An Evolutionary Arms Race Between Burkholderia pseudomallei and Host Immune System: What Do We Know?

Burkholderia pseudomallei and melioidosis

Burkholderia pseudomallei, the causative agent of melioidosis, is found in soil and water of tropical and subtropical regions globally. Modelled estimates of the global burden predict that melioidosis remains vastly under-reported, and a call has been made for it to be recognized as a neglected tropical disease by the World Health Organization. Severe weather events and environmental disturbance are associated with increased case numbers, and it is anticipated that, in some regions, cases will increase in association with climate change. Genomic epidemiological investigations have confirmed B. pseudomallei endemicity in newly recognized regions, including the southern United States. Melioidosis follows environmental exposure to B. pseudomallei and is associated with comorbidities that affect the immune response, such as diabetes, and with socioeconomic disadvantage. Several vaccine candidates are ready for phase I clinical trials. In this Review, we explore the global burden, epidemiology and pathophysiology of B. pseudomallei as well as current diagnostics, treatment recommendations and preventive measures, highlighting research needs and priorities.

The current landscape of microRNAs (miRNAs) in bacterial pneumonia: opportunities and challenges

MicroRNAs (miRNAs), which were initially discovered in Caenorhabditis elegans, can regulate gene expression by recognizing cognate sequences and interfering with the transcriptional or translational machinery. The application of bioinformatics tools for structural analysis and target prediction has largely driven the investigation of certain miRNAs. Notably, it has been found that certain miRNAs which are widely involved in the inflammatory response and immune regulation are closely associated with the occurrence, development, and outcome of bacterial pneumonia. It has been shown that certain miRNA techniques can be used to identify related targets and explore associated signal transduction pathways. This enhances the understanding of bacterial pneumonia, notably for “refractory” or drug-resistant bacterial pneumonia. Although these miRNA-based methods may provide a basis for the clinical diagnosis and treatment of this disease, they still face various challenges, such as low sensitivity, poor specificity, low silencing efficiency, off-target effects, and toxic reactions. The opportunities and challenges of these methods have been completely reviewed, notably in bacterial pneumonia. With the continuous improvement of the current technology, the miRNA-based methods may surmount the aforementioned limitations, providing promising support for the clinical diagnosis and treatment of “refractory” or drug-resistant bacterial pneumonia.

Pathogenicity and virulence of Burkholderia pseudomallei

The soil saprophyte, Burkholderia pseudomallei, is the causative agent of melioidosis, a disease endemic in South East Asia and northern Australia. Exposure to B. pseudomallei by either inhalation or inoculation can lead to severe disease. B. pseudomallei rapidly shifts from an environmental organism to an aggressive intracellular pathogen capable of rapidly spreading around the body. The expression of multiple virulence factors at every stage of intracellular infection allows for rapid progression of infection. Following invasion or phagocytosis, B. pseudomallei resists host-cell killing mechanisms in the phagosome, followed by escape using the type III secretion system. Several secreted virulence factors manipulate the host cell, while bacterial cells undergo a shift in energy metabolism allowing for overwhelming intracellular replication. Polymerisation of host cell actin into “actin tails” propels B. pseudomallei to the membranes of host cells where the type VI secretion system fuses host cells into multinucleated giant cells (MNGCs) to facilitate cell-to-cell dissemination. This review describes the various mechanisms used by B. pseudomallei to survive within cells.

Encapsulins

Subcellular compartmentalization is a defining feature of all cells. In prokaryotes, compartmentalization is generally achieved via protein-based strategies. The two main classes of microbial protein compartments are bacterial microcompartments and encapsulin nanocompartments. Encapsulins self-assemble into proteinaceous shells with diameters between 24 and 42 nm and are defined by the viral HK97-fold of their shell protein. Encapsulins have the ability to encapsulate dedicated cargo proteins, including ferritin-like proteins, peroxidases, and desulfurases. Encapsulation is mediated by targeting sequences present in all cargo proteins. Encapsulins are found in many bacterial and archaeal phyla and have been suggested to play roles in iron storage, stress resistance, sulfur metabolism, and natural product biosynthesis. Phylogenetic analyses indicate that they share a common ancestor with viral capsid proteins. Many pathogens encode encapsulins, and recent evidence suggests that they may contribute toward pathogenicity. The existing information on encapsulin structure, biochemistry, biological function, and biomedical relevance is reviewed here.

Co-evolutionary Signals Identify Burkholderia pseudomallei Survival Strategies in a Hostile Environment

The soil bacterium Burkholderia pseudomallei is the causative agent of melioidosis and a significant cause of human morbidity and mortality in many tropical and subtropical countries. The species notoriously survives harsh environmental conditions but the genetic architecture for these adaptations remains unclear. Here we employed a powerful combination of genome-wide epistasis and co-selection studies (2,011 genomes), condition-wide transcriptome analyses (82 diverse conditions), and a gene knockout assay to uncover signals of "co-selection"-that is a combination of genetic markers that have been repeatedly selected together through B. pseudomallei evolution. These enabled us to identify 13,061 mutation pairs under co-selection in distinct genes and noncoding RNA. Genes under co-selection displayed marked expression correlation when B. pseudomallei was subjected to physical stress conditions, highlighting the conditions as one of the major evolutionary driving forces for this bacterium. We identified a putative adhesin (BPSL1661) as a hub of co-selection signals, experimentally confirmed a BPSL1661 role under nutrient deprivation, and explored the functional basis of co-selection gene network surrounding BPSL1661 in facilitating the bacterial survival under nutrient depletion. Our findings suggest that nutrient-limited conditions have been the common selection pressure acting on this species, and allelic variation of BPSL1661 may have promoted B. pseudomallei survival during harsh environmental conditions by facilitating bacterial adherence to different surfaces, cells, or living hosts.

Genome data vs MLST for exploring intraspecific evolutionary history in bacteria: Much is not always better

Genome-based phylogeny has been proposed to be more accurate than phylogeny based in a few genes as MLST-based phylogeny. However, much is not always better. Here we analyzed 368 complete genomes corresponding to 9 bacterial species in order to address intraspecific phylogeny. The studied species were: Burkholderia pseudomallei, Campylobacter jejuni, Chlamydia trachomatis, Helicobacter pylori, Klebsiella pneumoniae, Listeria monocytogenes, Salmonella enterica, Staphylococcus aureus and Streptococcus pyogenes. The intra-specific phylogenies were inferred using the complete genome sequences of different strains of these species and their MLST schemes. A supermatrix approach was used to infer maximum likelihood phylogenies in both cases. The phylogenetic incongruence between the supermatrix-based genome or MLST tree and individual trees (constructed from genome fragments or MLST genes, respectively) was analyzed. In supermatrix-based trees for genomes, most branches showed a high branch support; however, a high number of branches also showed high percentage of topologically incongruent individual trees. Interestingly, genome and MLST trees showed similar levels of incongruence in the phylogeny for each bacteria specie. Both genome and MLST approaches showed that C. trachomatis and S. aureus have a tree-like evolutionary history (low levels of internal incongruence). Instead, B. pseudomallei and S. pyogenes show high levels of incongruence (network-like evolutionary story) probably caused by HGT (horizontal gene transfer). Concluding, our analysis showed that: high branch supports obtained in genome phylogenies could be an artifact probably caused by data size; MLST is valid to address intraspecific phylogenetic structure; and, each species has its own evolutionary history, which could be affected by HGT to different extents.

Large-scale computational discovery and analysis of virus-derived microbial nanocompartments

Encapsulins are a class of microbial protein compartments defined by the viral HK97-fold of their capsid protein, self-assembly into icosahedral shells, and dedicated cargo loading mechanism for sequestering specific enzymes. Encapsulins are often misannotated and traditional sequence-based searches yield many false positive hits in the form of phage capsids. Here, we develop an integrated search strategy to carry out a large-scale computational analysis of prokaryotic genomes with the goal of discovering an exhaustive and curated set of all HK97-fold encapsulin-like systems. We find over 6,000 encapsulin-like systems in 31 bacterial and four archaeal phyla, including two novel encapsulin families. We formulate hypotheses about their potential biological functions and biomedical relevance, which range from natural product biosynthesis and stress resistance to carbon metabolism and anaerobic hydrogen production. An evolutionary analysis of encapsulins and related HK97-type virus families shows that they share a common ancestor, and we conclude that encapsulins likely evolved from HK97-type bacteriophages.