Identification of Mycobacterium marinum virulence genes using signature-tagged mutagenesis and the goldfish model of mycobacterial pathogenesis

Mycobacteriosis and Infections with Non-tuberculous Mycobacteria in Aquatic Organisms: A Review

The Mycobacteriaceae constitute a family of varied Gram-positive organisms that include a large number of pathogenic bacteria. Among these, non-tuberculous mycobacteria are endemic worldwide and have been associated with infections in a large number of organisms, including humans and other mammals and reptiles, as well as fish. In this review, we summarize the most recent findings regarding this group of pathogens in fish. There, four species are most commonly associated with disease outbreaks: Mycobacterium marinum, the most common of these fish mycobacterial pathogens, Mycobacterium fortuitum, Mycobacterium gordonae, and Mycobacterium chelonae. These bacteria have a broad host range: they are zoonotic, and infections have been reported in a large number of fish species. The main route of entry of the bacterium into the fish is through the gastrointestinal route, and the disease is associated with ulcerative dermatitis as well as organomegaly and the development of granulomatous lesions in the internal organs. Mycobacteriaceae are slow-growing and fastidious and isolation is difficult and time consuming and diagnostic is mostly performed using serological and molecular tools. Control of the disease is also difficult: there is currently no effective vaccine and infections react poorly to antibiotherapy. For this reason, more research is needed on the subject of these vexing pathogens.

Mycobacterium marinum infection in fish and man: epidemiology, pathophysiology and management; a review

Mycobacterium marinum is an opportunistic pathogen inducing infection in fresh and marine water fish. This pathogen causes necrotizing granuloma like tuberculosis, morbidity and mortality in fish. The cell wall-associated lipid phthiocerol dimycocerosates, phenolic glycolipids and ESAT-6 secretion system 1 (ESX-1) are the conserved virulence determinant of the organism. Human infections with Mycobacterium marinum hypothetically are classified into four clinical categories (type I-type IV) and have been associated with the exposure of damaged skin to polluted water from fish pools or contacting objects contaminated with infected fish. Fish mycobacteriosis is clinically manifested and characterized in man by purple painless nodules, liable to develop into superficial crusting ulceration with scar formation. Early laboratory diagnosis of M. marinum including histopathology, culture and PCR is essential and critical as the clinical response to antibiotics requires months to be attained. The pathogenicity and virulence determinants of M. marinum need to be thoroughly and comprehensively investigated and understood. In spite of accumulating information on this pathogen, the different relevant data should be compared, connected and globally compiled. This article is reviewing the epidemiology, virulence factors, diagnosis and disease management in fish while casting light on the potential associated public health hazards.

Humoral and Cellular Immune Response of European Seabass Dicentrarchus labrax Vaccinated with Heat-Killed Mycobacterium marinum (iipA::kan Mutant)

No vaccine is yet commercially available against Mycobacterium marinum, the etiological agent of fish mycobacteriosis (also known as "fish tuberculosis"). The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to moderate M. marinum pathology in Zebrafish Danio rerio. Two doses of heat-killed, wild-type, virulent M. marinum and two doses of a heat-killed, avirulent M. marinum iipA::kan mutant strain were used in parallel to vaccinate European Seabass Dicentrarchus labrax. The fish were then challenged with live, virulent M. marinum, and the pathogenesis of the infection was monitored. High specific immunoglobulin M (IgM) response and an increase in cytokine tumor necrosis factor alpha (TNF-α) messenger RNA expression levels were observed in all vaccinated fish. At 1 month postchallenge, TNF-α expression levels increased in spleen tissues of fish vaccinated with the virulent type and in those of unvaccinated fish, whereas in the head kidney, expression was up-regulated only in unvaccinated fish. The expression then decreased, and at 2 months postchallenge, expression appeared similar in all vaccination types. The highest survival rate (75%) was recorded in the group of fish that were vaccinated with a high dose of avirulent iipA::kan mutant. The iipA::kan mutant induced a strong immune response accompanied by only modest tissue disruption. Coupled with an effective program of booster treatments, the iipA::kan mutant vaccine may be developed into a powerful preventive measure against fish mycobacteriosis.

Rv3272 encodes a novel Family III CoA transferase that alters the cell wall lipid profile and protects mycobacteria from acidic and oxidative stress

The availability of complete genome sequence of Mycobacterium tuberculosis has provided an important tool to understand the mycobacterial biology with respect to host-pathogen interaction, which is an unmet need of the hour owing to continuous increasing drug resistance. Hypothetical proteins are often an overlooked pool though half the genome encodes for such proteins of unknown function that could potentially play vital roles in mycobacterial biology. In this context, we report the structural and functional characterization of the hypothetical protein Rv3272. Sequence analysis classifies Rv3272 as a Family III CoA transferase with the classical two domain structure and conserved Aspartate residue (D175). The crystal structure of the wild type protein (2.2 Å) demonstrated the associated inter-locked dimer while that of the D175A mutant co-crystallized with octanoyl-CoA demonstrated relative movement between the two domains. Isothermal titration calorimetry studies indicate that Rv3272 binds to fatty acyl-CoAs of varying carbon chain lengths, with palmitoyl-CoA (C16:0) exhibiting maximum affinity. To determine the functional relevance of Rv3272 in mycobacterial biology, we ectopically expressed Rv3272 in M. smegmatis and assessed that its expression encodes significant alteration in cell surface with marked differences in triacylglycerol accumulation. Additionally, Rv3272 expression protects mycobacteria from acidic, oxidative and antibiotic stress under in vitro conditions. Taken together, these studies indicate a significant role for Rv3272 in host-pathogen interaction.

Interleukin 10 mutant zebrafish have an enhanced interferon gamma response and improved survival against a Mycobacterium marinum infection

Tuberculosis ranks as one of the world’s deadliest infectious diseases causing more than a million casualties annually. IL10 inhibits the function of Th1 type cells, and IL10 deficiency has been associated with an improved resistance against Mycobacterium tuberculosis infection in a mouse model. Here, we utilized M. marinum infection in the zebrafish (Danio rerio) as a model for studying Il10 in the host response against mycobacteria. Unchallenged, nonsense il10^e46/e46 mutant zebrafish were fertile and phenotypically normal. Following a chronic mycobacterial infection, il10^e46/e46 mutants showed enhanced survival compared to the controls. This was associated with an increased expression of the Th cell marker cd4-1 and a shift towards a Th1 type immune response, which was demonstrated by the upregulated expression of tbx21 and ifng1, as well as the down-regulation of gata3. In addition, at 8 weeks post infection il10^e46/e46 mutant zebrafish had reduced expression levels of proinflammatory cytokines tnfb and il1b, presumably indicating slower progress of the infection. Altogether, our data show that Il10 can weaken the immune defense against M. marinum infection in zebrafish by restricting ifng1 response. Importantly, our findings support the relevance of M. marinum infection in zebrafish as a model for tuberculosis.

Goldfish, Carassius auratus, as an infection model for studying the pathogenesis of Edwardsiella piscicida

This study demonstrates the feasibility of using goldfish as an infection model to investigate the pathogenesis of Edwardsiella piscicida. Goldfish were found to be susceptible to acute E. piscicida-induced disease and died in a dose-dependent manner. E. piscicida was further shown to replicate rapidly in the head kidneys and livers of infected goldfish from 1 d post-injection, and bacteria numbers were significantly decreased 5 d post-injection. Immune responses were successfully induced in goldfish injected with E. piscicida strains and 60% of goldfish inoculated with an attenuated E. piscicida strain were found to survive subsequent injection with a pathogenic strain. The results of differential leukocyte count experiments suggested that leukocytes were immediately recruited as an innate immune response against the infection. Thus, this well-characterized goldfish species is a suitable infection model for studying E. piscicida pathogenesis, and might be applicable to research on other fish diseases.

Mycobacterium marinum

Mycobacterium marinum is a well-known pathogenic mycobacterium for skin and soft tissue infections and is associated with fishes and water. Among nontuberculous mycobacteria (NTM), it is the leading cause of extrarespiratory human infections worldwide. In addition, there is a specific scientific interest in M. marinum because of its genetic relatedness to Mycobacterium tuberculosis and because experimental infection of M. marinum in fishes mimics tuberculosis pathogenesis. Microbiological characteristics include the fact that it grows in 7 to 14 days with photochromogenic colonies and is difficult to differentiate from Mycobacterium ulcerans and other mycolactone-producing NTM on a molecular basis. The diagnosis is highly suspected by the mode of infection, which is related to the hobby of fishkeeping, professional handling of marine shells, or swimming in nonchlorinated pools. Clinics distinguished skin and soft tissue lesions (typically sporotrichoid or subacute hand nodules) and lesions disseminated to joint and bone, often related with the local use of corticosteroids. In clinical microbiology, microscopy and culture are often negative because growth requires low temperature (30°C) and several weeks to succeed in primary cultivation. The treatment is not standardized, and no randomized control trials have been done. Therapy is a combination of surgery and antimicrobial agents such as cyclines and rifampin, with successful outcome in most of the skin diseases but less frequently in deep tissue infections. Prevention can be useful with hand protection recommendations for professionals and all persons manipulating fishes or fish tank water and use of alcohol disinfection after contact.

Transcriptomic Approaches in the Zebrafish Model for Tuberculosis-Insights Into Host- and Pathogen-specific Determinants of the Innate Immune Response

Mycobacterium marinum infection in zebrafish has become a well-established model of tuberculosis. Both embryonic and adult zebrafish infection studies have contributed to our knowledge of the development and function of tuberculous granulomas, which are typical of mycobacterial pathogenesis. In this review we discuss how transcriptome profiling studies have helped to characterize this infection process. We illustrate this using new RNA sequencing (RNA-Seq) data that reveals three main phases in the host response to M. marinum during the early stages of granuloma development in zebrafish embryos and larvae. The early phase shows induction of complement and transcription factors, followed by a relatively minor induction of pro-inflammatory cytokines within hours following phagocytosis of M. marinum. A minimal response is observed in the mid-phase, between 6 hours and 1day post infection, when the tissue dissemination of M. marinum begins. During subsequent larval development the granulomas expand and a late-phase response is apparent, which is characterized by progressively increasing induction of complement, transcription factors, pro-inflammatory cytokines, matrix metalloproteinases, and other defense and inflammation-related gene groups. This late-phase response shares common components with the strong and acute host transcriptome response that has previously been reported for Salmonella typhimurium infection in zebrafish embryos. In contrast, the early/mid-phase response to M. marinum infection, characterized by suppressed pro-inflammatory signaling, is strikingly different from the acute response to S. typhimurium infection. Furthermore, M. marinum infection shows a collective and strongly fluctuating regulation of lipoproteins, while S. typhimurium infection has pronounced effects on amino acid metabolism and glycolysis.

Mycobacterium tuberculosis effectors involved in host-pathogen interaction revealed by a multiple scales integrative pipeline

BACKGROUND

Mycobacterium tuberculosis (Mtb) has evolved multiple strategies to counter host immunity. Proteins are one important player in the host-pathogen interaction. A comprehensive list of such proteins will benefit our understanding of pathogenesis of Mtb.

METHODS

A genome-scale dataset was created from different sources of published data: global gene expression studies in disease models; genome-wide insertional mutagenesis defining gene essentiality under different conditions; genes lost in clinical isolates; subcellular localization analysis and non-homology analysis. Using data mining and meta-analysis, expressed proteins critical for intracellular survival of Mtb are first identified, followed by subcellular localization analysis, finally filtering a series of subtractive channel of analysis to find out promising drug target candidates.

RESULTS

The analysis found 54 potential candidates essential for the intracellular survival of the pathogen and non-homologous to host or gut flora, and might be promising drug targets.

CONCLUSION

Based on our meta-analysis and bioinformatics analysis, 54 hits were found from Mtb around 4000 open reading frames. These hits can be good candidates for further experimental investigation.

Biosynthesis of cell envelope-associated phenolic glycolipids in Mycobacterium marinum

Phenolic glycolipids (PGLs) are polyketide synthase-derived glycolipids unique to pathogenic mycobacteria. PGLs are found in several clinically relevant species, including various Mycobacterium tuberculosis strains, Mycobacterium leprae, and several nontuberculous mycobacterial pathogens, such as M. marinum. Multiple lines of investigation implicate PGLs in virulence, thus underscoring the relevance of a deep understanding of PGL biosynthesis. We report mutational and biochemical studies that interrogate the mechanism by which PGL biosynthetic intermediates (p-hydroxyphenylalkanoates) synthesized by the iterative polyketide synthase Pks15/1 are transferred to the noniterative polyketide synthase PpsA for acyl chain extension in M. marinum. Our findings support a model in which the transfer of the intermediates is dependent on a p-hydroxyphenylalkanoyl-AMP ligase (FadD29) acting as an intermediary between the iterative and the noniterative synthase systems. Our results also establish the p-hydroxyphenylalkanoate extension ability of PpsA, the first-acting enzyme of a multisubunit noniterative polyketide synthase system. Notably, this noniterative system is also loaded with fatty acids by a specific fatty acyl-AMP ligase (FadD26) for biosynthesis of phthiocerol dimycocerosates (PDIMs), which are nonglycosylated lipids structurally related to PGLs. To our knowledge, the partially overlapping PGL and PDIM biosynthetic pathways provide the first example of two distinct, pathway-dedicated acyl-AMP ligases loading the same type I polyketide synthase system with two alternate starter units to produce two structurally different families of metabolites. The studies reported here advance our understanding of the biosynthesis of an important group of mycobacterial glycolipids.

Antimicrobial responses of teleost phagocytes and innate immune evasion strategies of intracellular bacteria

During infection, macrophage lineage cells eliminate infiltrating pathogens through a battery of antimicrobial responses, where the efficacy of these innate immune responses is pivotal to immunological outcomes. Not surprisingly, many intracellular pathogens have evolved mechanisms to overcome macrophage defenses, using these immune cells as residences and dissemination strategies. With pathogenic infections causing increasing detriments to both aquacultural and wild fish populations, it is imperative to garner greater understanding of fish phagocyte antimicrobial responses and the mechanisms by which aquatic pathogens are able to overcome these teleost macrophage barriers. Insights into the regulation of macrophage immunity of bony fish species will lend to the development of more effective aquacultural prophylaxis as well as broadening our understanding of the evolution of these immune processes. Accordingly, this review focuses on recent advances in the understanding of teleost macrophage antimicrobial responses and the strategies by which intracellular fish pathogens are able to avoid being killed by phagocytes, with a focus on Mycobacterium marinum.

Analysis of the immune response in infections of the goldfish (Carassius auratus L.) with Mycobacterium marinum

The rapid doubling time and genetic relatedness of the fish pathogen Mycobacterium marinum to Mycobacterium tuberculosis has rendered the former an attractive model for investigating mycobacterial host-pathogen interactions. We employed the M. marinum-goldfish infection model to investigate the in vivo immune responses to this pathogen in the context of a natural host. Histological analysis revealed mycobacterial infiltrates in goldfish kidney and spleen tissues, peaking 28 days post infections (dpi). Quantitative gene expression analysis showed significant increases of mRNA levels of pro-inflammatory cytokines (IFNγ, IL-12p40, IL-1β1) and cytokine receptors (IFNGR1-1, TNFR2) at 7 dpi. Conversely, the gene expression levels of key anti-inflammatory cytokines TGFβ and IL-10 were elevated at 14 dpi. Furthermore, M. marinum infections markedly increased the cytokine-primed oxidative burst responses of isolated kidney phagocytes at 7 but not 56 dpi. We believe that the M. marinum-goldfish infection model will be invaluable in furthering the understanding of the mycobacterium host-pathogen interface.

Mycoketide: a CD1c-presented antigen with important implications in mycobacterial infection

Mycobacterium tuberculosis and related mycobacteria species are unique in that the acid-fast bacilli possess a highly lipid-rich cell wall that not simply confers resistance to treatment with acid alcohol, but also controls their survival and virulence. It has recently been established that a fraction of the cell wall lipid components of mycobacteria can function as antigens targeted by the acquired immunity of the host. Human group 1 CD1 molecules (CD1a, CD1b, and CD1c) bind a pool of lipid antigens expressed by mycobacteria and present them to specific T cells, thereby mediating an effective pathway for host defense against tuberculosis. The contrasting and mutually complementary functions of CD1a and CD1b molecules in terms of the repertoire of antigens they bind have been well appreciated, but it remains to be established how CD1c may play a unique role. Nevertheless, recent advances in our understanding of the CD1c structure as well as the biosynthetic pathway of a CD1c-presented antigen, mannose-1, β-phosphomycoketide, expressed by pathogenic mycobacteria now unravel a new aspect of the group 1 CD1 biology that has not been appreciated in previous studies of CD1a and CD1b molecules.

The mycobacterial acyltransferase PapA5 is required for biosynthesis of cell wall-associated phenolic glycolipids

Phenolic glycolipids (PGLs) are non-covalently bound components of the outer membrane of many clinically relevant mycobacterial pathogens, and play important roles in pathogen biology. We report a mutational analysis that conclusively demonstrates that the conserved acyltransferase-encoding gene papA5 is essential for PGL production. In addition, we provide an in vitro acyltransferase activity analysis that establishes proof of principle for the competency of PapA5 to utilize diol-containing polyketide compounds of mycobacterial origin as acyl-acceptor substrates. Overall, the results reported herein are in line with a model in which PapA5 catalyses the acylation of diol-containing polyketides to form PGLs. These studies advance our understanding of the biosynthesis of an important group of mycobacterial glycolipids and suggest that PapA5 might be an attractive target for exploring the development of antivirulence drugs.

Expression of common fluorescent reporters may modulate virulence for Mycobacterium marinum: dramatic attenuation results from Gfp over-expression

Mycobacterium marinum is an established surrogate pathogen for Mycobacterium tuberculosis because of its strong conservation of thousands of orthologous genes, lower risk to researchers and similar pathology in fish. This pathogen causes TB-like chronic disease in a wide variety of fish species. As in human TB, the microbe grows within the host macrophages, can mount life-long chronic infections and produces granulomatous lesions in target organs. One of the fish species known to manifest chronic "fish TB" is the small laboratory fish, Japanese ricefish (medaka; Oryzias latipes). Our laboratory is currently characterizing the disease progression in medaka using fluorescent reporter systems that are introduced into engineered strains of M. marinum. While conducting these studies we observed differences in growth, plasmid stability, and virulence depending on which fluorescent reporter construct was present. Here, we describe large negative effects on virulence and organ colonization that occurred with a commonly used plasmid pG13, that expresses green fluorescent protein (Gfp). The studies presented here, indicate that Gfp over-expression was the basis for the reduced virulence in this reporter construct. We also show that these negative effects could be reversed by significantly reducing Gfp expression levels or by using low-expression constructs of Rfp.

Analysis of the antimicrobial responses of primary phagocytes of the goldfish (Carassius auratus L.) against Mycobacterium marinum

The slow growth rate of Mycobacterium spp. that infect humans coupled with a lack of reliable in vitro infection model systems has hindered the progress of research in host cell-mycobacteria interactions. Recent studies have utilized the relatively fast growing Mycobacterium marinum to examine the host-pathogen interface in natural fish hosts. Here we describe the use of primary goldfish monocyte and mature macrophage cultures to investigate the immune cell-M. marinum interactions. Live and heat-killed M. marinum abrogated the recombinant goldfish (rg)TNFα2 and rgIFNγ-induced monocyte reactive oxygen production. Live but not heat-killed M. marinum also ablated rgIFNγrel and rg-TNFα2 induced macrophage nitric oxide production. M. marinum induced significant changes in gene expression of select NADPH oxidase components and inflammatory cytokine receptors and up-regulated the expression of immunosuppressive genes IL-10, TGFβ1 and SOCS-3. The exposure of monocytes and mature macrophages to M. marinum caused an increase in the mRNA levels of several pro-inflammatory genes. Stimulation of monocytes and macrophages with rgTNFα2, rgIFNγ, or rgIFNγrel reduced the survival of intracellular mycobacteria. The characterization of the interaction between M. marinum and natural host-derived primary phagocyte cultures will enable future studies on the host-pathogen interactions in mycobacterial infections.

Inactivation of tesA reduces cell wall lipid production and increases drug susceptibility in mycobacteria

Phthiocerol dimycocerosates (PDIMs) and phenolic glycolipids (PGLs) are structurally related lipids noncovalently bound to the outer cell wall layer of Mycobacterium tuberculosis, Mycobacterium leprae, and several opportunistic mycobacterial human pathogens. PDIMs and PGLs are important effectors of virulence. Elucidation of the biosynthesis of these complex lipids will not only expand our understanding of mycobacterial cell wall biosynthesis, but it may also illuminate potential routes to novel therapeutics against mycobacterial infections. We report the construction of an in-frame deletion mutant of tesA (encoding a type II thioesterase) in the opportunistic human pathogen Mycobacterium marinum and the characterization of this mutant and its corresponding complemented strain control in terms of PDIM and PGL production. The growth and antibiotic susceptibility of these strains were also probed and compared with the parental wild-type strain. We show that deletion of tesA leads to a mutant that produces only traces of PDIMs and PGLs, has a slight growth yield increase and displays a substantial hypersusceptibility to several antibiotics. We also provide a robust model for the three-dimensional structure of M. marinum TesA (TesAmm) and demonstrate that a Ser-to-Ala substitution in the predicted catalytic Ser of TesAmm renders a mutant that recapitulates the phenotype of the tesA deletion mutant. Overall, our studies demonstrate a critical role for tesA in mycobacterial biology, advance our understanding of the biosynthesis of an important group of polyketide synthase-derived mycobacterial lipids, and suggest that drugs aimed at blocking PDIM and/or PGL production might synergize with antibiotic therapy in the control of mycobacterial infections.

Construction of targeted mycobacterial mutants by homologous recombination

The ability to select genes to knock out of mycobacterial genomes has greatly improved our understanding of mycobacteria. This chapter describes a method for doing this. The gene (including a 1-kb flanking region) is cloned into a pNIL series vector and disrupted by deletion or insertion of a cassette. A selection of marker genes obtained from the pGOAL series of vectors are inserted into the pNIL vector to create a suicide delivery system. This delivery vector is introduced into mycobacteria where the disrupted version of the gene replaces the wild-type version by a two-step homologous recombination process. The method involves selecting for a single crossover event followed by selection of double crossovers. Single crossovers have incorporated plasmid marker genes and are sucrose(S), kanamycin(R) and blue on media containing X-gal. Double crossovers have lost plasmid markers and are sucrose(R), kanamycin(S) and white on media containing X-gal.

Yersinia ruckeri genes that attenuate survival in rainbow trout (Oncorhynchus mykiss) are identified using signature-tagged mutants

To identify genes that enable the enteric redmouth disease bacterium, Yersinia ruckeri, to persist in salmonid fish, 1056 signature-tagged mini-Tn5Km2 transposon mutants of a serotype 1 strain of Y. ruckeri, RS1154, were screened in rainbow trout by immersion infection. Two rounds of screening in fish identified 25 mutants that were not re-isolated from the kidney, 7 days post-infection. Six mutants were tested a third time in fish, in 1:1 competitive challenges with the parent strain; 4 failed to establish in kidney and 2 were present at low levels compared to the parent. Sequence analyses from the single transposon insertion sites in each of the 25 mutants identified genes with sequence homologies to genes for ZnuA, a periplasmic zinc-binding protein of ZnuABC transporter; the UvrY response regulator of BarA-UvrY two-component system; a PtrA protease of the insulin-degrading enzyme family; the RcpA protein of type IV bundle-forming pili; the ParA ATPase of a ParAB DNA-partitioning system; a Wzy polymerase; a polysaccharide deacetylase; a transporter belonging to the major facilitator superfamily and 7 hypothetical proteins of unknown function. The products of 5 of these mutated genes have predicted functions associated with cell surfaces or membranes, which could be important for survival of Y. ruckeri in rainbow trout, while other putative gene products could contribute to infection and invasion processes.

Cooperation between a coenzyme A-independent stand-alone initiation module and an iterative type I polyketide synthase during synthesis of mycobacterial phenolic glycolipids

Several Mycobacterium tuberculosis strains, Mycobacterium leprae, and other mycobacterial pathogens produce a group of small-molecule virulence factors called phenolic glycolipids (PGLs). PGLs play key roles in pathogenicity and host−pathogen interaction. Thus, elucidation of the PGL biosynthetic pathway will not only expand our understanding of natural product biosynthesis, but may also illuminate routes to novel therapeutics to afford alternative lines of defense against mycobacterial infections. In this study, we report an investigation of the enzymatic requirements for the production of long-chain p-hydroxyphenylalkanoate intermediates of PGL biosynthesis. We demonstrate a functional cooperation between a coenzyme A-independent stand-alone didomain initiation module (FadD22) and a 6-domain reducing iterative type I polyketide synthase (Pks15/1) for production of p-hydroxyphenylalkanoate intermediates in in vitro and in vivo FadD22-Pks15/1 reconstituted systems. Our results suggest that Pks15/1 is an iterative type I polyketide synthase with a relaxed control of catalytic cycle iterations, a mechanistic property that explains the origin of a characteristic alkyl chain length variability seen in mycobacterial PGLs. The FadD22-Pks15/1 reconstituted systems lay an initial foundation for future efforts to unveil the mechanism of iterative catalysis control by which the structures of the final products of Pks15/1 are defined, and to scrutinize the functional partnerships of the FadD22-Pks15/1 system with downstream enzymes of the PGL biosynthetic pathway.

Identification of a novel multidrug efflux pump of Mycobacterium tuberculosis

The impermeability of the outer membrane in combination with drug efflux are major determinants of the natural drug resistance of mycobacteria. beta-Lactams are the most widely used antibiotics for treatment of bacterial infections. However, it is unknown how beta-lactams enter Mycobacterium tuberculosis and whether efflux pumps exist that can export these drugs out of the cell. To identify the molecular mechanisms of M. tuberculosis resistance to beta-lactams, a library of 7,500 transposon mutants was generated in the model organism Mycobacterium bovis BCG. Thirty-three unique insertion sites were determined that conferred medium or high-level (> or =2,000 microg/ml) resistance to ampicillin. Three mutants in sulfolipid synthesis or transport were highly resistant to ampicillin, indicating an indirect effect of the lipid composition on the outer membrane permeability of M. bovis BCG to ampicillin. Mutants with insertions in genes encoding surface molecules such as PPE proteins or lipoarabinomannan were also completely resistant to ampicillin, thus suggesting a lack of transport across the outer membrane. Insertion of the transposon in front of bcg0231 increased transcription of the gene and concomitantly the resistance of M. bovis BCG to ampicillin, streptomycin, and chloramphenicol by 32- to 64-fold. Resistance to vancomycin and tetracycline was increased four- to eightfold. Bcg0231 and Rv0194 are almost identical ATP-binding cassette transporters. Expression of rv0194 significantly reduced accumulation of ethidium bromide and conferred multidrug resistance to Mycobacterium smegmatis. Both effects were abrogated in the presence of the efflux pump inhibitor reserpine. These results demonstrate that Rv0194 is a novel multidrug efflux pump of M. tuberculosis.

Use of gene dosage effects for a whole-genome screen to identify Mycobacterium marinum macrophage infection loci

We recently identified two loci, mel1 and mel2, that affect macrophage infection by Mycobacterium marinum. The ability of these loci to confer enhanced infection in trans is presumably due to gene dosage effects since their presence on plasmids increases expression from five- to eightfold. Reasoning that this phenomenon would allow identification of other mycobacterial genes involved in macrophage infection, we conducted a screen of an M. marinum DNA library that provides 2.6-fold coverage of the entire genome for clones that affect macrophage infection. Our preliminary screen identified 76 plasmids that carry loci affecting macrophage infection. We eliminated plasmids that do not confer the expected phenotype when retransformed (70%), that have identical physical maps (5%), or that carry either of the mel1 or mel2 loci (14%) from further consideration. Four loci that confer enhanced infection (mel) and four that confer repressed infection (mrl) of macrophages were identified, and two of each group were chosen for detailed analysis. Saturating transposon mutagenesis was used to identify the loci responsible, and M. marinum mutants were constructed in the genes involved. We expect these genes to provide insight into how mycobacteria parasitize macrophages, an important component of innate immunity.

Comparative pathogenesis of Mycobacterium marinum and Mycobacterium tuberculosis

A thorough understanding of Mycobacterium tuberculosis pathogenesis in humans has been elusive in part because of imperfect surrogate laboratory hosts, each with its own idiosyncrasies. Mycobacterium marinum is the closest genetic relative of the M. tuberculosis complex and is a natural pathogen of ectotherms. In this review, we present evidence that the similar genetic programmes of M. marinum and M. tuberculosis and the corresponding host immune responses reveal a conserved skeleton of Mycobacterium host-pathogen interactions. While both species have made niche-specific refinements, an essential framework has persisted. We highlight genetic comparisons of the two organisms and studies of M. marinum in the developing zebrafish. By pairing M. marinum with the simplified immune system of zebrafish embryos, many of the defining mechanisms of mycobacterial pathogenesis can be distilled and investigated in a tractable host/pathogen pair.

Mycobacterial phenolic glycolipid virulence factor biosynthesis: mechanism and small-molecule inhibition of polyketide chain initiation

Phenolic glycolipids (PGLs) are polyketide-derived virulence factors produced by Mycobacterium tuberculosis, M. leprae, and other mycobacterial pathogens. We have combined bioinformatic, genetic, biochemical, and chemical biology approaches to illuminate the mechanism of chain initiation required for assembly of the p-hydroxyphenyl-polyketide moiety of PGLs. Our studies have led to the identification of a stand-alone, didomain initiation module, FadD22, comprised of a p-hydroxybenzoic acid adenylation domain and an aroyl carrier protein domain. FadD22 forms an acyl-S-enzyme covalent intermediate in the p-hydroxyphenyl-polyketide chain assembly line. We also used this information to develop a small-molecule inhibitor of PGL biosynthesis. Overall, these studies provide insights into the biosynthesis of an important group of small-molecule mycobacterial virulence factors and support the feasibility of targeting PGL biosynthesis to develop new drugs to treat mycobacterial infections.

Genes required for Lactococcus garvieae survival in a fish host

Lactococcus garvieae is considered an emergent pathogen in aquaculture and it is also associated with mastitis in domestic animals as well as human endocarditis and septicaemia. In spite of this, the pathogenic mechanisms of this bacterium are poorly understood. Signature-tagged mutagenesis was used to identify virulence factors and to establish the basis of pathogen-host interactions. A library of 1250 L. garvieae UNIUD074-tagged Tn917 mutants in 25 pools was screened for the ability to grow in fish. Among them, 29 mutants (approx. 2.4 %) were identified which could not be recovered from rainbow trout following infection. Sequence analysis of the tagged Tn917-interrupted genes in these mutants indicated the participation in pathogenesis of the transcriptional regulatory proteins homologous to GidA and MerR; the metabolic enzymes asparagine synthetase A and alpha-acetolactate synthase; the ABC transport system of glutamine and a calcium-transporting ATPase; the dltA locus involved in alanylation of teichoic acids; and hypothetical proteins containing EAL and Eis domains, among others. Competence index experiments in several of the selected mutants confirmed the relevance of the Tn917-interrupted genes in the development of the infection process. The results suggested some of the metabolic routes and enzymic systems necessary for the complete virulence of this bacterium. This work is believed to represent the first report of a genome-wide scan for virulence factors in L. garvieae. The identified genes will further our understanding of the pathogenesis of L. garvieae infections and may provide targets for intervention or lead to the development of novel therapies.

Invasion and persistence of Mycobacterium avium subsp. paratuberculosis during early stages of Johne's disease in calves

Infection with Mycobacterium avium subsp. paratuberculosis causes Johne's disease in cattle and is a serious problem for the dairy industry worldwide. Development of models to mimic aspects of Johne's disease remains an elusive goal because of the chronic nature of the disease. In this report, we describe a surgical approach employed to characterize the very early stages of infection of calves with M. avium subsp. paratuberculosis. To our surprise, strains of M. avium subsp. paratuberculosis were able to traverse the intestinal tissues within 1 h of infection in order to colonize distant organs, such as the liver and lymph nodes. Both the ileum and the mesenteric lymph nodes were persistently infected for months following intestinal deposition of M. avium subsp. paratuberculosis despite a lack of fecal shedding of mycobacteria. During the first 9 months of infection, humoral immune responses were not detected. Nonetheless, using flow cytometric analysis, we detected a significant change in the cells participating in the inflammatory responses of infected calves compared to cells in a control animal. Additionally, the levels of cytokines detected in both the ileum and the lymph nodes indicated that there were TH1-type-associated cellular responses but not TH2-type-associated humoral responses. Finally, surgical inoculation of a wild-type strain and a mutant M. avium subsp. paratuberculosis strain (with an inactivated gcpE gene) demonstrated the ability of the model which we developed to differentiate between the wild-type strain and a mutant strain of M. avium subsp. paratuberculosis deficient in tissue colonization and invasion. Overall, novel insights into the early stages of Johne's disease were obtained, and a practical model of mycobacterial invasiveness was developed. A similar approach can be used for other enteric bacteria.

Mycobacterium marinum produces long-term chronic infections in medaka: a new animal model for studying human tuberculosis

Human infection by Mycobacterium tuberculosis is endemic, with approximately 2 billion infected and is the most common cause of adult death due to an infectious agent. Because of the slow growth rate of M. tuberculosis and risk to researchers, other species of Mycobacterium have been employed as alternative model systems to study human tuberculosis (TB). Mycobacterium marinum may be a good surrogate pathogen, conferring TB-like chronic infections in some fish. Medaka (Oryzias latipes) has been established for over five decades as a laboratory fish model for toxicology, genotoxicity, teratogenesis, carcinogenesis, classical genetics and embryology. We are investigating if medaka might also serve as a host for M. marinum in order to model human TB. We show that both acute and chronic infections are inducible in a dose dependent manner. Colonization of target organs and systemic granuloma formation has been demonstrated through the use of histology. M. marinum expressing green fluorescent protein (Gfp) was used to monitor bacterial colonization of these organs in fresh tissues as well as in intact animals. Moreover, we have employed the See-Through fish line, a variety of medaka devoid of major pigments, to monitor real-time disease progression, in living animals. We have also compared the susceptibility of another prominent fish model, zebrafish (Danio rerio), to our medaka-M. marinum model. We determined the course of infections in zebrafish is significantly more severe than in medaka. Together, these results indicate that the medaka-M. marinum model provides unique advantages for studying chronic mycobacteriosis.

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Identification of Mycobacterium marinum macrophage infection mutants

Mycobacterium marinum is an important pathogen of humans, amphibians and fish. Most pathogenic mycobacteria, including M. marinum, infect, survive and replicate primarily intracellularly within macrophages. We constructed a transposon mutant library in M. marinum using Tn5367 delivered by phage transduction in the shuttle phasmid phAE94. We screened 529 clones from the transposon library directly in macrophage infection assays. All clones were screened for their ability to initially infect macrophages as well as survive and replicate intracellularly. We identified 19 mutants that fit within three classes: class I) defective for growth in association with macrophages (42%), class II) defective for macrophage infection (21%) and class III) defective for infection of and growth in association with macrophages (37%). Although 14 of the macrophage infection mutants (Mim) carry insertions in genes that have not been previously identified, five are associated with virulence of mycobacteria in animal models. These observations confirm the utility of mutant screens directly in association with macrophages to identify new virulence determinants in mycobacteria. We complemented four of the Mim mutants with their M. tuberculosis homologue, demonstrating that secondary mutations are not responsible for the observed defect in macrophage infection. The genes we identified provide insight into the molecular mechanisms of macrophage infection by M. marinum.

Signature-tagged mutagenesis: technical advances in a negative selection method for virulence gene identification

Signature-tagged mutagenesis (STM) is a powerful negative selection method, predominantly used to identify the genes of a pathogen that are required for the successful colonization of an animal host. Since its first description a decade ago, STM has been applied to screen a vast amount of transposon insertion mutants in 31 bacterial species. This has led to the identification of over 1,700 bacterial genes that are involved in virulence. Despite the preservation of the basic design, the STM method has been developed further owing to recent advances including different designs of the signature-tags and profound changes in the mode of detection. These advances promoted substantially the application range and versatility of the STM method.

Identification of two Mycobacterium marinum loci that affect interactions with macrophages

Mycobacterium marinum is closely related to Mycobacterium tuberculosis, the cause of tuberculosis in humans. M. marinum has become an important model system for the study of the molecular mechanisms involved in causing tuberculosis in humans. Through molecular genetic analysis of the differences between pathogenic and nonpathogenic mycobacteria, we identified two loci that affect the ability of M. marinum to infect macrophages, designated mel(1) and mel(2). In silico analyses of the 11 putative genes in these loci suggest that mel(1) encodes secreted proteins that include a putative membrane protein and two putative transglutaminases, whereas mel(2) is involved in secondary metabolism or biosynthesis of fatty acids. Interestingly, mel(2) is unique to M. marinum and the M. tuberculosis complex and not present in any other sequenced mycobacterial species. M. marinum mutants with mutations in mel(1) and mel(2), constructed by allelic exchange, are defective in the ability to infect both murine and fish macrophage cell lines. These data suggest that the genes in mel(1) and mel(2) are important for the ability of M. marinum to infect host cells.