Identification of a novel DNase of Streptococcus suis (EndAsuis) important for neutrophil extracellular trap degradation during exponential growth.

The porcine and human pathogen Streptococcus suis induces and degrades neutrophil extracellular traps (NETs) in vitro. In this study, we investigated the working hypothesis that NET degradation is mediated not only by the known secreted S. suis nuclease A (SsnA) but also by a so-far undescribed putative endonuclease A of S. suis (designated EndAsuis) homologous to the pneumococcal endonuclease A (EndA). Comparative analysis was conducted to identify differences in localization, expression and function of EndAsuis and SsnA. In contrast to ssnA, endAsuis RNA expression was not substantially different during exponential and stationary growth. Modelling of the 3D structure confirmed a putative DRGH-motif-containing ßßα-metal finger catalytic core in EndAsuis. Accordingly, nuclease activity of recombinant EndAsuis with a point-mutated H165 was rescued through imidazol treatment. In accordance with a putative membrane anchor, nuclease activity caused by endAsuis was not detectable in the supernatant. Importantly, endAsuis determined nuclease activity of S. suis prominently during exponential growth. This activity depended on the presence of Mg(2+) but, in contrast to SsnA activity, not on Ca(2+). A pH of 5.4 did not inhibit endAsuis-encoded nuclease activity during exponential growth. NET degradation of S. suis harvested during exponential growth was significantly attenuated in the endAsuis mutant. In contrast to SsnA, mutagenesis of endAsuis did not result in a significantly higher susceptibility against the antimicrobial effect mediated by NETs. As degradation of bacterial DNA caused by S. suis depended on ssnA and endAsuis, further functions of both factors in the host-pathogen interaction might be envisioned.

Mycobacterium tuberculosis PE/PPE proteins enhance the production of reactive oxygen species and formation of neutrophil extracellular traps.

Introduction: Neutrophil granulocytes predominate in the lungs of patients infected with Mycobacterium tuberculosis (Mtb) in earlier stages of the disease. During infection, neutrophils release neutrophil extracellular traps (NETs), an antimicrobial mechanism by which a DNA-backbone spiked with antimicrobial components traps the mycobacteria. However, the specific mycobacterial factors driving NET formation remain unclear. Proteins from the proline-glutamic acid (PE)/proline-proline-glutamic acid (PPE) family are critical to Mtb pathophysiology and virulence. Methods: Here, we investigated NET induction by PE18, PPE26, and PE31 in primary human blood-derived neutrophils. Neutrophils were stimulated with the respective proteins for 3h, and NET formation was subsequently assessed using confocal fluorescence microscopy. Intracellular ROS levels and cell necrosis were estimated by flow cytometry. Additionally, the influence of phorbol-12-myristate-13-acetate (PMA), a known NADPH oxidase enhancer, on NET formation was examined. Neutrophil integrity following incubation with the PE/PPE proteins was evaluated using transmission electron microscopy. Results: For the first time, we report that stimulation of primary human blood-derived neutrophils with Mtb proteins PE18, PPE26, and PE31 resulted in the formation of NETs, which correlated with an increase in intracellular ROS levels. Notably, the presence of PMA further amplified this effect. Following incubation with the PE/PPE proteins, neutrophils were found to remain viable and structurally intact, as verified through transmission electron microscopy, indicating the occurrence of vital NET formation. Discussion: These findings offer valuable insights that contribute to a better understanding of host-pathogen interactions during Mtb infection. Moreover, they underscore the significance of these particular Mtb antigens in triggering NET formation, representing a distinctive and previously unrecognized function of PE/PPE antigens.

Spore-FP1 tuberculosis mucosal vaccine candidate is highly protective in guinea pigs but fails to improve on BCG-conferred protection in non-human primates.

Tuberculosis remains a major health threat globally and a more effective vaccine than the current Bacillus Calmette Guerin (BCG) is required, either to replace or boost it. The Spore-FP1 mucosal vaccine candidate is based on the fusion protein of Ag85B-Acr-HBHA/heparin-binding domain, adsorbed on the surface of inactivated Bacillus subtilis spores. The candidate conferred significant protection against Mycobacterium. tuberculosis challenge in naïve guinea pigs and markedly improved protection in the lungs and spleens of animals primed with BCG. We then immunized rhesus macaques with BCG intradermally, and subsequently boosted with one intradermal and one aerosol dose of Spore-FP1, prior to challenge with low dose aerosolized M. tuberculosis Erdman strain. Following vaccination, animals did not show any adverse reactions and displayed higher antigen specific cellular and antibody immune responses compared to BCG alone but this did not translate into significant improvement in disease pathology or bacterial burden in the organs.

The Mycobacterium tuberculosis Ag85A is a novel diacylglycerol acyltransferase involved in lipid body formation.

Mycobacterium tuberculosis accumulates large amounts of triacylglycerol (TAG) which acts as storage compounds for energy and carbon. The mycobacterial triacylglycerols stored in the form of intracellular lipid droplets are essential for long-term survival of M. tuberculosis during a dormant state. We report here that when the M. tuberculosis mycolytransferase Ag85A is overexpressed in Mycobacterium smegmatis mc(2)155, cell morphology was changed and the cells became grossly enlarged. A massive formation of lipid bodies and a change in lipid pattern was observed simultaneously. We suspected a possible role of Ag85A in the acyl lipid metabolism and discovered that the enzyme possesses acyl-CoA:diacylglycerol acyltransferase (DGAT) activity in addition to its well-known function as mycolyltransferase. Ag85A mediates the transesterification of diacylglycerol using long-chain acyl-CoA as acyl donors. The K(m) and K(cat) values for palmitoleoyl-coenzyme A were 390 µM and 55.54 min(-1) respectively. A docking model suggests that palmitoleoyl-coenzyme A and 1,2-dipalmitin occupy the same active site as trehalose 6,6'-dimycolate and trehalose 6'-monomycolate. The site-directed Ser126Ala mutation of the active site proved that this residue is involved in the catalytic activity of this enzyme. Although not proven conclusively for dormant stage of M. tuberculosis, our novel finding about the synthesis of TAGs by Ag85A strongly suggests that Ag85A may play a significant role in the formation of lipid storage bodies and thus also in the establishment and maintenance of a persistent tuberculosis infection.

Nanofluidic Immobilization and Growth Detection of Escherichia coli in a Chip for Antibiotic Susceptibility Testing.

Infections with antimicrobial resistant bacteria are a rising threat for global healthcare as more and more antibiotics lose their effectiveness against bacterial pathogens. To guarantee the long-term effectiveness of broad-spectrum antibiotics, they may only be prescribed when inevitably required. In order to make a reliable assessment of which antibiotics are effective, rapid point-of-care tests are needed. This can be achieved with fast phenotypic microfluidic tests, which can cope with low bacterial concentrations and work label-free. Here, we present a novel optofluidic chip with a cross-flow immobilization principle using a regular array of nanogaps to concentrate bacteria and detect their growth label-free under the influence of antibiotics. The interferometric measuring principle enabled the detection of the growth of Escherichia coli in under 4 h with a sample volume of 187.2 µL and a doubling time of 79 min. In proof-of-concept experiments, we could show that the method can distinguish between bacterial growth and its inhibition by antibiotics. The results indicate that the nanofluidic chip approach provides a very promising concept for future rapid and label-free antimicrobial susceptibility tests.

Crystallization and preliminary X-ray analysis of the small subunit (R2F) of native ribonucleotide reductase from Corynebacterium ammoniagenes.

Ribonucleotide reduction, the unique step in DNA-precursor biosynthesis, involves radical-dependent redox chemistry and diverse metallo-cofactors. The metallo-cofactor (R2F) encoded by the nrdF (nucleotide reduction) gene in Corynebacterium ammoniagenes ATCC 6872 was isolated after homologous expression and a new crystal form of ribonucleotide reductase R2F was obtained. R2F was crystallized at 277 K using the vapour-diffusion method with PEG as the precipitating agent. A data set was collected to 1.36 A resolution from a single crystal at 100 K using synchrotron radiation. The crystal belonged to space group C2, with unit-cell parameters a = 96.21, b = 87.68, c = 83.25 A, beta = 99.29 degrees. The crystal contained two molecules per asymmetric unit, with a Matthews coefficient (V(M)) of 2.69 A(3) Da(-1); the solvent content was estimated to be 54.3%. X-ray fluorescence spectroscopy and MAD diffraction data indicated the presence of manganese in the molecule and the absence of iron.

Charles River altered Schaedler flora (CRASF) remained stable for four years in a mouse colony housed in individually ventilated cages.

As recommendations for specific pathogen-free housing change, mouse facilities need to re-derive their colonies repeatedly in order to eliminate specified bacteria or viruses. This paper describes the establishment of a new mouse facility using as starting point a small colony of CD-1 mice colonized with the Charles River altered Schaedler flora (CRASF) housed in individually ventilated cages (IVCs). The import of new strains was performed exclusively via embryo transfer using CD-1 mice as recipients. The integrity of the CRASF in caecum samples of the original CD-1 colony and of three inbred mouse lines imported into the colony was proven by a quantitative realtime polymerase chain reaction approach. Furthermore, we searched for bacterial contaminants in the gut flora using non-specific 16S rRNA primers. The bacterial sequences found were closely related to but not exclusively sequences of altered Schaedler flora (ASF) members, suggesting that the ASF is heterogeneous rather than restricted to the eight defined bacteria. Moreover, no pathogens were found, neither using the non-specific 16S rRNA primers nor in routine quarterly health monitoring. As one effect of this defined gut flora, interleukin-10 knockout mice are devoid of colitis in our facility. In conclusion, our approach building up a mouse facility using foster mothers and embryo transfer as well as a strict barrier system and IVCs is suitable to maintain a colony free from contaminating bacteria over the long term. CRASF remained stable for seven mouse generations and was efficiently transferred to the imported mouse strains.

Cross-Flow Filtration of Escherichia coli at a Nanofluidic Gap for Fast Immobilization and Antibiotic Susceptibility Testing.

Infections with antimicrobial-resistant (AMR) bacteria are globally on the rise. In the future, multi-resistant infections will become one of the major problems in global health care. In order to enable reserve antibiotics to retain their effect as long as possible, broad-spectrum antibiotics must be used sparingly. This can be achieved by a rapid microfluidic phenotypic antibiotic susceptibility test, which provides the information needed for a targeted antibiotic therapy in less time than conventional tests. Such microfluidic tests must cope with a low bacteria concentration. On-chip filtering of the samples to accumulate bacteria can shorten the test time. By means of fluorescence microscopy, we examined a novel nanogap filtration principle to hold back Escherichia coli and to perform cultivation experiments with and without antibiotics present. Microfluidic chips based on the nanogap flow principle showed to be useful for the concentration and cultivation of E. coli. With a concentration of 106 cells/mL, a specific growth rate of 0.013 min-1 and a doubling time of 53 min were achieved. In the presence of an antibiotic, no growth was observed. The results prove that this principle can, in future, be used in fast and marker-free antimicrobial susceptibility testing (AST).

A Specific Blood Signature Reveals Higher Levels of S100A12: A Potential Bladder Cancer Diagnostic Biomarker Along With Urinary Engrailed-2 Protein Detection.

Urothelial carcinoma of the urinary bladder (UCB) or bladder cancer remains a major health problem with high morbidity and mortality rates, especially in the western world. UCB is also associated with the highest cost per patient. In recent years numerous markers have been evaluated for suitability in UCB detection and surveillance. However, to date none of these markers can replace or even reduce the use of routine tools (cytology and cystoscopy). Our current study described UCB's extensive expression profile and highlighted the variations with normal bladder tissue. Our data revealed that JUP, PTGDR, KLRF1, MT-TC, and RNU6-135P are associated with prognosis in patients with UCB. The microarray expression data identified also S100A12, S100A8, and NAMPT as potential UCB biomarkers. Pathway analysis revealed that natural killer cell mediated cytotoxicity is the most involved pathway. Our analysis showed that S100A12 protein may be useful as a biomarker for early UCB detection. Plasma S100A12 has been observed in patients with UCB with an overall sensitivity of 90.5% and a specificity of 75%. S100A12 is highly expressed preferably in high-grade and high-stage UCB. Furthermore, using a panel of more than hundred urine samples, a prototype lateral flow test for the transcription factor Engrailed-2 (EN2) also showed reasonable sensitivity (85%) and specificity (71%). Such findings provide confidence to further improve and refine the EN2 rapid test for use in clinical practice. In conclusion, S100A12 and EN2 have shown potential value as biomarker candidates for UCB patients. These results can speed up the discovery of biomarkers, improving diagnostic accuracy and may help the management of UCB.

Magnetic biosensor for the detection of Yersinia pestis.

A novel type of magnetic-beads based magnetic biosensor is described for the detection of Yersinia pestis. Experiments were performed with the antigen fraction F1 of these bacteria. The magnetic sensor platform offers easy and reliable detection of Y. pestis by the use of magnetic beads for labelling and quantification in a single step due to their paramagnetic features. The system uses antiYPF1 antibodies as capture element on ABICAP columns as core element of the magnetic sensor. Several immobilization methods for antibodies on polyethylene were exploited. The established biosensor has a linear detection range of 25-300 ng/ml Y. pestis antigen F1 and a detection limit of 2.5 ng/ml in buffer and human blood serum. The presented sensor system is small, simple, portable and therefore usable as off-lab detection unit for medical and warfare analytes.

Pyrazinamide: the importance of uncovering the mechanisms of action in mycobacteria.

Pyrazinamide (PZA) is still one of the key drugs used in current therapeutic regimens for tuberculosis (TB). Despite its importance for TB therapy, the mode of action of PZA remains unknown. PZA has to be converted to its active form pyrazinoic acid (POA) by the nicotinamidase PncA and is then excreted by an unknown efflux pump. At acidic conditions, POA is protonated to HPOA and is reabsorbed into the cell where it causes cellular damage. For a long time, it has been thought that PZA/POA has no defined target of action, but recent studies have shown that both PZA and POA have several different targets interfering with diverse biochemical pathways, especially in the NAD(+) and energy metabolism. PZA resistance seems to depend not only on a defective pyrazinamidase but is also rather a result of the interplay of many different enzyme targets and transport mechanisms.

NrdH-redoxin of Corynebacterium ammoniagenes forms a domain-swapped dimer.

NrdH-redoxins constitute a family of small redox proteins, which contain a conserved CXXC sequence motif, and are characterized by a glutaredoxin-like amino acid sequence but a thioredoxin-like activity profile. Here we report the structure of Corynebacterium ammoniagenes NrdH at 2.7 A resolution, determined by molecular replacement using E. coli NrdH as model. The structure is the first example of a domain-swapped dimer from the thioredoxin family. The domain-swapped structure is formed by an inter-chain two-stranded anti-parallel beta-sheet and is stabilized by electrostatic interactions at the dimer interface. Size exclusion chromatography, and MALDI-ESI experiments revealed however, that the protein exists as a monomer in solution. Similar to E. coli NrdH-redoxin and thioredoxin, C. ammoniagenes NrdH-redoxin has a wide hydrophobic pocket at the surface that could be involved in binding to thioredoxin reductase. However, the loop between alpha2 and beta3, which is complementary to a crevice in the reductase in the thioredoxin-thioredoxin reductase complex, is the hinge for formation of the swapped dimer in C. ammoniagenes NrdH-redoxin. C. ammoniagenes NrdH-redoxin has the highly conserved sequence motif W61-S-G-F-R-P-[DE]67 which is unique to the NrdH-redoxins and which determines the orientation of helix alpha3. An extended hydrogen-bond network, similar to that in E. coli NrdH-redoxin, determines the conformation of the loop formed by the conserved motif.

Filling the pipeline - new drugs for an old disease.

Tuberculosis is a major global health problem. In the middle of the last century several laboratories identified, developed and synthesized several substances which were active against Mycobacterium tuberculosis, the causative agent of the disease. In the 1980s the standard oral treatment regimen was introduced with isoniazid, rifampicin, pyrazinamide, and ethambutol. In combination with the DOTS strategy it was possible treat TB within 6-8 months. But with the emergence of drug resistant strains, the formerly successful regiment became ineffective for MDR and XDR TB patients. Even more alarming, the rapidly increasing HIV epidemic also increases the number of HIV-related TB. Facing these facts, it became evident that novel strategies and antibiotics were needed to treat the new forms of TB. But over the last 60 years no novel TB drug was developed or even in the drug pipeline. But during the last ten years several novel substances have been developed to combat the deadly disease. For the first time in decades the TB drug pipeline is filled again with several promising compounds and many of them have reached Phase II and Phase III clinical trials. Several laboratories and companies all over the world currently are developing and evaluating these substances. This review presents novel substances, which were for the first time exclusively developed for TB such as bedaquilines, nitroimidazoles and the diamine SQ109. We also summarize the present knowledge about enzymes and biosynthesis pathways which offer potential targets for drug discovery against M. tuberculosis.

Mycobacterium tuberculosis isolates from Rio de Janeiro reveal unusually low correlation between pyrazinamide resistance and mutations in the pncA gene.

It has been widely accepted, that pyrazinamide (PZA) resistance in Mycobacterium tuberculosis is correlated with mutations in the pncA gene. But since years researchers have been puzzled by the fact that up to 30% of PZA resistant strains do not show any correlation between PZA resistance and mutations in the pncA gene, and thus may vary with geographic area. The objective of the study was to investigate the correlation between PZA susceptibility and mutations in pncA gene in M. tuberculosis isolates from individuals living in a highly endemic area. Therefore we analyzed drug resistant and multidrug resistant (MDR) isolates from patients in Rio de Janeiro, Brazil. From a total of 97 clinical isolates of M. tuberculosis 35 were identified as PZA resistant, 24/35 strains did not show PZase activity and 15/24 (62.5%) strains possess mutation in the pncA gene. This is a low correlation between PZA resistance and PZase activity (68.6%) and even lower correlation between PZA resistance and the presence of mutation in pncA gene (45.7%). Most of the mutations found were conserved near the active site or metal binding site of PZase. The 146A>C mutation was found both in PZA resistant and susceptible isolates, suggesting that this mutation may not fully associated with PZA resistance. Of the mutations found, three have not been previously described. The insertions 192-193 TCCTCGTC and 388-389 AGGTCGATG, although found before, here was found to be a short tandem repeat and in one strain, insertion of the IS6110 was observed 55nt upstream of the gene. All PZA resistant isolates had no mutation in the gene coding ribosomal protein S1 (rpsA), which has recently been proposed as alternate target for pyrazinoic acid (POA). The results show a low association of PZA resistance and pncA gene mutations in a selected patient group from an highly endemic area. Our findings point out that the phenotypic susceptibility testing remains important for the detection of PZA-resistant M. tuberculosis.

Lipid Droplets and Mycobacterium leprae Infection.

Leprosy is a chronic infectious disease and is a major source of morbidity in developing countries. Leprosy is caused by the obligate intracellular bacterium Mycobacterium leprae, which infects as primary target Schwann cells. Lepromatous leprosy exhibits multiple lesions of the skin, eyes, nerves, and lymph nodes. The sites of infection are characterized by the presence of foamy macrophages, fully packed with lipid droplets (LDs), which are induced by M. leprae. In the last years, it has become evident that M. tuberculosis imports lipids from foamy macrophages and is dependent on fatty acids for growth in infected macrophages. M. leprae seems to have similar mechanisms for scavenging lipids from the host. But due to the inability to culture M. leprae on laboratory media, research progresses only slowly. However, in the last years, substantial progress has been made in the field of lipid metabolism in M. leprae. Herein, we will present and summarize the lipid droplets formation and the metabolism of lipids during M. leprae infection.

Cytosolic lipid inclusions formed during infection by viral and bacterial pathogens.

Lipid inclusions play an important role in several pathological processes. Intracellular bacterial pathogens, such as members of the Mycobacterium and Chlamydia species are able to trigger the formation of lipid-laden foamy macrophages. Lipid droplet accumulation in the host constitutes a reservoir used by the bacilli for long-term persistence. Viruses need lipid droplets as assembly platform. We present the current knowledge about structural, functional and regulatory aspects of lipid inclusions.

Fitness of Mycobacterium tuberculosis strains of the W-Beijing and Non-W-Beijing genotype.

BACKGROUND: Multidrug resistant tuberculosis (MDR-TB) is a major threat for global tuberculosis control. The W-Beijing Mycobacterium tuberculosis genotype has been associated with drug resistance. Elucidation of the mechanisms underlying this epidemiological finding may have an important role in the control of MDR-TB. The aim of this study was to evaluate the fitness of drug-susceptible and MDR M. tuberculosis strains of the W-Beijing genotype compared with that of Non-W-Beijing strains. METHODOLOGY/PRINCIPAL FINDINGS: Fitness of M. tuberculosis strains was determined by evaluating the difference in the growth curves obtained in the MGIT960 automated system and assessing the competitive growth capacity between W-Beijing and non-W-Beijing strains. The W-Beijing MDR strains had a significant longer lag phase duration compared to the other strains but did not present a significant fitness cost. When grown in competition they had an advantage only in medium containing 0.1% Tween 80. CONCLUSIONS/SIGNIFICANCE: It was not possible to confirm a selective advantage of W-Beijing strains to grow, except for differences in their resistance to Tween 80. Further studies are needed to elucidate the putative advantage of W-Beijing strains compared to other genotypes.

The mycolyltransferase 85A, a putative drug target of Mycobacterium tuberculosis: development of a novel assay and quantification of glycolipid-status of the mycobacterial cell wall.

The enzymes of the antigen 85 complex (Ag85A, B, and C) possess mycolyltransferase activity and catalyze the synthesis of the most abundant glycolipid of the mycobacterial cell wall, the cord factor. The cord factor (trehalose 6,6'-dimycolate, TDM) is essential for the integrity of the mycobacterial cell wall and pathogenesis of the bacillus. Thus, TDM biosynthesis is regarded as a potential drug target for control of Mycobacterium tuberculosis infections. Trehalose 6,6'-dimycolate (TDM) is synthesized from two molecules of trehalose-6'-monomycolate (TMM) by antigen 85A. We report here a novel enzyme assay using the natural substrate TMM. The novel colorimetric assay is based on the quantification of glucose from the degradation of trehalose, which is the product from catalytic activity of antigen 85A. Using the new assay, K(m) and K(cat) were determined with values of 129.6+/-8.1 microM and 65.4+/-4.1 min(-1), respectively. This novel assay is also suitable for robust high-throughput screening (HTS) for compound library screening against mycolyltransferase (antigen 85A). The assay is significantly faster and more convenient to use than all assays currently in use. The assay has a very low coefficient of variance (0.04) in 96-well plates and shows a Z' factor of 0.67-0.73, indicating the robustness of the assay. In addition, this new assay is highly suitable for the quantification of total TMM of the mycobacterial cell envelope.

Three-dimensional structures of apo- and holo-L-alanine dehydrogenase from Mycobacterium tuberculosis reveal conformational changes upon coenzyme binding.

L-alanine dehydrogenase from Mycobacterium tuberculosis catalyzes the NADH-dependent reversible conversion of pyruvate and ammonia to L-alanine. Expression of the gene coding for this enzyme is up-regulated in the persistent phase of the organism, and alanine dehydrogenase is therefore a potential target for pathogen control by antibacterial compounds. We have determined the crystal structures of the apo- and holo-forms of the enzyme to 2.3 and 2.0 A resolution, respectively. The enzyme forms a hexamer of identical subunits, with the NAD-binding domains building up the core of the molecule and the substrate-binding domains located at the apical positions of the hexamer. Coenzyme binding stabilizes a closed conformation where the substrate-binding domains are rotated by about 16 degrees toward the dinucleotide-binding domains, compared to the open structure of the apo-enzyme. In the structure of the abortive ternary complex with NAD+ and pyruvate, the substrates are suitably positioned for hydride transfer between the nicotinamide ring and the C2 carbon atom of the substrate. The approach of the nucleophiles water and ammonia to pyruvate or the reaction intermediate iminopyruvate, respectively, is, however, only possible through conformational changes that make the substrate binding site more accessible. The crystal structures identified the conserved active-site residues His96 and Asp270 as potential acid/base catalysts in the reaction. Amino acid replacements of these residues by site-directed mutagenesis led to inactive mutants, further emphasizing their essential roles in the enzymatic reaction mechanism.

Structure of the inactive variant C60S of Mycobacterium tuberculosis thiol peroxidase.

The genome of Mycobacterium tuberculosis encodes several peroxiredoxins (Prxs) thought to be active against organic and inorganic peroxides. The open reading frame Rv1932 encodes a 165-residue thiol peroxidase (Tpx), which belongs to the atypical 2-Cys peroxiredoxin family. The crystal structure of the C60S mutant of M. tuberculosis Tpx (MtTpx) crystallized in space group P3(1)21, with unit-cell parameters a = 106.08, b = 106.08, c = 65.33 A. The structure has been refined to an R value of 17.1% (R(free) = 24.9%) at 2.1 A resolution. MtTpx is structurally homologous to other peroxiredoxins, including the mycobacterial AhpC and AhpE. The inactive MtTpx C60S mutant structure closely resembles the structure of Streptococcus pneumoniae Tpx (SpTpx) and thus represents the reduced enzyme state. The mutated active-site serine is electrostatically linked to Arg130 and hydrogen bonded to Thr57, practically identical to the cysteine in SpTpx. A cocrystallized acetate molecule mimics the position of the substrate and interacts with Ser60, Arg130 and Thr57.