Regulation of the 18 kDa heat shock protein in Mycobacterium ulcerans: an alpha-crystallin orthologue that promotes biofilm formation

Is the lipochaperone activity of sHSP a key to the stress response encoded in its primary sequence?

Several strategies have been put in place by organisms to adapt to their environment. One of these strategies is the production of stress proteins such as sHSPs, which have been widely described over the last 30 years for their role as molecular chaperones. Some sHSPs have, in addition, the particularity to exert a lipochaperone role by interacting with membrane lipids to maintain an optimal membrane fluidity. However, the mechanisms involved in this sHSP-lipid interaction remain poorly understood and described rather sporadically in the literature. This review gathers the information concerning the structure and function of these proteins available in the literature in order to highlight the mechanism involved in this interaction. In addition, analysis of primary sequence data of sHSPs available in database shows that sHSPs can interact with lipids via certain amino acid residues present on some β sheets of these proteins. These residues could have a key role in the structure and/or oligomerization dynamics of sHPSs, which is certainly essential for interaction with membrane lipids and consequently for maintaining optimal cell membrane fluidity.

Non-Tuberculous Mycobacteria: Molecular and Physiological Bases of Virulence and Adaptation to Ecological Niches

Non-tuberculous mycobacteria (NTM) are paradigmatic colonizers of the total environment, circulating at the interfaces of the atmosphere, lithosphere, hydrosphere, biosphere, and anthroposphere. Their striking adaptive ecology on the interconnection of multiple spheres results from the combination of several biological features related to their exclusive hydrophobic and lipid-rich impermeable cell wall, transcriptional regulation signatures, biofilm phenotype, and symbiosis with protozoa. This unique blend of traits is reviewed in this work, with highlights to the prodigious plasticity and persistence hallmarks of NTM in a wide diversity of environments, from extreme natural milieus to microniches in the human body. Knowledge on the taxonomy, evolution, and functional diversity of NTM is updated, as well as the molecular and physiological bases for environmental adaptation, tolerance to xenobiotics, and infection biology in the human and non-human host. The complex interplay between individual, species-specific and ecological niche traits contributing to NTM resilience across ecosystems are also explored. This work hinges current understandings of NTM, approaching their biology and heterogeneity from several angles and reinforcing the complexity of these microorganisms often associated with a multiplicity of diseases, including pulmonary, soft-tissue, or milliary. In addition to emphasizing the cornerstones of knowledge involving these bacteria, we identify research gaps that need to be addressed, stressing out the need for decision-makers to recognize NTM infection as a public health issue that has to be tackled, especially when considering an increasingly susceptible elderly and immunocompromised population in developed countries, as well as in low- or middle-income countries, where NTM infections are still highly misdiagnosed and neglected.

High antibody titres induced by protein subunit vaccines using Mycobacterium ulcerans antigens Hsp18 and MUL_3720 with a TLR-2 agonist fail to protect against Buruli ulcer in mice

Background

Mycobacterium ulcerans is the causative agent of a debilitating skin and soft tissue infection known as Buruli ulcer (BU). There is no vaccine against BU. The purpose of this study was to investigate the vaccine potential of two previously described immunogenic M. ulcerans proteins, MUL_3720 and Hsp18, using a mouse tail infection model of BU.

Methods

Recombinant versions of the two proteins were each electrostatically coupled with a previously described lipopeptide adjuvant. Seven C57BL/6 and seven BALB/c mice were vaccinated and boosted with each of the formulations. Vaccinated mice were then challenged with M. ulcerans via subcutaneous tail inoculation. Vaccine performance was assessed by time-to-ulceration compared to unvaccinated mice.

Results

The MUL_3720 and Hsp18 vaccines induced high titres of antigen-specific antibodies that were predominately subtype IgG₁. However, all mice developed ulcers by day-40 post-M. ulcerans challenge. No significant difference was observed in the time-to-onset of ulceration between the experimental vaccine groups and unvaccinated animals.

Conclusions

These data align with previous vaccine experiments using Hsp18 and MUL_3720 that indicated these proteins may not be appropriate vaccine antigens. This work highlights the need to explore alternative vaccine targets and different approaches to understand the role antibodies might play in controlling BU.

The extracellular matrix of mycobacterial biofilms: could we shorten the treatment of mycobacterial infections?

A number of non-tuberculous mycobacterium species are opportunistic pathogens and ubiquitously form biofilms. These infections are often recalcitrant to treatment and require therapy with multiple drugs for long duration. The biofilm resident bacteria also display phenotypic drug tolerance and thus it has been hypothesized that the drug unresponsiveness in vivo could be due to formation of biofilms inside the host. We have discussed the biofilms of several pathogenic non-tuberculous mycobacterium (NTM) species in context to the in vivo pathologies. Besides pathogenic NTMs, Mycobacterium smegmatis is often used as a model organism for understanding mycobacterial physiology and has been studied extensively for understanding the mycobacterial biofilms. A number of components of the mycobacterial cell wall such as glycopeptidolipids, short chain mycolic acids, monomeromycolyl diacylglycerol, etc. have been shown to play an important role in formation of pellicle biofilms. It shall be noted that these components impart a hydrophobic character to the mycobacterial cell surface that facilitates cell to cell interaction. However, these components are not necessarily the constituents of the extracellular matrix of mycobacterial biofilms. In the end, we have described the biofilms of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. Three models of Mtb biofilm formation have been proposed to study the factors regulating biofilm formation, the physiology of the resident bacteria, and the nature of the biomaterial that holds these bacterial masses together. These models include pellicle biofilms formed at the liquid-air interface of cultures, leukocyte lysate-induced biofilms, and thiol reductive stressinduced biofilms. All the three models offer their own advantages in the study of Mtb biofilms. Interestingly, lipids (mainly keto-mycolic acids) are proposed to be the primary component of extracellular polymeric substance (EPS) in the pellicle biofilm, whereas the leukocyte lysate-induced and thiol reductive stress-induced biofilms possess polysaccharides as the primary component of EPS. Both models also contain extracellular DNA in the EPS. Interestingly, thiol reductive stressinduced Mtb biofilms are held together by cellulose and yet unidentified structural proteins. We believe that a better understanding of the EPS of Mtb biofilms and the physiology of the resident bacteria will facilitate the development of shorter regimen for TB treatment.

Comparative Genomics Shows That Mycobacterium ulcerans Migration and Expansion Preceded the Rise of Buruli Ulcer in Southeastern Australia

Since 2000, cases of the neglected tropical disease Buruli ulcer, caused by infection with Mycobacterium ulcerans, have increased 100-fold around Melbourne (population 4.4 million), the capital of Victoria, in temperate southeastern Australia. The reasons for this increase are unclear. Here, we used whole-genome sequence comparisons of 178 M. ulcerans isolates obtained primarily from human clinical specimens, spanning 70 years, to model the population dynamics of this pathogen from this region. Using phylogeographic and advanced Bayesian phylogenetic approaches, we found that there has been a migration of the pathogen from the east end of the state, beginning in the 1980s, 300 km west to the major human population center around Melbourne. This move was then followed by a significant increase in M. ulcerans population size. These analyses inform our thinking around Buruli ulcer transmission and control, indicating that M. ulcerans is introduced to a new environment and then expands, rather than it being from the awakening of a quiescent pathogen reservoir.IMPORTANCE Buruli ulcer is a destructive skin and soft tissue infection caused by Mycobacterium ulcerans and is characterized by progressive skin ulceration, which can lead to permanent disfigurement and long-term disability. Despite the majority of disease burden occurring in regions of West and central Africa, Buruli ulcer is also becoming increasingly common in southeastern Australia. Major impediments to controlling disease spread are incomplete understandings of the environmental reservoirs and modes of transmission of M. ulcerans The significance of our research is that we used genomics to assess the population structure of this pathogen at the Australian continental scale. We have then reconstructed a historical bacterial spread and modeled demographic dynamics to reveal bacterial population expansion across southeastern Australia. These findings provide explanations for the observed epidemiological trends with Buruli ulcer and suggest possible management to control disease spread.

A Sero-epidemiological Approach to Explore Transmission of Mycobacterium ulcerans

The debilitating skin disease Buruli ulcer (BU) is caused by infection with Mycobacterium ulcerans. While various hypotheses on potential reservoirs and vectors of M. ulcerans exist, the mode of transmission has remained unclear. Epidemiological studies have indicated that children below the age of four are less exposed to the pathogen and at lower risk of developing BU than older children. In the present study we compared the age at which children begin to develop antibody responses against M. ulcerans with the age pattern of responses to other pathogens transmitted by various mechanisms. A total of 1,352 sera from individuals living in the BU endemic Offin river valley of Ghana were included in the study. While first serological responses to the mosquito transmitted malaria parasite Plasmodium falciparum and to soil transmitted Strongyloides helminths emerged around the age of one and two years, sero-conversion for M. ulcerans and for the water transmitted trematode Schistosoma mansoni occurred at around four and five years, respectively. Our data suggest that exposure to M. ulcerans intensifies strongly at the age when children start to have more intense contact with the environment, outside the small movement range of young children. Further results from our serological investigations in the Offin river valley also indicate ongoing transmission of Treponema pallidum, the causative agent of yaws.

The Porphyromonas gingivalis ferric uptake regulator orthologue binds hemin and regulates hemin-responsive biofilm development

Porphyromonas gingivalis is a Gram-negative pathogen associated with the biofilm-mediated disease chronic periodontitis. P. gingivalis biofilm formation is dependent on environmental heme for which P. gingivalis has an obligate requirement as it is unable to synthesize protoporphyrin IX de novo, hence P. gingivalis transports iron and heme liberated from the human host. Homeostasis of a variety of transition metal ions is often mediated in Gram-negative bacteria at the transcriptional level by members of the Ferric Uptake Regulator (Fur) superfamily. P. gingivalis has a single predicted Fur superfamily orthologue which we have designated Har (heme associated regulator). Recombinant Har formed dimers in the presence of Zn²⁺ and bound one hemin molecule per monomer with high affinity (K_d of 0.23 µM). The binding of hemin resulted in conformational changes of Zn(II)Har and residue ⁹⁷Cys was involved in hemin binding as part of a predicted -⁹⁷C-⁹⁸P-⁹⁹L- hemin binding motif. The expression of 35 genes was down-regulated and 9 up-regulated in a Har mutant (ECR455) relative to wild-type. Twenty six of the down-regulated genes were previously found to be up-regulated in P. gingivalis grown as a biofilm and 11 were up-regulated under hemin limitation. A truncated Zn(II)Har bound the promoter region of dnaA (PGN_0001), one of the up-regulated genes in the ECR455 mutant. This binding decreased as hemin concentration increased which was consistent with gene expression being regulated by hemin availability. ECR455 formed significantly less biofilm than the wild-type and unlike wild-type biofilm formation was independent of hemin availability. P. gingivalis possesses a hemin-binding Fur orthologue that regulates hemin-dependent biofilm formation.

Late onset of the serological response against the 18 kDa small heat shock protein of Mycobacterium ulcerans in children

A previous survey for clinical cases of Buruli ulcer (BU) in the Mapé Basin of Cameroon suggested that, compared to older age groups, very young children may be less exposed to Mycobacterium ulcerans. Here we determined serum IgG titres against the 18 kDa small heat shock protein (shsp) of M. ulcerans in 875 individuals living in the BU endemic river basins of the Mapé in Cameroon and the Densu in Ghana. While none of the sera collected from children below the age of four contained significant amounts of 18 kDa shsp specific antibodies, the majority of sera had high IgG titres against the Plasmodium falciparum merozoite surface protein 1 (MSP-1). These data suggest that exposure to M. ulcerans increases at an age which coincides with the children moving further away from their homes and having more intense environmental contact, including exposure to water bodies at the periphery of their villages.

Although M. ulcerans, the causative agent of Buruli ulcer (BU), was identified in 1948, its transmission pathways and environmental reservoirs remain poorly understood. The occurrence of M. ulcerans infections in endemic countries in West and Central Africa is highly focal and associated with stagnant and slow flowing water bodies. BU is often described as a disease mainly affecting children <15 years of age. However, taking the population age distribution into account, our recent longitudinal survey for BU in the Mapé Dam Region of Cameroon revealed that clinical cases of BU among children <5 years are relatively rare. In accordance with these findings, data of the present sero-epidemiological study indicate that children <4 years old are less exposed to M. ulcerans than older children. Sero-conversion is associated with age, which may be due to age-related changes in behavioural factors, such as a wider movement radius of older children, including more frequent contact with water bodies at the periphery of their villages.

The cell wall-associated mycolactone polyketide synthases are necessary but not sufficient for mycolactone biosynthesis

Mycolactones are polyketide-derived lipid virulence factors made by the slow-growing human pathogen, Mycobacterium ulcerans. Three unusually large and homologous plasmid-borne genes (mlsA1: 51 kb, mlsB: 42 kb and mlsA2: 7 kb) encode the mycolactone type I polyketide synthases (PKS). The extreme size and low sequence diversity of these genes has posed significant barriers for exploration of the genetic and biochemical basis of mycolactone synthesis. Here, we have developed a truncated, more tractable 3-module version of the 18-module mycolactone PKS and we show that this engineered PKS functions as expected in the natural host M. ulcerans to produce an additional polyketide; a triketide lactone (TKL). Cell fractionation experiments indicated that this 3-module PKS and the putative accessory enzymes encoded by mup045 and mup038 associated with the mycobacterial cell wall, a finding supported by confocal microscopy. We then assessed the capacity of the faster growing, Mycobacterium marinum to harbor and express the 3-module Mls PKS and accessory enzymes encoded by mup045 and mup038. RT-PCR, immunoblotting, and cell fractionation experiments confirmed that the truncated Mls PKS multienzymes were expressed and also partitioned with the cell wall material in M. marinum. However, this heterologous host failed to produce TKL. The systematic deconstruction of the mycolactone PKS presented here suggests that the Mls multienzymes are necessary but not sufficient for mycolactone synthesis and that synthesis is likely to occur (at least in part) within the mycobacterial cell wall. This research is also the first proof-of-principle demonstration of the potential of this enzyme complex to produce tailored small molecules through genetically engineered rearrangements of the Mls modules.

Environmental distribution and seasonal prevalence of Mycobacterium ulcerans in Southern Louisiana

Mycobacterium ulcerans is an emerging environmental pathogen that causes debilitating, ulcerative disease in humans and other vertebrates. The majority of human cases occur in tropical and temperate regions of Africa and Australia, and outbreaks of piscine mycobacteriosis caused by M. ulcerans have been reported in disparate geographic locations spanning the globe. While exposure to a natural body of water is the most common risk factor for human infection, the environmental distribution of M. ulcerans in aquatic habitats has not been extensively studied. Although no human cases have been reported in the United States, a strain of M. ulcerans has been identified as the cause of a piscine mycobacteriosis in Striped bass (Morone saxatilis) within the Chesapeake Bay. Infected fish exhibit bright red ventral and lateral dermal lesions. We observed a possible outbreak causing similar lesions on red drum (Sciaenops ocellatus) in wetlands of southern Louisiana and detected M. ulcerans-specific genetic markers in lesion samples from these fish. Based on these findings, we studied the geographic and seasonal prevalence of these markers across southern Louisiana. M. ulcerans was detected in each of the nine areas sampled across the state. M. ulcerans prevalence was significantly lower in the fall samples, and the low prevalence coincided with decreased nutrient levels and an increase in water temperature. To our knowledge, this is the first study of M. ulcerans biomarkers in the southern United States.

On the origin of Mycobacterium ulcerans, the causative agent of Buruli ulcer

Background

Mycobacterium ulcerans is an unusual bacterial pathogen with elusive origins. While closely related to the aquatic dwelling M. marinum, M. ulcerans has evolved the ability to produce the immunosuppressive polyketide toxin mycolactone and cause the neglected tropical disease Buruli ulcer. Other mycolactone-producing mycobacteria (MPM) have been identified in fish and frogs and given distinct species designations (M. pseudoshottsii, M. shinshuense, M. liflandii and M. marinum), however the evolution of M. ulcerans and its relationship to other MPM has not been defined. Here we report the comparative analysis of whole genome sequences from 30 MPM and five M. marinum.

Results

A high-resolution phylogeny based on genome-wide single nucleotide polymorphisms (SNPs) showed that M. ulcerans and all other MPM represent a single clonal group that evolved from a common M. marinum progenitor. The emergence of the MPM was driven by the acquisition of the pMUM plasmid encoding genes for the biosynthesis of mycolactones. This change was accompanied by the loss of at least 185 genes, with a significant overrepresentation of genes associated with cell wall functions. Cell wall associated genes also showed evidence of substantial adaptive selection, suggesting cell wall remodeling has been critical for the survival of MPM. Fine-grain analysis of the MPM complex revealed at least three distinct lineages, one of which comprised a highly clonal group, responsible for Buruli ulcer in Africa and Australia. This indicates relatively recent transfer of M. ulcerans between these continents, which represent the vast majority of the global Buruli ulcer burden. Our data provide SNPs and gene sequences that can differentiate M. ulcerans lineages, suitable for use in the diagnosis and surveillance of Buruli ulcer.

Conclusions

M. ulcerans and all mycolactone-producing mycobacteria are specialized variants of a common Mycobacterium marinum progenitor that have adapted to live in restricted environments. Examination of genes lost or retained and now under selective pressure suggests these environments might be aerobic, and extracellular, where slow growth, production of an immune suppressor, cell wall remodeling, loss or modification of cell wall antigens, and biofilm-forming ability provide a survival advantage. These insights will guide our efforts to find the elusive reservoir(s) of M. ulcerans and to understand transmission of Buruli ulcer.

Biotechnical applications of small heat shock proteins from bacteria

The stress responses of most bacteria are thought to involve the upregulation of small heat shock proteins. We describe here some of the most pertinent aspects of small heat shock proteins, to highlight their potential for use in various applications. Bacterial species have between one and 13 genes encoding small heat shock proteins, the precise number depending on the species considered. Major efforts have recently been made to characterize the protein protection and membrane stabilization mechanisms involving small heat shock proteins in bacteria. These proteins seem to be involved in the acquisition of cellular heat tolerance. They could therefore potentially be used to maintain cell viability under unfavorable conditions, such as heat shock or chemical treatments. This review highlights the potential roles of applications of small heat shock proteins in stabilizing overproduced heterologous proteins in Escherichia coli, purified bacterial small heat shock proteins in protein biochip technology, proteomic analysis and food technology and the potential impact of these proteins on some diseases. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.