Characterization of a haemolysin from Mycobacterium tuberculosis with homology to a virulence factor of Serpulina hyodysenteriae

Virulence genotype and phenotype of two clinical isolates of Arcobacter butzleri obtained from patients with different pathologies

The surge in human arcobacteriosis has increased interest in determining the mechanisms involved in the pathogenesis of Arcobacter butzleri. Here, genomic analyses and in vitro Caco-2 infection, motility, urease and antimicrobial susceptibility testing (AST) assays were used to characterise the virulence and antimicrobial resistance (AMR) determinants of strains HC-1, isolated from a patient with travellers' diarrhoea, and HC-2, isolated from another with pruritus. AMR determinants conferring resistance to tetracycline (tetO, present in both genomes) and to ampicillin and amoxicillin-clavulanic acid (bla3, present in HC-2) were identified. The same determinants associated with flagellum, chemotaxis, adhesion and invasion were detected in both, but HC-1 lacked eight flagellar genes. The urease cluster was only present in HC-1. Motility and urease tests confirmed the genetic differences between strains, but no genetic marker related to the inability of HC-2 to adhere and invade was identified. This inability could be conditioning the patient's pathology.

Virulence-Associated Secretion in Mycobacterium abscessus

Non-tuberculous mycobacteria (NTM) are a heterogeneous group of originally environmental organi3sms, increasingly recognized as pathogens with rising prevalence worldwide. Knowledge of NTM’s mechanisms of virulence is lacking, as molecular research of these bacteria is challenging, sometimes more than that of M. tuberculosis (Mtb), and far less resources are allocated to their investigation. While some of the virulence mechanisms are common to several mycobacteria including Mtb, others NTM species-specific. Among NTMs, Mycobacterium abscessus (Mabs) causes some of the most severe and difficult to treat infections, especially chronic pulmonary infections. Mabs survives and proliferates intracellularly by circumventing host defenses, using multiple mechanisms, many of which remain poorly characterized. Some of these immune-evasion mechanisms are also found in Mtb, including phagosome pore formation, inhibition of phagosome maturation, cytokine response interference and apoptosis delay. While much is known of the role of Mtb-secreted effector molecules in mediating the manipulation of the host response, far less is known of the secreted effector molecules in Mabs. In this review, we briefly summarize the knowledge of secreted effectors in Mtb (such as ESX secretion, SecA2, TAT and others), and draw the parallel pathways in Mabs. We also describe pathways that are unique to Mabs, differentiating it from Mtb. This review will assist researchers interested in virulence-associated secretion in Mabs by providing the knowledge base and framework for their studies.

Mycobacterium tuberculosis low molecular weight T-cell antigen Mtb8.4 has heme-binding and fiber-forming properties

Mtb8.4, a secretory T-cell antigen of Mycobacterium tuberculosis, is important for providing an antigen-specific immune response. In this study, we showed Mtb8.4 to have both heme-binding and fibril-forming properties, using experimental and in silico methods. High absorbance at 410 nm and interaction with hemin-agarose demonstrated its heme-binding nature. Titration of Mtb8.4 with heme resulted in 1:1 stoichiometry. The heme-binding pocket in Mtb8.4 was identified by molecular modeling, and binding residues were predicted using molecular docking. The molecular dynamics simulations of apo- and heme-bound Mtb8.4 confirmed that the heme group forms a stable complex. Transmission electron microscopy analyses and dye-binding assays showed that Mtb8.4 forms fibers. Computational studies predicted that the C-terminal sequence (⁹³ AAQYIGLVESV¹⁰³ ) is important for forming fibers. In silico analyses further anticipated the probable epitope (⁸² AMAAQLQAV⁹⁰ ) of Mtb8.4. The fiber-forming properties of Mtb8.4 could be advantageous from a vaccine perspective for aggregate/fibril-based vaccine delivery or it might influence the epitope presentation of Mtb8.4.

16S rRNA Methyltransferases as Novel Drug Targets Against Tuberculosis

Tuberculosis (TB) is an airborne infectious disease caused by Mycobacterium tuberculosis (M.tb) whose natural history traces back to 70,000 years. TB remains a major global health burden. Methylation is a type of post-replication, post-transcriptional and post-translational epi-genetic modification involved in transcription, translation, replication, tissue specific expression, embryonic development, genomic imprinting, genome stability and chromatin structure, protein protein interactions and signal transduction indicating its indispensable role in survival of a pathogen like M.tb. The pathogens use this epigenetic mechanism to develop resistance against certain drug molecules and survive the lethality. Drug resistance has become a major challenge to tackle and also a major concern raised by WHO. Methyltransferases are enzymes that catalyze the methylation of various substrates. None of the current TB targets belong to methyltransferases which provides therapeutic opportunities to develop novel drugs through studying methyltransferases as potential novel targets against TB. Targeting 16S rRNA methyltransferases serves two purposes simultaneously: a) translation inhibition and b) simultaneous elimination of the ability to methylate its substrates hence stopping the emergence of drug resistance strains. There are ~ 40 different rRNA methyltransferases and 13 different 16S rRNA specific methyltransferases which are unexplored and provide a huge opportunity for treatment of TB.

Mutational analysis of TlyA from Brachyspira hampsonii reveals two key residues conserved in pathogenic bacteria responsible for oligomerization and hemolytic activity

BACKGROUND

TlyA proteins are expressed in a variety of pathogenic bacteria and possess dual hemolytic and ribosomal RNA methyltransferase functions. While the mechanism of TlyA mediated rRNA methylation is well understood, relatively little is known about the mechanism of TlyA induced hemolysis.

METHODS

TlyA protein from the pig pathogen Brachyspira hampsonii was heterologously expressed and purified from an E. coli host. Hemolytic activity and rRNA methylation were assessed in vitro. Site-directed mutagenesis was used to mutate amino acids believed to be involved in TlyA mediated hemolysis.

RESULTS

Purified TlyA-His protein exhibited both hemolytic and rRNA methyltransferase activities in vitro, with partial inhibition of hemolysis observed under reducing conditions. Mutation of cysteine 80 to alanine impaired hemolytic activity. A C27A/C93A mutant was capable of dimerizing under non-reducing conditions, indicating that a C80-C80 disulfide bond is involved in TlyA oligomerization. A mutation conserved in several avirulent Brachyspira species (S9K) completely abolished hemolytic activity of TlyA. This loss of activity was attributed to impaired oligomerization in the S9K mutant, as assessed by ITC and size-exclusion chromatography experiments.

CONCLUSIONS

Oligomeric assembly and hemolytic activity of TlyA from Brachyspira hampsonii is dependent on the formation of an intermolecular C80-C80 disulfide bond and noncovalent interactions involving serine 9. The conservation of these amino acids in TlyA proteins from pathogenic bacteria suggests a correlation between tlyA gene mutations and bacterial virulence.

GENERAL SIGNIFICANCE

Our results further elucidate the mechanisms underlying TlyA mediated hemolysis and provide evidence of a conserved mechanism of oligomerization for TlyA family proteins.

Characterization of Arcobacter spp. Isolated from human diarrheal, non-diarrheal and food samples in Thailand

Arcobacter butzleri is an emerging zoonotic food-borne and water-borne pathogen that can cause diarrhea in humans. The global prevalence of A. butzleri infection is underestimated, and little is known about their phenotypic and genotypic characterization. The aim of this study was to determine antimicrobial susceptibility (AST) profiles, detect related virulence genes, and classify sequence type (ST) of A. butzleri isolates obtained from human stool and food samples. A total of 84 A. butzleri isolates were obtained from human diarrheal (n = 25), non-diarrheal (n = 24) stool, and food (n = 35) samples in Thailand. They were evaluated for phenotypic identification by conventional microbiological procedures and AST by Kirby-Bauer disc diffusion method as well as virulence genes detection. Representative isolates from each origin were selected based on the presence of virulence genes and AST profiles to analyze genetic diversity by multilocus sequence typing (MLST). All isolates showed resistance to nalidixic acid 40.5% (34/84), ciprofloxacin 11.9% (10/84), azithromycin 8.3% (7/84), and erythromycin 3.6% (3/84). Regarding the ten virulence genes detected, cj1349, mviN and pldA had the highest prevalence 100% (84/84), followed by tlyA 98.8% (83/84), cadF 97.6% (82/84), ciaB 71.4% (60/84), hecA and hecB 22.6% (19/84), iroE 15.5% (13/84) and irgA 10.7% (9/84), respectively. Three virulence genes were present among A. butzleri isolates of human diarrheal stool and food samples, with a significant difference observed among isolates; hecB [36% (9/25) and 8.6% (3/35)], hecA [36% (9/25) and 5.7% (2/35)], and irgA [24% (6/25) and 2.9% (1/35)] (p < 0.05), respectively. The hecA and hecB virulence genes functions are related to the mechanism of hemolysis, while irgA supports a bacterial nutritional requirement. MLST analysis of 26 A. butzleri isolates revealed that 16 novel STs exhibited high genetic diversity. The results of this study is useful for understanding potentially pathogenic and antimicrobial-resistant A. butzleri in Thailand. The pathogenic virulence markers hecB, hecA, and irgA have the potential to be developed for rapid diagnostic detection in human diarrheal stool. No significant relationships among STs and sources of origin were observed. Little is known about A. butzleri, the mechanism of action of these virulence genes, is a topic that needs further investigation.

Host Cell Targets of Released Lipid and Secreted Protein Effectors of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is a very successful pathogen, strictly adapted to humans and the cause of tuberculosis. Its success is associated with its ability to inhibit host cell intrinsic immune responses by using an arsenal of virulence factors of different nature. It has evolved to synthesize a series of complex lipids which form an outer membrane and may also be released to enter host cell membranes. In addition, secreted protein effectors of Mtb are entering the host cell cytosol to interact with host cell proteins. We briefly discuss the current model, involving the ESX-1 type seven secretion system and the Mtb lipid phthiocerol dimycoserosate (PDIM), of how Mtb creates pores in the phagosomal membrane to allow Mtb proteins to access to the host cell cytosol. We provide an exhaustive list of Mtb secreted proteins that have effector functions. They modify (mostly inhibit but sometimes activate) host cell pathways such as: phagosome maturation, cell death, cytokine response, xenophagy, reactive oxygen species (ROS) response via NADPH oxidase 2 (NOX2), nitric oxide (NO) response via NO Synthase 2 (NOS2) and antigen presentation via MHC class I and class II molecules. We discuss the host cell targets for each lipid and protein effector and the importance of the Mtb effector for virulence of the bacterium.

Virulence properties of Campylobacter jejuni are enhanced by displaying a mycobacterial TlyA methylation pattern in its rRNA

Campylobacter jejuni is a bacterial pathogen that is generally acquired as a zoonotic infection from poultry and animals. Adhesion of C. jejuni to human colorectal epithelial cells is weakened after loss of its cj0588 gene. The Cj0588 protein belongs to the type I group of TlyA (TlyA^I) enzymes, which 2′‐O‐methylate nucleotide C1920 in 23S rRNA. Slightly longer TlyA^II versions of the methyltransferase are found in actinobacterial species including Mycobacterium tuberculosis, and methylate not only C1920 but also nucleotide C1409 in 16S rRNA. Loss of TlyA function attenuates virulence of both M. tuberculosis and C. jejuni. We show here that the traits impaired in C. jejuni null strains can be rescued by complementation not only with the original cj0588 (tlyA^I) but also with a mycobacterial tlyA^II gene. There are, however, significant differences in the recombinant phenotypes. While cj0588 restores motility, biofilm formation, adhesion to and invasion of human epithelial cells and stimulation of IL‐8 production in a C. jejuni null strain, several of these properties are further enhanced by the mycobacterial tlyA^II gene, in some cases to twice the original wild‐type level. These findings strongly suggest that subtle changes in rRNA modification patterns can affect protein synthesis in a manner that has serious consequences for bacterial pathogenicity.

Iron Supplementation Therapy, A Friend and Foe of Mycobacterial Infections?

Iron is an essential element that is required for oxygen transfer, redox, and metabolic activities in mammals and bacteria. Mycobacteria, some of the most prevalent infectious agents in the world, require iron as growth factor. Mycobacterial-infected hosts set up a series of defense mechanisms, including systemic iron restriction and cellular iron distribution, whereas mycobacteria have developed sophisticated strategies to acquire iron from their hosts and to protect themselves from iron’s harmful effects. Therefore, it is assumed that host iron and iron-binding proteins, and natural or synthetic chelators would be keys targets to inhibit mycobacterial proliferation and may have a therapeutic potential. Beyond this hypothesis, recent evidence indicates a host protective effect of iron against mycobacterial infections likely through promoting remodeled immune response. In this review, we discuss experimental procedures and clinical observations that highlight the role of the immune response against mycobacteria under various iron availability conditions. In addition, we discuss the clinical relevance of our knowledge regarding host susceptibility to mycobacteria in the context of iron availability and suggest future directions for research on the relationship between host iron and the immune response and the use of iron as a therapeutic agent.

Iron Acquisition in Mycobacterium tuberculosis

The highly contagious disease tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis (Mtb), which has been evolving drug resistance at an alarming rate. Like all human pathogens, Mtb requires iron for growth and virulence. Consequently, Mtb iron transport is an emerging drug target. However, the development of anti-TB drugs aimed at these metabolic pathways has been restricted by the dearth of information on Mtb iron acquisition. In this Review, we describe the multiple strategies utilized by Mtb to acquire ferric iron and heme iron. Mtb iron uptake is a complex process, requiring biosynthesis and subsequent export of Mtb siderophores, followed by ferric iron scavenging and ferric-siderophore import into Mtb. Additionally, Mtb possesses two possible heme uptake pathways and an Mtb-specific mechanism of heme degradation that yields iron and novel heme-degradation products. We conclude with perspectives for potential therapeutics that could directly target Mtb heme and iron uptake machineries. We also highlight how hijacking Mtb heme and iron acquisition pathways for drug import may facilitate drug transport through the notoriously impregnable Mtb cell wall.

Biofilm Formation and Motility Are Promoted by Cj0588-Directed Methylation of rRNA in Campylobacter jejuni

Numerous bacterial pathogens express an ortholog of the enzyme TlyA, which is an rRNA 2′-O-methyltransferase associated with resistance to cyclic peptide antibiotics such as capreomycin. Several other virulence traits have also been attributed to TlyA, and these appear to be unrelated to its methyltransferase activity. The bacterial pathogen Campylobacter jejuni possesses the TlyA homolog Cj0588, which has been shown to contribute to virulence. Here, we investigate the mechanism of Cj0588 action and demonstrate that it is a type I homolog of TlyA that 2′-O-methylates 23S rRNA nucleotide C1920. This same specific function is retained by Cj0588 both in vitro and also when expressed in Escherichia coli. Deletion of the cj0588 gene in C. jejuni or substitution with alanine of K⁸⁰, D¹⁶², or K¹⁸⁸ in the catalytic center of the enzyme cause complete loss of 2′-O-methylation activity. Cofactor interactions remain unchanged and binding affinity to the ribosomal substrate is only slightly reduced, indicating that the inactivated proteins are folded correctly. The substitution mutations thus dissociate the 2′-O-methylation function of Cj0588/TlyA from any other putative roles that the protein might play. C. jejuni strains expressing catalytically inactive versions of Cj0588 have the same phenotype as cj0588-null mutants, and show altered tolerance to capreomycin due to perturbed ribosomal subunit association, reduced motility and impaired ability to form biofilms. These functions are reestablished when methyltransferase activity is restored and we conclude that the contribution of Cj0588 to virulence in C. jejuni is a consequence of the enzyme's ability to methylate its rRNA.

A Novel Motif for S-Adenosyl-l-methionine Binding by the Ribosomal RNA Methyltransferase TlyA from Mycobacterium tuberculosis

Capreomycin is a potent ribosome-targeting antibiotic that is an essential component of current antituberculosis treatments, particularly in the case of multidrug-resistant Mycobacterium tuberculosis (Mtb). Optimal capreomycin binding and Mtb ribosome inhibition requires ribosomal RNA methylation in both ribosome subunits by TlyA (Rv1694), an enzyme with dual 2'-O-methytransferase and putative hemolytic activities. Despite the important role of TlyA in capreomycin sensitivity and identification of inactivating mutations in the corresponding Mtb gene tlyA, which cause resistance to capreomycin, our current structural and mechanistic understanding of TlyA action remains limited. Here, we present structural and functional analyses of Mtb TlyA interaction with its obligatory co-substrate for methyltransferase activity, S-adenosyl-l-methionine (SAM). Despite adopting a complete class I methyltransferase fold containing conserved SAM-binding and catalytic motifs, the isolated TlyA carboxyl-terminal domain exhibits no detectable affinity for SAM. Further analyses identify a tetrapeptide motif (RXWV) in the TlyA interdomain linker as indispensable for co-substrate binding. Our results also suggest that structural plasticity of the RXWV motif could contribute to TlyA domain interactions, as well as specific recognition of its two structurally distinct ribosomal RNA targets. Our findings thus reveal a novel motif requirement for SAM binding by TlyA and set the stage for future mechanistic studies of TlyA substrate recognition and modification that underpin Mtb sensitivity to capreomycin.

Genomes of Gardnerella Strains Reveal an Abundance of Prophages within the Bladder Microbiome

Bacterial surveys of the vaginal and bladder human microbiota have revealed an abundance of many similar bacterial taxa. As the bladder was once thought to be sterile, the complex interactions between microbes within the bladder have yet to be characterized. To initiate this process, we have begun sequencing isolates, including the clinically relevant genus Gardnerella. Herein, we present the genomic sequences of four Gardnerella strains isolated from the bladders of women with symptoms of urgency urinary incontinence; these are the first Gardnerella genomes produced from this niche. Congruent to genomic characterization of Gardnerella isolates from the reproductive tract, isolates from the bladder reveal a large pangenome, as well as evidence of high frequency horizontal gene transfer. Prophage gene sequences were found to be abundant amongst the strains isolated from the bladder, as well as amongst publicly available Gardnerella genomes from the vagina and endometrium, motivating an in depth examination of these sequences. Amongst the 39 Gardnerella strains examined here, there were more than 400 annotated prophage gene sequences that we could cluster into 95 homologous groups; 49 of these groups were unique to a single strain. While many of these prophages exhibited no sequence similarity to any lytic phage genome, estimation of the rate of phage acquisition suggests both vertical and horizontal acquisition. Furthermore, bioinformatic evidence indicates that prophage acquisition is ongoing within both vaginal and bladder Gardnerella populations. The abundance of prophage sequences within the strains examined here suggests that phages could play an important role in the species’ evolutionary history and in its interactions within the complex communities found in the female urinary and reproductive tracts.

The Forgotten Virulence Factor: The 'non-conventional' Hemolysin TlyA And Its Role in Helicobacter pylori Infection

Helicobacter pylori is a human-specific Gram-negative pathogenic bacterium which colonizes the gastric mucosal layer in the stomach causing diseases such as peptic ulcer, adenocarcinoma, and gastric lymphoma. It is estimated that approximately half of the world's population is infected with H. pylori making it the most intensively characterized microbial pathogen up to now. Hemolysis has been suggested to significantly contribute to colonization of the stomach and disease progression by H. pylori. In a number of earlier studies, TlyA was characterized as a putative pore-forming cytolysin. Although a few observations in the literature suggest a role for TlyA as significant virulence factor of H. pylori, the molecular and structural characterization of this protein is much curtailed at present. Given the intensive characterization of numerous H. pylori virulence factors over the past decade, surprisingly little information exists for the TlyA toxin and its significance for pathogenesis. This review provides a brief overview on microbial hemolysis and its role for pathogenesis and discusses recent research efforts aimed at an improved understanding of the role of the 'non-conventional' hemolysin and its associated RNA methyltransferase TlyA from H. pylori.

Variation in hemolytic activity of Brachyspira hyodysenteriae strains from pigs

Brachyspira hyodysenteriae is the primary cause of swine dysentery, which is responsible for major economic losses to the pig industry worldwide. The hemolytic activity of 10 B. hyodysenteriae strains isolated from stools of pigs with mild to mucohemorrhagic diarrhea was compared and seven hemolysis associated genes were sequenced. Hemolysis induced by these strains varied from strong to near absent. One weakly hemolytic B. hyodysenteriae strain showed sequence changes in five hemolysis associated genes (tlyA, tlyB, hemolysin III, hemolysin activation protein and hemolysin III channel protein) resulting in amino acid substitutions. The occurrence of weakly hemolytic strains identifiable as B. hyodysenteriae should be taken into account in swine dysentery diagnostics. The presence of these strains may affect herd dysentery status, with great impact on a farms trading opportunities.

Comparative Genomic Analysis Reveals a Possible Novel Non-Tuberculous Mycobacterium Species with High Pathogenic Potential

Mycobacteria have been reported to cause a wide range of human diseases. We present the first whole-genome study of a Non-Tuberculous Mycobacterium, Mycobacterium sp. UM_CSW (referred to hereafter as UM_CSW), isolated from a patient diagnosed with bronchiectasis. Our data suggest that this clinical isolate is likely a novel mycobacterial species, supported by clear evidence from molecular phylogenetic, comparative genomic, ANI and AAI analyses. UM_CSW is closely related to the Mycobacterium avium complex. While it has characteristic features of an environmental bacterium, it also shows a high pathogenic potential with the presence of a wide variety of putative genes related to bacterial virulence and shares very similar pathogenomic profiles with the known pathogenic mycobacterial species. Thus, we conclude that this possible novel Mycobacterium species should be tightly monitored for its possible causative role in human infections.

Toll-Like Receptor-4 Dependent Intestinal Gene Expression During Arcobacter Butzleri Infection of Gnotobiotic Il-10 Deficient Mice

We have previously shown that Arcobacter butzleri infection induces Toll-like receptor (TLR) -4 dependent immune responses in perorally infected gnotobiotic IL-10^–/– mice. Here, we analyzed TLR-4-dependent expression of genes encoding inflammatory mediators and matrix-degrading gelatinases MMP-2 and -9 in the small and large intestines of gnotobiotic TLR-4-deficient IL-10^–/– mice that were perorally infected with A. butzleri strains CCUG 30485 or C1, of human and chicken origin, respectively. At day 6 following A. butzleri infection, colonic mucin-2 mRNA, as integral part of the intestinal mucus layer, was downregulated in the colon, but not ileum, of IL-10^–/– but not TLR-4^–/– IL-10^–/– mice. CCUG 30485 strain-infected TLR-4-deficient IL-10^–/– mice displayed less distinctly upregulated IFN-γ, IL-17A, and IL-1β mRNA levels in ileum and colon, which was also true for colonic IL-22. These changes were accompanied by upregulated colonic MMP-2 and ileal MMP-9 mRNA exclusively in IL-10^–/– mice. In conclusion, TLR-4 is essentially involved in A. butzleri mediated modulation of gene expression in the intestines of gnotobiotic IL-10^–/– mice.

Survey of small intestinal and systemic immune responses following murine Arcobacter butzleri infection

BACKGROUND

Arcobacter (A.) butzleri has been described as causative agent for sporadic cases of human gastroenteritis with abdominal pain and acute or prolonged watery diarrhea. In vitro studies revealed distinct adhesive, invasive and cytotoxic properties of A. butzleri. Information about the underlying immunopathological mechanisms of infection in vivo, however, are scarce. The aim of this study was to investigate the immunopathological properties of two different A. butzleri strains in a well-established murine infection model.

RESULTS

Gnotobiotic IL-10(-/-) mice, in which the intestinal microbiota was depleted by broad-spectrum antibiotic treatment, were perorally infected with two different A. butzleri strains isolated from a diseased patient (CCUG 30485) or fresh chicken meat (C1), respectively. Eventhough bacteria of either strain could stably colonize the intestinal tract at day 6 and day 16 postinfection (p.i.), mice did not exert infection induced symptoms such as diarrhea or wasting. In small intestines of infected mice, however, increased numbers of apoptotic cells could be detected at day 16, but not day 6 following infection with either strain. A strain-dependent influx of distinct immune cell populations such as T and B cells as well as of regulatory T cells could be observed upon A. butzleri infection which was accompanied by increased small intestinal concentrations of pro-inflammatory cytokines such as TNF, IFN-γ, MCP-1 and IL-6. Remarkably, inflammatory responses following A. butzleri infection were not restricted to the intestinal tract, given that the CCUG 30485 strain induced systemic immune responses as indicated by increased IFN-γ concentrations in spleens at day 6, but not day 16 following infection.

CONCLUSION

Upon peroral infection A. butzleri stably colonized the intestinal tract of gnotobiotic IL-10(-/-) mice. The dynamics of distinct local and systemic inflammatory responses could be observed in a strain-dependent fashion pointing towards an immunopathogenic potential of A. butzleri in vivo. These results indicate that gnotobiotic IL-10(-/-) mice are well suited to further investigate the molecular mechanisms underlying arcobacteriosis in vivo.

Mycobacterial tlyA gene product is localized to the cell-wall without signal sequence

The mycobacterial tlyA gene product, Rv1694 (MtbTlyA), has been annotated as “hemolysin” which was re-annotated as 2′-O rRNA methyl transferase. In order to function as a hemolysin, it must reach the extracellular milieu with the help of signal sequence(s) and/or transmembrane segment(s). However, the MtbTlyA neither has classical signals sequences that signify general/Sec/Tat pathways nor transmembrane segments. Interestingly, the tlyA gene appears to be restricted to pathogenic strains such as H37Rv, M. marinum, M. leprae, than M. smegmatis, M. vaccae, M. kansasii etc., which highlights the need for a detailed investigation to understand its functions. In this study, we have provided several evidences which highlight the presence of TlyA on the surface of M. marinum (native host) and upon expression in M. smegmatis (surrogate host) and E. coli (heterologous host). The TlyA was visualized at the bacterial-surface by confocal microscopy and accessible to Proteinase K. In addition, sub-cellular fractionation has revealed the presence of TlyA in the membrane fractions and this sequestration is not dependent on TatA, TatC or SecA2 pathways. As a consequence of expression, the recombinant bacteria exhibit distinct hemolysis. Interestingly, the MtbTlyA was also detected in both membrane vesicles secreted by M. smegmatis and outer membrane vesicles secreted by E. coli. Our experimental evidences unambiguously confirm that the mycobacterial TlyA can reach the extra cellular milieu without any signal sequence. Hence, the localization of TlyA class of proteins at the bacterial surface may highlight the existence of non-classical bacterial secretion mechanisms.

Modulation of phagolysosome maturation by bacterial tlyA gene product

The pathogenic traits of TlyA proteins of Mycobacterium tuberculosis are not known. Expressions of TlyA in bacteria that do not express endogenous TlyA adhere better to RAW264.7 macrophages and get phagocytosed efficiently. The internalized bacteria avoid acidification to the extent of greater than 65 percent in the case of both TlyA-expressing E. coli and M. smegmatis. Consistent with this observation, we have observed decreased co-localizaton of Lysosomal Membrane Associated Protein-1 (approx. 35 percent), Early Endosomal Antigen-1 (approx. 34 percent), Rab5 (approx. 30 percent) and Rab7 (approx. 35 percent) and enhanced colocalizaton of Rab14 (approx. 80 percent) on both TlyA-expressing bacteria as well as on TlyA-coated latex beads. These results suggest that the mycobacterial TlyA, in general, can modulate phagolysosome maturation pathway immediately after entry into macrophages, while other important molecules may aid the bacterium for long-term, intracellular survival at later point of time.

Helicobacter pylori TlyA Forms Amyloid-like Aggregates with Potent Cytotoxic Activity

Helicobacter pylori is a potent human gastric pathogen. It is known to be associated with several gastroenteric disorders, including gastritis, peptic ulcer, and gastric cancer. The H. pylori genome encodes a gene product TlyA that has been shown to display potent membrane damaging properties and cytotoxic activity. On the basis of such properties, TlyA is considered as a potential virulence factor of H. pylori. In this study, we show that the H. pylori TlyA protein has a strong propensity to convert into the amyloid-like aggregated assemblies, upon exposure to elevated temperatures. Even at the physiological temperature of 37 °C, TlyA shows a strong amyloidogenic property. TlyA aggregates that are generated upon exposure at temperatures of ≥37 °C show prominent binding to dyes like thioflavin T and Nile Red. Transmission electron microscopy also demonstrates the presence of typical amyloid-like fibrils in the TlyA aggregates generated at 37 °C. Conversion of TlyA into the amyloid-like aggregates is found to be associated with major alterations in the secondary and tertiary structural organization of the protein. Finally, our study shows that the preformed amyloid-like aggregates of TlyA are capable of exhibiting potent cytotoxic activities against human gastric adenocarcinoma cells. Altogether, such a propensity of H. pylori TlyA to convert into the amyloid-like aggregated assemblies with cytotoxic activity suggests potential implications for the virulence functionality of the protein.

Molecular analysis of the tlyA gene in Campylobacter lari

Full-length tlyA gene and its adjacent genetic loci from the urease-positive thermophilic Campylobacter (UPTC) CF89-12 [approximately 15,000 base pairs (bp) in length], as well as a reference strain Campylobacter lari RM2100 (approximately 9,000 bp), were analyzed. The possible open-reading frame of tlyA from UPTC CF89-12 was shown to have 720 bp with a calculated molecular mass of approximately 26.7 kDa. Using a primer pair designed in silico, a total of approximately 1.1 kbp consisting of putative promoter region, structural gene for tlyA, and its adjacent genetic loci were identified in all 17 C. lari isolates [n = 13 for UPTC; n = 4 for urease-negative (UN) C. lari]. Although sequence differences were demonstrated at approximately 20 loci within the 90 bp non-coding (NC) region, including the putative promoter structure candidates immediately upstream of the tlyA gene among the 18 isolates including C. lari RM2100, no sequence differences were identified within the NC region among the five UN C. lari isolates examined. A start codon ATG and a probable ribosome-binding site, AGGC(T)GG(A), for the tlyA gene were identified in all 18 isolates, including C. lari RM2100. The putative intrinsic ρ-independent transcriptional terminator structure candidate was also identified for the tlyA gene in both UPTC CF89-12 and C. lari RM2100. Additionally, the hemolysis assay was performed with some of the C. lari isolates. The tlyA gene nucleotide sequence data may possibly be useful for discrimination between UN C. lari and UPTC organisms, as well as for the differentiation among the four thermophilic Campylobacter species.

Helicobacter pylori TlyA agglutinates liposomes and induces fusion and permeabilization of the liposome membranes

Helicobacter pylori TlyA is a pore-forming hemolysin with potent cytotoxic activity. To explore the potential membrane-damaging activity of H. pylori TlyA, we have studied its interaction with the synthetic liposome vesicles. In our study, H. pylori TlyA shows a prominent ability to associate with the liposome vesicles without displaying an obligatory requirement for any protein receptor on the liposome membranes. Interaction of TlyA triggers agglutination of the liposome vesicles. Such agglutinating activity of TlyA could also be observed with erythrocytes before the induction of its pore-forming hemolytic activity. In addition to its agglutinating activity against liposomes, TlyA also induces fusion and disruption of the liposome membranes. Altogether, our study highlights novel membrane-damaging properties of H. pylori TlyA that have not been documented previously with any other TlyA family protein.

Insights on how the Mycobacterium tuberculosis heme uptake pathway can be used as a drug target

Mycobacterium tuberculosis (Mtb) acquires non-heme iron through salicylate-derived siderophores termed mycobactins whereas heme iron is obtained through a cascade of heme uptake proteins. Three proteins are proposed to mediate Mtb heme iron uptake, a secreted heme transporter (Rv0203), and MmpL3 and MmpL11, which are potential transmembrane heme transfer proteins. Furthermore, MhuD, a cytoplasmic heme-degrading enzyme, has been identified. Rv0203, MmpL3 and MmpL11 are mycobacteria-specific proteins, making them excellent drug targets. Importantly, MmpL3, a necessary protein, has also been implicated in trehalose monomycolate export. Recent drug-discovery efforts revealed that MmpL3 is the target of several compounds with antimycobacterial activity. Inhibition of the Mtb heme uptake pathway has yet to be explored. We propose that inhibitor design could focus on heme analogs, with the goal of blocking specific steps of this pathway. In addition, heme uptake could be hijacked as a method of importing drugs into the mycobacterial cytosol.

Functional characterization of Helicobacter pylori TlyA: pore-forming hemolytic activity and cytotoxic property of the protein

Helicobacter pylori is a human specific gastric pathogen. H. pylori pathogenesis process involves a number of well-studied virulence factors that include the 'vacuolating cytotoxin' and the 'cytotoxin associated gene A'. Analysis of the H. pylori genome, however, indicates presence of additional virulence factors that are yet to be characterized in molecular detail. For example, H. pylori genome harbors a gene that has potential to encode a protein with sequence similarity to those of the TlyA-like proteins of several pathogenic bacteria. Earlier studies have indicated potential association of this H. pylori tlyA gene in the virulence mechanism of the organism. Despite such notions, however, the TlyA-like protein of H. pylori has not been studied previously in molecular detail. In particular, purified form of H. pylori TlyA has never been studied before toward exploring its functional properties. Here, we report characterization of the H. pylori TlyA protein purified from the recombinant over-expression system in Escherichia coli. Purified form of the recombinant TlyA exhibits prominent hemolytic activity against human erythrocytes, presumably via formation of pores of specific diameter in the cell membrane. Purified TlyA also triggers prominent cytotoxic responses in human gastric adenocarcinoma cells. Altogether, our study establishes H. pylori TlyA as a potential virulence factor of the organism.

Molecular modeling and in silico characterization of Mycobacterium tuberculosis TlyA: possible misannotation of this tubercle bacilli-hemolysin

Background

The TlyA protein has a controversial function as a virulence factor in Mycobacterium tuberculosis (M. tuberculosis). At present, its dual activity as hemolysin and RNA methyltransferase in M. tuberculosis has been indirectly proposed based on in vitro results. There is no evidence however for TlyA relevance in the survival of tubercle bacilli inside host cells or whether both activities are functionally linked. A thorough analysis of structure prediction for this mycobacterial protein in this study shows the need for reevaluating TlyA's function in virulence.

Results

Bioinformatics analysis of TlyA identified a ribosomal protein binding domain (S4 domain), located between residues 5 and 68 as well as an FtsJ-like methyltranferase domain encompassing residues 62 and 247, all of which have been previously described in translation machinery-associated proteins. Subcellular localization prediction showed that TlyA lacks a signal peptide and its hydrophobicity profile showed no evidence of transmembrane helices. These findings suggested that it may not be attached to the membrane, which is consistent with a cytoplasmic localization. Three-dimensional modeling of TlyA showed a consensus structure, having a common core formed by a six-stranded β-sheet between two α-helix layers, which is consistent with an RNA methyltransferase structure. Phylogenetic analyses showed high conservation of the tlyA gene among Mycobacterium species. Additionally, the nucleotide substitution rates suggested purifying selection during tlyA gene evolution and the absence of a common ancestor between TlyA proteins and bacterial pore-forming proteins.

Conclusion

Altogether, our manual in silico curation suggested that TlyA is involved in ribosomal biogenesis and that there is a functional annotation error regarding this protein family in several microbial and plant genomes, including the M. tuberculosis genome.

Discovery and characterization of a unique mycobacterial heme acquisition system

Mycobacterium tuberculosis must import iron from its host for survival, and its siderophore-dependent iron acquisition pathways are well established. Here we demonstrate a newly characterized pathway, whereby M. tuberculosis can use free heme and heme from hemoglobin as an iron source. Significantly, we identified the genomic region, Rv0202c-Rv0207c, responsible for the passage of heme iron across the mycobacterial membrane. Key players of this heme uptake system were characterized including a secreted protein and two transmembrane proteins, all three specific to mycobacteria. Furthermore, the crystal structure of the key heme carrier protein Rv0203 was found to have a unique fold. The discovery of a unique mycobacterial heme acquisition pathway opens new avenues of exploration into mycobacterial therapeutics.

Mycobacterium tuberculosis can utilize heme as an iron source

Most iron in mammals is found within the heme prosthetic group. Consequently, many bacterial pathogens possess heme acquisition systems to utilize iron from the host. Here, we demonstrate that Mycobacterium tuberculosis can utilize heme as an iron source, suggesting that M. tuberculosis possesses a yet-unknown heme acquisition system.

Molecular characterization of tlyA gene product, Rv1694 of Mycobacterium tuberculosis: a non-conventional hemolysin and a ribosomal RNA methyl transferase

Background

Mycobacterium tuberculosis is a virulent bacillus causing tuberculosis, a disease responsible for million deaths each year worldwide. In order to understand its mechanism of pathogenesis in humans and to help control tuberculosis, functions of numerous Mycobacterium tuberculosis genes are being characterized. In this study we report the dual functionality of tlyA gene product of Mycobacterium tuberculosis annotated as Rv1694, a 268 amino acid long basic protein.

Results

The recombinant purified Rv1694 protein was found to exhibit hemolytic activity in vitro. It showed concentration and time-dependent hemolysis of rabbit and human erythrocytes. Multiple oligomeric forms (dimers to heptamers) of this protein were seen on the membranes of the lysed erythrocytes. Like the oligomers of conventional, well-known, pore-forming toxins, the oligomers of Rv1694 were found to be resistant to heat and SDS, but were susceptible to reducing agents like β-mercaptoethanol as it had abolished the hemolytic activity of Rv1694 indicating the role of disulfide bond(s). The Rv1694 generated de novo by in vitro transcription and translation also exhibited unambiguous hemolysis confirming the self assembly and oligomerization properties of this protein. Limited proteolytic digestion of this protein has revealed that the amino terminus is susceptible while in solution but is protected in presence of membrane. Striking feature of Rv1694 is its presence on the cell wall of E. coli as visualized by confocal microscopy. The surface expression is consistent with the contact dependent haemolytic ability of E. coli expressing this protein. Also, immune serum specific to this protein inhibits the contact dependent hemolysis. Moreover, Rv1694 protein binds to and forms stable oligomers on the macrophage phagosomal membranes. In addition to all these properties, E. coli expressing Rv1694 was found to be susceptible to the antibiotic capreomycin as its growth was significantly slower than mock vector transformed E. coli. The S30 extract of E. coli expressing the Rv1694 had poor translational activity in presence of capreomycin, further confirming its methylation activity. Finally, incorporation of methyl group of [³H]-S-adenosylmethionine in isolated ribosomes also confirmed its methylation activity.

Conclusions

The Rv1694 has an unusual dual activity. It appears to contain two diverse functions such as haemolytic activity and ribosomal RNA methylation activity. It is possible that the haemolytic activity might be relevant to intra-cellular compartments such as phagosomes rather than cell lysis of erythrocytes and the self-assembly trait may have a potential role after successful entry into macrophages by Mycobacterium tuberculosis.

[Novel type of antimicrobial mechanism in host macrophages against mycobacterial infections]

Macrophages (M(phi)s) play a central role as anti-microbial effector cells in the expression of host resistance to mycobacterial infections. With respect to antimicrobial effector molecules of host M(phi) against mycobacterial pathogens, recent studies suggest the possibility that the reactive nitrogen intermediates (RNI)--and reactive oxygen intermediates-independent antimycobacterial mechanism(s) may be crucial for the antimycobacterial function of host M(phi). In this context, we previously found that free fatty acids (FFAs) such as arachidonic acid (AA) and linolenic acid exhibited potent antimicrobial activity against mycobacterial organisms, including Mycobacterium tuberculosis (MTB) and Mycobacterium avium complex (MAC). In addition, FFAs in combination with RNI played critical roles in manifestation of the activity of M(phi) against mycobacterial organisms. Moreover, our recent studies have shown the following findings. First, anti-MTB activity of IFN-gamma-activated M(phi)s was specifically blocked by arachidonyl trifluoromethylketone (aTFMK), an inhibitor of cytosolic phospholipase A2 (cPLA2). Second, ATP potentiated the anti-MAC bactericidal activity of M(phi)s cultivated in the presence of clarithromycin and rifamycin. This effect of ATP was closely related to intracellular Ca2+ mobilization and was specifically blocked by aTFMK. Third, intramacrophage translocation of membranous AA molecules to MAC-containing phagosomes was also specifically blocked by aTFMK. In the confocal microscopic observation of MAC-infected M(phi)s, ATP enhanced the intracellular translocation of cPLA2 into MAC-containing phagosomes. These findings suggest that FFAs (especially AA) produced by the enzymatic action of cPLA2 play important roles as antimycobacterial effectors in the expression of M(phi) antimicrobial activity against mycobacterial pathogens.

Functional analysis of the Campylobacter jejuni cj0183 and cj0588 genes

The cj0183 and cj0588 genes identified in the Campylobacter jejuni NCTC 11168 genome encode proteins with amino acid sequences predicted to be homologous to other bacterial hemolysins. The Cj0183 protein exhibits homology to Brachyspira hyodysenteriae TlyC protein, whereas the cj0588 gene product is homologous to TlyA proteins Brachyspira hyodysenteriae, Helicobacter pylori, and Mycobacterium tuberculosis, which play a crucial role in bacterial virulence. The aim of our work was to examine the hemolytic activity and determine the role of cj0183- and cj0588-encoded proteins on the adherence of chosen C. jejuni strains to the Caco-2 cell line by constructing deletion mutants in the mentioned genes. We found out there is no difference in hemolytic activity between both mutants in gene cj0183 and cj0588 and the wild strains. However, Cj0588 protein but not Cj0183 is involved in adherence to the Caco-2 cells.

Capreomycin Binds across the Ribosomal Subunit Interface Using tlyA-Encoded 2′-O-Methylations in 16S and 23S rRNAs

The cyclic peptide antibiotics capreomycin and viomycin are generally effective against the bacterial pathogen Mycobacterium tuberculosis. However, recent virulent isolates have become resistant by inactivation of their tlyA gene. We show here that tlyA encodes a 2'-O-methyltransferase that modifies nucleotide C1409 in helix 44 of 16S rRNA and nucleotide C1920 in helix 69 of 23S rRNA. Loss of these previously unidentified rRNA methylations confers resistance to capreomycin and viomycin. Many bacterial genera including enterobacteria lack a tlyA gene and the ensuing methylations and are less susceptible than mycobacteria to capreomycin and viomycin. We show that expression of recombinant tlyA in Escherichia coli markedly increases susceptibility to these drugs. When the ribosomal subunits associate during translation, the two tlyA-encoded methylations are brought into close proximity at interbridge B2a. The location of these methylations indicates the binding site and inhibitory mechanism of capreomycin and viomycin at the ribosome subunit interface.

Mutation of tlyA confers capreomycin resistance in Mycobacterium tuberculosis

Capreomycin, an important drug for the treatment of multidrug-resistant tuberculosis, is a macrocyclic peptide antibiotic produced by Saccharothrix mutabolis subspecies capreolus. The basis of resistance to this drug was investigated by isolating and characterizing capreomycin-resistant strains of Mycobacterium smegmatis and Mycobacterium tuberculosis. Colonies resistant to capreomycin were recovered from a library of transposon-mutagenized M. smegmatis. The transposon insertion site of one mutant was mapped to an open reading frame in the unfinished M. smegmatis genome corresponding to the tlyA gene (Rv1694) in the M. tuberculosis H37Rv genome. In M. smegmatis spontaneous capreomycin-resistant mutants, the tlyA gene was disrupted by one of three different naturally occurring insertion elements. Genomic DNAs from pools of transposon mutants of M. tuberculosis H37Rv were screened by PCR by using primers to the tlyA gene and the transposon to detect mutants with an insertion in the tlyA gene. One capreomycin-resistant mutant was recovered that contained the transposon inserted at base 644 of the tlyA gene. Complementation with the wild-type tlyA gene restored susceptibility to capreomycin in the M. smegmatis and M. tuberculosis tlyA transposon mutants. Mutations were found in the tlyA genes of 28 spontaneous capreomycin-resistant mutants generated from three different M. tuberculosis strains and in the tlyA genes of capreomycin-resistant clinical isolates. In in vitro transcription-translation assays, ribosomes from tlyA mutant but not tlyA(+) strains resist capreomycin inhibition of transcription-translation. Therefore, TlyA appears to affect the ribosome, and mutation of tlyA confers capreomycin resistance.

The Campylobacter jejuni dccRS two-component system is required for optimal in vivo colonization but is dispensable for in vitro growth

A Campylobacter jejuni two-component signal transduction system (TCSTS), designated dccR-dccS (diminished capacity to colonize; Cj1223c-Cj1222c), has been found to be important for in vivo colonization but dispensable for in vitro growth. A DeltadccR response regulator mutant generated using the virulent strain 81-176 background exhibited significantly reduced colonization of immunocompetent limited flora (I-LF) mice, severe combined immunodeficient limited flora (SCID-LF) mice, and 1-day-old chicks. A DeltadccS sensor kinase mutant was likewise defective for colonization in the I-LF mouse model. DeltadccR-infected SCID-LF mice also exhibited dramatically reduced inflammation relative to wild type-infected SCID-LF mice. Despite this diminished colonization capacity, the DeltadccRS mutants were indistinguishable from wild type for growth under numerous in vitro conditions as well as for various phenotypes. Microarray analysis identified several genes encoding putative periplasmic and membrane proteins as being regulated by this two-component system; binding of purified His-tagged DccR to the promoter region of two of these genes supports a direct protein-DNA interaction. A conserved repeat sequence was identified in the promoter regions of these genes and in three other promoter regions in the genome, including that of an operon encoding a putative type I secretion system. Two of the regulated target genes were found to be essential for optimal colonization. Both the two-component system and the putative regulated genes have uncharacterized homologues in other Campylobacter and Helicobacter spp., suggesting that they may perform an important function in colonization among a variety of related pathogenic species.

Most human isolates of Mycobacterium avium Mav-A and Mav-B are strong producers of hemolysin, a putative virulence factor

Hemolysin was quantified in 58 isolates of Mycobacterium avium from human, animal, and environmental sources. Human Mav-A and Mav-B isolates were the strongest producers; in contrast, animal and environmental Mav-A isolates and human, animal, and environmental Mav-C organisms were low-level producers. Hemolysin production was not restricted to isolates causing invasive infections.

Human CD8+ T cells recognize epitopes of the 28-kDa hemolysin and the 38-kDa antigen of Mycobacterium tuberculosis

Mycobacterium tuberculosis antigens that are recognized by human CD8+ T cells are potentially important vaccine target molecules. We used a motif-based strategy to screen selected proteins of M. tuberculosis for peptides predicted to bind to human leukocyte antigen (HLA)-A*0201. We identified two 10 amino acid peptides that elicited cytolytic T lymphocyte activity and interferon-gamma production by CD8+ T cells from HLA-A*0201+ healthy tuberculin reactors. These peptides were derived from the 38-kDa antigen and the 28-kDa hemolysin, the latter being a novel target for CD8+ T cells. We speculate that hemolysins may alter the phagosomal membrane surrounding intracellular M. tuberculosis, allowing themselves and other antigens to gain access to the major histocompatibility complex class I processing pathway.

Comparative profiles of intramacrophage behavior of Mycobacterium tuberculosis and Mycobacterium avium complex with different levels of virulence

Mycobacterium tuberculosis (MTB) and M. avium complex (MAC) strains with different levels of virulence in mice were examined for profiles of interaction with murine peritoneal macrophages (Mphis). Their growth rates in Mphis were in these orders: H37Ra strain (attenuated) > H37Rv strain (virulent) for MTB, and N-260 strain (moderate virulence) > MAC N-444 strain (low virulence) for MAC. MTB but not MAC caused the necrotic death of host Mphis in terms of increased release of lactate dehydrogenase from infected Mphis. The MTB H37Ra strain induced a greater production of reactive nitrogen intermediates (RNI) by Mphis than the MTB H37Rv strain did. However, this phenomenon was not observed with MAC, implying less important roles of RNI in the expression of Mphi antimicrobial activity against MAC organisms.

LsaA, an antigen involved in cell attachment and invasion, is expressed by Lawsonia intracellularis during infection in vitro and in vivo

Lawsonia intracellularis has been identified recently as the etiological agent of proliferative enteropathies, which are characterized by intestinal epithelial hyperplasia and associated moderate immune responses. This disease complex has been reported in a broad range of animals, prevalently in pigs, and L. intracellularis has been linked with ulcerative colitis in humans. L. intracellularis is an obligate intracellular bacterium, and the pathogenic mechanisms used to cause disease are unknown. Using in vitro-grown organisms as a source of genomic DNA, we identified a Lawsonia gene which encodes a surface antigen, LsaA (for Lawsonia surface antigen), associated with attachment to and entry into cells. The deduced amino acid sequence of this protein showed some similarity to members of a novel protein family identified in a number of other bacterial pathogens but for which roles are not fully defined. Transcription of this gene was detected by reverse transcription-PCR in L. intracellularis grown in vitro in IEC18 cells and in bacteria present in ileal tissue from infected animals. Immunohistochemistry with specific monoclonal antibody and immunoblotting with sera from infected animals demonstrated that LsaA protein is synthesized by L. intracellularis during infection. Expression of this gene during infection in vitro and in vivo suggests that this surface antigen is involved during infection, and phenotypic analysis indicated a role during L. intracellularis attachment to and entry into intestinal epithelial cells

Virulence factors of Mycobacterium bovis

Virulence factors of Mycobacterium bovis are the special properties that enable it to infect, survive, multiply and cause disease in an animal host. An understanding of these factors will lead to new strategies including an effective vaccine to control bovine tuberculosis. A few factors have already been identified and two broadly different approaches to discover other virulence factors are now being used. In the first approach, libraries of random M. bovis mutants are produced, the likely attenuated mutants are identified using a screening technique and the interrupted genes in selected mutants are identified. In the second approach, genes encoding putative virulence factors are selected by a range of different methods and then inactivated, usually by allelic exchange, to produce likely attenuated mutants of M. bovis. In both approaches, loss of virulence by a mutant must be determined in an animal model. Subsequently, the mutant must be complemented back to virulence with an active form of the identified gene in order to demonstrate that loss of virulence was not due to polar effects of the mutation on nearby genes. It is almost certain that most of the virulence factors of M. bovis are the same as those of the classical human tuberculosis organism, Mycobacterium tuberculosis, as both organisms can cause identical clinical disease in humans and are genetically very similar. Many putative virulence genes are now being investigated and only the inherent slowness with which mycobacterial work proceeds, delays the inevitable arrival of an exciting new phase in the understanding of mycobacterial disease.

Vaccination with the T cell antigen Mtb 8.4 protects against challenge with Mycobacterium tuberculosis

The development of an effective vaccine against Mycobacterium tuberculosis is a research area of intense interest. Mounting evidence suggests that protective immunity to M. tuberculosis relies on both MHC class II-restricted CD4(+) T cells and MHC class I-restricted CD8(+) T cells. By purifying polypeptides present in the culture filtrate of M. tuberculosis and evaluating these molecules for their ability to stimulate PBMC from purified protein derivative-positive healthy individuals, we previously identified a low-m.w. immunoreactive T cell Ag, Mtb 8.4, which elicited strong Th1 T cell responses in healthy purified protein derivative-positive human PBMC and in mice immunized with recombinant Mtb 8.4. Herein we report that Mtb 8.4-specific T cells can be detected in mice immunized with the current live attenuated vaccine, Mycobacterium bovis-bacillus Calmette-Guérin as well as in mice infected i.v. with M. tuberculosis. More importantly, immunization of mice with either plasmid DNA encoding Mtb 8.4 or Mtb 8.4 recombinant protein formulated with IFA elicited strong CD4(+) T cell and CD8(+) CTL responses and induced protection on challenge with virulent M. tuberculosis. Thus, these results suggest that Mtb 8.4 is a potential candidate for inclusion in a subunit vaccine against TB.

The complete genome sequence of the murine respiratory pathogen Mycoplasma pulmonis

Mycoplasma pulmonis is a wall-less eubacterium belonging to the Mollicutes (trivial name, mycoplasmas) and responsible for murine respiratory diseases. The genome of strain UAB CTIP is composed of a single circular 963 879 bp chromosome with a G + C content of 26.6 mol%, i.e. the lowest reported among bacteria, Ureaplasma urealyticum apart. This genome contains 782 putative coding sequences (CDSs) covering 91.4% of its length and a function could be assigned to 486 CDSs whilst 92 matched the gene sequences of hypothetical proteins, leaving 204 CDSs without significant database match. The genome contains a single set of rRNA genes and only 29 tRNAs genes. The replication origin oriC was localized by sequence analysis and by using the G + C skew method. Sequence polymorphisms within stretches of repeated nucleotides generate phase-variable protein antigens whilst a recombinase gene is likely to catalyse the site-specific DNA inversions in major M.pulmonis surface antigens. Furthermore, a hemolysin, secreted nucleases and a glyco-protease are predicted virulence factors. Surprisingly, several of the genes previously reported to be essential for a self-replicating minimal cell are missing in the M.pulmonis genome although this one is larger than the other mycoplasma genomes fully sequenced until now.

Regulation of virulence genes in Mycobacterium tuberculosis

Mycobacterium tuberculosis has demonstrated remarkable ability to survive in diverse conditions encountered during the infection process. These involve surviving the bactericidal stresses within the macrophage, the anaerobic and nutritionally altered environment of the granuloma, and the metabolically inactive latent state. Understanding the molecular basis of this adaptive behavior lies in the identification of genes (or virulence determinants) specifically expressed under these varied conditions. Transcriptional control plays a key role in regulating gene expression in response to environmental signals. However, even after decades of investigation our knowledge about the function of these regulatory mechanisms in mycobacteria remains meagre. But the elucidation of the genome sequence and implementation of sophisticated molecular genetic approaches to this organism have made a revolutionary impact on the study of mycobacterial pathogenesis. Deletion and complementation of individual genes can be done at will facilitating the comparative analysis of mutants and wild-type strains. Novel and powerful technologies such as DNA microarrays, fluorescent beacons and proteomics have made possible the analysis of the expression levels of multiple genes in in vitro systems. More technically challenging uses of these techniques is being undertaken to explore pathogen gene expression within the host. This will lead to the identification of virulence factors and give definitive insight into their regulatory signals.

M. tuberculosis: immunology and vaccination

Tuberculosis is increasing. Current treatment regimens require at least 6 months, because latent or stationary phase organisms are difficult to kill. Such regimens do not achieve full compliance, and "directly observed therapy short course" (DOTS) is having less impact than expected. This worrying situation is aggravated by coinfection with human immunodeficiency virus (HIV), and by the increase in drug-resistant strains. We need new insights that lead to more rapid therapies and immunotherapies, and more reliable vaccines. Recent insights have come from: understanding of the relationship between Mycobacterium tuberculosis and macrophages; the multiple T cell types that recognise mycobacterial peptides, lipids and glycolipids; the critical role of interferon-gamma (IFNgamma) and interleukin-12 (IL-12) in human mycobacterial infection revealed by genetically defective children; quantitation of the presence and importance of Th2 lymphocyte activation in human tuberculosis; the role of local conversion of inactive cortisone to active cortisol in the lesions; the recognition that some effective prophylactic vaccines also work as immumotherapeutics whereas others do not. In the longer term the recent sequencing of the M. tuberculosis genome will lead to further advances. In the short term, effective immunotherapy remains the most accessible breakthrough in the management of tuberculosis. The types of practical advance that will result from sequencing the genome are discussed speculatively, but cannot yet be predicted with certainty.

Helicobacter pylori pore-forming cytolysin orthologue TlyA possesses in vitro hemolytic activity and has a role in colonization of the gastric mucosa

Hemolysins have been found to possess a variety of functions in bacteria, including a role in virulence. Helicobacter pylori demonstrates hemolytic activity when cultured on unlysed blood agar plates which is increased under iron-limiting conditions. However, the role of an H. pylori hemolysin in virulence is unclear. Scrutiny of the H. pylori 26695 genome sequence suggests the presence of at least two distinct hemolysins, HP1086 and HP1490, in this strain. Previous studies have shown that the in vitro hemolytic activity of H. pylori is reduced when it is coincubated with dextran 5000, suggesting the presence of a pore-forming cytolysin. HP1086 has homology to pore-forming cytolysins (TlyA) from other bacterial species, and the introduction of the cloned H. pylori tlyA gene into a nonhemolytic Escherichia coli strain conferred hemolytic activity. An H. pylori tlyA defined mutant showed reduced in vitro hemolytic activity, which appears to be due to pore formation, as the hemolytic activity of the wild-type strain is reduced to the same level as the tlyA mutant by the addition of dextran 5000. The mutant also showed reduced adhesion to human gastric adenocarcinoma cells and failed to colonize the gastric mucosa of mice. These data clearly suggest a role in virulence for H. pylori TlyA, contrary to the suggestion that hemolytic activity is an in vitro phenomenon for this pathogen.

Human CD8+ CTL specific for the mycobacterial major secreted antigen 85A

The role of CD8(+) CTL in protection against tuberculosis in human disease is unclear. In this study, we stimulated the peripheral blood mononuclear cells of bacillus Calmette-Guérin (BCG)-vaccinated individuals with live Mycobacterium bovis BCG bacilli to establish short-term cell lines and then purified the CD8(+) T cells. A highly sensitive enzyme-linked immunospot (ELISPOT) assay for single cell IFN-gamma release was used to screen CD8(+) T cells with overlapping peptides spanning the mycobacterial major secreted protein, Ag85A. Three peptides consistently induced a high frequency of IFN-gamma responsive CD8(+) T cells, and two HLA-A*0201 binding motifs, P(48-56) and P(242-250), were revealed within the core sequences. CD8(+) T cells responding to the 9-mer epitopes were visualized within fresh blood by ELISPOT using free peptide or by binding of HLA-A*0201 tetrameric complexes. The class I-restricted CD8(+) T cells were potent CTL effector cells that efficiently lysed an HLA-A2-matched monocyte cell line pulsed with peptide as well as autologous macrophages infected with Mycobacterium tuberculosis or recombinant vaccinia virus expressing the whole Ag85A protein. Tetramer assays revealed a 6-fold higher frequency of peptide-specific T cells than IFN-gamma ELISPOT assays, indicating functional heterogeneity within the CD8(+) T cell population. These results demonstrate a previously unrecognized, MHC class I-restricted, CD8(+) CTL response to a major secreted Ag of mycobacteria and supports the use of Ag85A as a candidate vaccine against tuberculosis.

Necrosis of lung epithelial cells during infection with Mycobacterium tuberculosis is preceded by cell permeation

Mycobacterium tuberculosis establishes infection, progresses towards disease, and is transmitted from the alveolus of the lung. However, the role of the alveolar epithelium in any of these pathogenic processes of tuberculosis is unclear. In this study, lung epithelial cells (A549) were used as a model in which to examine cytotoxicity during infection with either virulent or avirulent mycobacteria in order to further establish the role of the lung epithelium during tuberculosis. Infection of A549 cells with M. tuberculosis strains Erdman and CDC1551 demonstrated significant cell monolayer clearing, whereas infection with either Mycobacterium bovis BCG or Mycobacterium smegmatis LR222 did not. Clearing of M. tuberculosis-infected A549 cells correlated to necrosis, not apoptosis. Treatment of M. tuberculosis-infected A549 cells with streptomycin, but not cycloheximide, demonstrated a significant reduction in the necrosis of A549 cell monolayers. This mycobacterium-induced A549 necrosis did not correlate to higher levels of intracellular or extracellular growth by the mycobacteria during infection. Staining of infected cells with propidium iodide demonstrated that M. tuberculosis induced increased permeation of A549 cell membranes within 24 h postinfection. Quantitation of lactate dehydrogenase (LDH) release from infected cells further demonstrated that cell permeation was specific to M. tuberculosis infection and correlated to A549 cellular necrosis. Inactivated M. tuberculosis or its subcellular fractions did not result in A549 necrosis or LDH release. These studies demonstrate that lung epithelial cell cytotoxicity is specific to infection by virulent mycobacteria and is caused by cellular necrosis. This necrosis is not a direct correlate of mycobacterial growth or of the expression of host cell factors, but is preceded by permeation of the A549 cell membrane and requires infection with live bacilli.

The complete sequence of the mucosal pathogen Ureaplasma urealyticum

The comparison of the genomes of two very closely related human mucosal pathogens, Mycoplasma genitalium and Mycoplasma pneumoniae, has helped define the essential functions of a self-replicating minimal cell, as well as what constitutes a mycoplasma. Here we report the complete sequence of a more distant phylogenetic relative of those bacteria, Ureaplasma urealyticum (parvum biovar), which is also a mucosal pathogen of humans. It is the third mycoplasma to be sequenced, and has the smallest sequenced prokaryotic genome except for M. genitalium. Although the U. urealyticum genome is similar to the two sequenced mycoplasma genomes, features make this organism unique among mycoplasmas and all bacteria. Almost all ATP synthesis is the result of urea hydrolysis, which generates an energy-producing electrochemical gradient. Some highly conserved eubacterial enzymes appear not to be encoded by U. urealyticum, including the cell-division protein FtsZ, chaperonins GroES and GroEL, and ribonucleoside-diphosphate reductase. U. urealyticum has six closely related iron transporters, which apparently arose through gene duplication, suggesting that it has a kind of respiration system not present in other small genome bacteria The genome is only 25.5% G+C in nucleotide content, and the G+C content of individual genes may predict how essential those genes are to ureaplasma survival.