosphate synthase (M. C. Schulbach, P. J. Brennan, and D. C. Crick, J. Biol. Chem. 275:22876-22881, 2000). We have now purified the enzyme to near homogeneity by using an Escherichia coli expression system and have shown that the product of this enzyme is decaprenyl diphosphate. Rv2361c has an absolute requirement for divalent cations and an optimal pH range of 7.5 to 8.5, and the activity is stimulated by both detergent and dithiothreitol. The enzyme catalyzes the addition of isopentenyl diphosphate to geranyl diphosphate, neryl diphosphate, ω,E,E-farnesyl diphosphate, ω,E,Z-farnesyl diphosphate, or ω,E,E,E-geranylgeranyl diphosphate, with Km values for the allylic substrates of 490, 29, 84, 290, and 40 μM, respectively. The Km value for isopentenyl diphosphate is 89 μM. The catalytic efficiency is greatest when ω,E,Z-farnesyl diphosphate is used as the allylic acceptor, suggesting that this is the natural substrate in vivo, a conclusion that is supported by previous structural studies of decaprenyl phosphoryl mannose isolated from M. tuberculosis. This is the first report of a bacterial Z-prenyl diphosphate synthase that preferentially utilizes an allylic diphosphate primer having the α-isoprene unit in the Z configuration, indicating that Rv1086 (ω,E,Z-farnesyl diphosphate synthase) and Rv2361c act sequentially in the biosynthetic pathway that leads to the formation of decaprenyl phosphate in M. tuberculosis.