Increased Neurotropic Threat from Burkholderia pseudomallei Strains with a B. mallei-like Variation in the bimA Motility Gene, Australia

Burkholderia pseudomallei Complex Subunit and Glycoconjugate Vaccines and Their Potential to Elicit Cross-Protection to Burkholderia cepacia Complex

Burkholderia are a group of Gram-negative bacteria that can cause a variety of diseases in at-risk populations. B. pseudomallei and B. mallei, the etiological agents of melioidosis and glanders, respectively, are the two clinically relevant members of the B. pseudomallei complex (Bpc). The development of vaccines against Bpc species has been accelerated in recent years, resulting in numerous promising subunits and glycoconjugate vaccines incorporating a variety of antigens. However, a second group of pathogenic Burkholderia species exists known as the Burkholderia cepacia complex (Bcc), a group of opportunistic bacteria which tend to affect individuals with weakened immunity or cystic fibrosis. To date, there have been few attempts to develop vaccines to Bcc species. Therefore, the primary goal of this review is to provide a broad overview of the various subunit antigens that have been tested in Bpc species, their protective efficacy, study limitations, and known or suspected mechanisms of protection. Then, we assess the reviewed Bpc antigens for their amino acid sequence conservation to homologous proteins found in Bcc species. We propose that protective Bpc antigens with a high degree of Bpc-to-Bcc sequence conservation could serve as components of a pan-Burkholderia vaccine capable of protecting against both disease-causing groups.

In Vitro Roles of Burkholderia Intracellular Motility A (BimA) in Infection of Human Neuroblastoma Cell Line

The bacterial pathogen Burkholderia pseudomallei causes human melioidosis, which can infect the brain, leading to encephalitis and brain abscesses. Infection of the nervous system is a rare condition but is associated with an increased risk of mortality. Burkholderia intracellular motility A (BimA) was reported to play an important role in the invasion and infection of the central nervous system in a mouse model. Thus, to gain insight of the cellular mechanisms underlying the pathogenesis of neurological melioidosis, we explored the human neuronal proteomics to identify the host factors that are up- and downregulated during Burkholderia infection. When infected the SH-SY5Y cells with B. pseudomallei K96243 wild-type (WT), 194 host proteins showed a fold change of >2 compared with uninfected cells. Moreover, 123 proteins showed a fold change of >2 when infected with a knockout bimA mutant (ΔbimA) mutant compared with WT. The differentially expressed proteins were mainly associated with metabolic pathways and pathways linked to human diseases. Importantly, we observed the downregulation of proteins in the apoptosis and cytotoxicity pathway, and in vitro investigation with the ΔbimA mutant revealed the association of BimA with the induction of these pathways. Additionally, we disclosed that BimA was not required for invasion into the neuron cell line but was necessary for effective intracellular replication and multinucleated giant cell (MNGC) formation. These findings show the extraordinary capacity of B. pseudomallei in subverting and interfering with host cellular systems to establish infection and extend our understanding of B. pseudomallei BimA involvement in the pathogenesis of neurological melioidosis. IMPORTANCE Neurological melioidosis, caused by Burkholderia pseudomallei, can result in severe neurological damage and enhance the mortality rate of melioidosis patients. We investigate the involvement of the virulent factor BimA, which mediates actin-based motility, in the intracellular infection of neuroblastoma SH-SY5Y cells. Using proteomics-based analysis, we provide a list of host factors exploited by B. pseudomallei. The expression level of selected downregulated proteins in neuron cells infected with the ΔbimA mutant was determined by quantitative reverse transcription-PCR and was consistent with our proteomic data. The role of BimA in the apoptosis and cytotoxicity of SH-SY5Y cells infected by B. pseudomallei was uncovered in this study. Additionally, our research demonstrates that BimA is required for successful intracellular survival and cell fusion upon infection of neuron cells. Our findings have significant implications for understanding the pathogenesis of B. pseudomallei infections and developing novel therapeutic strategies to combat this deadly disease.

Cycle-Inhibiting Factor Is Associated with Burkholderia pseudomallei Invasion in Human Neuronal Cells

Burkholderia pseudomallei is a pathogenic bacterium that causes human melioidosis, which is associated with a high mortality rate. However, the underlying mechanisms of B. pseudomallei pathogenesis are largely unknown. In this study, we examined the infection of human neuronal SH-Sy5y cells by several clinically relevant B. pseudomallei strains. We found that all tested B. pseudomallei strains can invade SH-Sy5y cells, undergo intracellular replication, cause actin-tail formation, and form multinucleated giant cells. Additionally, a deletion mutant of B. pseudomallei cycle-inhibiting factor (cif) was constructed that exhibited reduced invasion in SH-Sy5y cells. Complementation of cif restored invasion of the B. pseudomallei cif-deleted mutant. Our findings enhance understanding of B. pseudomallei pathogenicity in terms of the virulence factor Cif and demonstrate the function of Cif in neurological melioidosis. This may eventually lead to the discovery of novel targets for treatment and a strategy to control the disease.

Clinical Burkholderia pseudomallei isolates from north Queensland carry diverse bimABm genes that are associated with central nervous system disease and are phylogenomically distinct from other Australian strains

Background

Burkholderia pseudomallei is an environmental gram-negative bacterium that causes the disease melioidosis and is endemic in many countries of the Asia-Pacific region. In Australia, the mortality rate remains high at approximately 10%, despite curative antibiotic treatment being available. The bacterium is almost exclusively found in the endemic region, which spans the tropical Northern Territory and North Queensland, with clusters occasionally present in more temperate climates. Despite being endemic to North Queensland, these infections remain understudied compared to those of the Northern Territory.

Methodology/Principal findings

This study aimed to assess the prevalence of central nervous system (CNS) disease associated variant bimA_Bm, identify circulating antimicrobial resistance mutations and genetically distinct strains from Queensland, via comparative genomics. From 76 clinical isolates, we identified the bimA_Bm variant in 20 (26.3%) isolates and in 9 (45%) of the isolates with documented CNS infection (n = 18). Explorative analysis suggests a significant association between isolates carrying the bimA_Bm variant and CNS disease (OR 2.8, 95% CI 1.3–6.0, P = 0.009) compared with isolates carrying the wildtype bimA_Bp. Furthermore, 50% of isolates were identified as novel multi-locus sequence types, while the bimA_Bm variant was more commonly identified in isolates with novel sequence types, compared to those with previously described. Additionally, mutations associated with acquired antimicrobial resistance were only identified in 14.5% of all genomes.

Conclusions/Significance

The findings of this research have provided clinically relevant genomic data of B. pseudomallei in Queensland and suggest that the bimA_Bm variant may enable risk stratification for the development CNS complications and be a potential therapeutic target.

Melioidosis is a life-threatening infection, caused by the Gram-negative bacterium Burkholderia pseudomallei, which is endemic to tropical regions in Australia. Variants of the bimA gene have been proposed as a virulence factor associated with more severe disease. In a genomic analysis of 76 clinical B. pseudomallei isolates from Queensland, Australia, we identified that the bimA_Bm variant was associated with infection involving the central nervous system (odds ratio 2.8, 95% Confidence Interval: 1.3–6.0, P = 0.009), compared to isolates with the wild-type allele bimA_Bp. Half of the isolates from this region were novel multi-locus sequence types, and bimA_Bm was more commonly seen in these novel sequence types. Early genomic characterisation to identify virulence factors such as bimA_Bm, may be useful as an early marker of more complex disease that could guide further investigation and help determine optimal treatment. Further investigation of a “genomics-guided” approach to the clinical management of this complex infectious disease are warranted.

Melioidosis of the central nervous system; impact of the bimABm allele on patient presentation and outcome

BACKGROUND

The autotransporter protein Burkholderia intracellular motility A (BimA) facilitates the entry of Burkholderia pseudomallei into the central nervous system (CNS) in mouse models of melioidosis. Its role in the pathogenesis of human cases of CNS melioidosis is incompletely defined.

METHODS

Consecutive culture-confirmed cases of melioidosis at two sites in tropical Australia after 1989 were reviewed. Demographic, clinical and radiological data of the patients with CNS melioidosis were recorded. The bimA allele (bimABm or bimABp) of the B. pseudomallei isolated from each patient was determined.

RESULTS

Of the 1587 cases diagnosed at the two sites during the study period, 52 (3.3%) had confirmed CNS melioidosis; 20 (38.5%) had a brain abscess, 18 (34.6%) had encephalomyelitis, 4 (7.7%) had isolated meningitis and 10 (19.2%) had extra-meningeal disease. Among the 52 patients, there were 8 (15.4%) deaths; 17/44 (38.6%) survivors had residual disability. The bimA allele was characterized in 47/52; 17/47 (36.2%) had the bimABm allele and 30 (63.8%) had the bimABp allele. Patients with a bimABm variant were more likely to have a predominantly neurological presentation (odds ratio (OR) (95% confidence interval (CI)): 5.60 (1.52-20.61), p=0.01), to have brainstem involvement (OR (95%CI): 7.33 (1.92-27.95), p=0.004) and to have encephalomyelitis (OR (95%CI): 4.69 (1.30-16.95), p=0.02. Patients with a bimABm variant were more likely to die or have residual disability (odds ratio (95%CI): 4.88 (1.28-18.57), p=0.01).

CONCLUSIONS

The bimA allele of B. pseudomallei has a significant impact on the clinical presentation and outcome of patients with CNS melioidosis.

Biogeography and genetic diversity of clinical isolates of Burkholderia pseudomallei in Sri Lanka

Background

Melioidosis is a potentially fatal infectious disease caused by Burkholderia pseudomallei and the disease is endemic in Southeast Asia and Northern Australia. It has been confirmed as endemic in Sri Lanka. Genomic epidemiology of B. pseudomallei in Sri Lanka is largely unexplored. This study aims to determine the biogeography and genetic diversity of clinical isolates of B. pseudomallei and the phylogenetic and evolutionary relationship of Sri Lankan sequence types (STs) to those found in other endemic regions of Southeast Asia and Oceania.

Methods

The distribution of variably present genetic markers [Burkholderia intracellular motility A (bimA) gene variants bimA_BP/bimA_BM, filamentous hemagglutinin 3 (fhaB3), Yersinia-like fimbrial (YLF) and B. thailandensis-like flagellum and chemotaxis (BTFC) gene clusters and lipopolysaccharide O-antigen type A (LPS type A)] was examined among 310 strains. Multilocus sequence typing (MLST) was done for 84 clinical isolates. The phylogenetic and evolutionary relationship of Sri Lankan STs within Sri Lanka and in relation to those found in other endemic regions of Southeast Asia and Oceania were studied using e BURST, PHYLOViZ and minimum evolutionary analysis.

Results

The Sri Lankan B. pseudomallei population contained a large proportion of the rare BTFC clade (14.5%) and bimA_BM allele variant (18.5%) with differential geographic distribution. Genotypes fhaB3 and LPSA were found in 80% and 86% respectively. This study reported 43 STs (including 22 novel). e-BURST analysis which include all Sri Lankan STs (71) resulted in four groups, with a large clonal group (group 1) having 46 STs, and 17 singletons. ST1137 was the commonest ST. Several STs were shared with India, Bangladesh and Cambodia.

Conclusion

This study demonstrates the usefulness of high-resolution molecular typing to locate isolates within the broad geographical boundaries of B. pseudomallei at a global level and reveals that Sri Lankan isolates are intermediate between Southeast Asia and Oceania.

Burkholderia pseudomallei is an important cause of community acquired pneumonia, septicemia and abscesses in Sri Lanka. The risk of infection is increased after flooding following heavy rainfall. Risk groups include rice farmers and rural populations engaged in subsistence cultivation in home gardens. Nationwide surveillance has been carried out since 2006 and the state public health system offers free diagnostics and free antibiotic therapy. The incidence of melioidosis in Sri Lanka has increased in tandem with increased awareness among clinicians. This study reports the genetic diversity among Sri Lankan B. pseudomallei clinical isolates and shows that some variably present genes are regionally distributed. The population is intermediate between Southeast Asia and Oceania. This may reflect its past geological history.

Identification of defective early immune responses to Burkholderia pseudomallei infection in a diet-induced murine model of type 2 diabetes

Co-occurrence of bacterial infections with type 2 diabetes (T2D) is a global problem. Melioidosis caused by Burkholderia pseudomallei is 10 times more likely to occur in patients with T2D, than in normoglycemic individuals. Using an experimental model of T2D, we observed that greater susceptibility in T2D was due to differences in proportions of infiltrating leucocytes and reduced levels of MCP-1, IFN-γ and IL-12 at sites of infection within 24 h post-infection. However, by 72 h the levels of inflammatory cytokines and bacteria were markedly higher in visceral tissue and blood in T2D mice. In T2D, dysregulated early immune responses are responsible for the greater predisposition to B. pseudomallei infection.

Detection and differentiation of Burkholderia species with pathogenic potential in environmental soil samples

The Burkholderia pseudomallei phylogenetic cluster includes B. pseudomallei, B. mallei, B. thailandensis, B. oklahomensis, B. humptydooensis and B. singularis. Regarded as the only pathogenic members of this group, B. pseudomallei and B. mallei cause the diseases melioidosis and glanders, respectively. Additionally, variant strains of B. pseudomallei and B. thailandensis exist that include the geographically restricted B. pseudomallei that express a B. mallei-like BimA protein (BPBM), and B. thailandensis that express a B. pseudomallei-like capsular polysaccharide (BTCV). To establish a PCR-based assay for the detection of pathogenic Burkholderia species or their variants, five PCR primers were designed to amplify species-specific sequences within the bimA (Burkholderiaintracellular motility A) gene. Our multiplex PCR assay could distinguish pathogenic B. pseudomallei and BPBM from the non-pathogenic B. thailandensis and the BTCV strains. A second singleplex PCR successfully discriminated the BTCV from B. thailandensis. Apart from B. humptydooensis, specificity testing against other Burkholderia spp., as well as other Gram-negative and Gram-positive bacteria produced a negative result. The detection limit of the multiplex PCR in soil samples artificially spiked with known quantities of B. pseudomallei and B. thailandensis were 5 and 6 CFU/g soil, respectively. Furthermore, comparison between standard bacterial culture and the multiplex PCR to detect B. pseudomallei from 34 soil samples, collected from an endemic area of melioidosis, showed high sensitivity and specificity. This robust, sensitive, and specific PCR assay will be a useful tool for epidemiological study of B. pseudomallei and closely related members with pathogenic potential in soil.

Melioidosis in Australia

Melioidosis is a potentially fatal bacterial infection caused by the Gram-negative bacillus, Burkholderia pseudomallei following contact with a contaminated environmental source, normally soil or water in tropical and subtropical locations. The disease spectrum varies from rapidly progressive bacteraemic infection with or without pneumonia, to focal lesions in deep soft tissues and internal organs to superficial soft tissue infection and asymptomatic seroconversion with possible long-term dormancy. Most infections occur with a background of chronic illness such as diabetes, chronic kidney disease and alcoholic liver disease. Improvements in diagnosis, targeted antimicrobial treatment and long term follow up have improved clinical outcomes. Environmental controls following rare point source case clusters and heightened awareness of melioidosis appear to have reduced the disease burden in some parts of northern Australia. However, the impact of climate change on dispersal of environmental B. pseudomallei, and changing land use in tropical Australia is expected to change the epidemiology of melioidosis in future.

Implications of environmental and pathogen-specific determinants on clinical presentations and disease outcome in melioidosis patients

BACKGROUND

Melioidosis is gaining recognition as an emerging infectious disease with diverse clinical manifestations and high-case fatality rates worldwide. However, the molecular epidemiology of the disease outside the endemic regions such as northeast part of Thailand and northern Australia remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Clinical data and B. pseudomallei isolates obtained from 199 culture-confirmed cases of melioidosis diagnosed during 2006-2016 in South India were used to elucidate the host and pathogen specific variable virulence determinants associated with clinical presentations and disease outcome. Further, we determined the temporal variations and the influence of ecological factors on B.pseudomallei Lipopolysaccharide (LPS) genotypes causing infections. Severe forms of the disease were observed amongst 169 (85%) patients. Renal dysfunction and infection due to B.pseudomallei harboring BimABm variant had significant associations with severe forms of the disease. Diabetes mellitus, septicemic melioidosis and infection due to LPSB genotype were independent risk factors for mortality. LPSB (74%) and LPSA (20.6%) were the prevalent genotypes causing infections. Both genotypes demonstrated temporal variations and had significant correlations with rainfall and humidity.

CONCLUSION/SIGNIFICANCE

Our study findings suggest that the pathogen specific virulence traits under the influence of ecological factors are the key drivers for geographical variations in the molecular epidemiology of melioidosis.

Suspected cases of intracontinental Burkholderia pseudomallei sequence type homoplasy resolved using whole-genome sequencing

Burkholderia pseudomallei is a Gram-negative environmental bacterium that causes melioidosis, a disease of high mortality in humans and animals. Multilocus sequence typing (MLST) is a popular and portable genotyping method that has been used extensively to characterise the genetic diversity of B. pseudomallei populations. MLST has been central to our understanding of the underlying phylogeographical signal present in the B. pseudomallei genome, revealing distinct populations on both the intra- and the inter-continental level. However, due to its high recombination rate, it is possible for B. pseudomallei isolates to share the same multilocus sequence type (ST) despite being genetically and geographically distinct, with two cases of ‘ST homoplasy’ recently reported between Cambodian and Australian B. pseudomallei isolates. This phenomenon can dramatically confound conclusions about melioidosis transmission patterns and source attribution, a critical issue for bacteria such as B. pseudomallei that are of concern due to their potential for use as bioweapons. In this study, we used whole-genome sequencing to identify the first reported instances of intracontinental ST homoplasy, which involved ST-722 and ST-804 B. pseudomallei isolates separated by large geographical distances. In contrast, a third suspected homoplasy case was shown to be a true long-range (460 km) dispersal event between a remote Australian island and the Australian mainland. Our results show that, whilst a highly useful and portable method, MLST can occasionally lead to erroneous conclusions about isolate origin and disease attribution. In cases where a shared ST is identified between geographically distant locales, whole-genome sequencing should be used to resolve strain origin.

Burkholderia pseudomallei-loaded cells act as a Trojan horse to invade the brain during endotoxemia

Neurologic melioidosis occurs in both human and animals; however, the mechanism by which the pathogen Burkholderia pseudomallei invades the central nervous system (CNS) remains unclear. B. pseudomallei-loaded Ly6C cells have been suggested as a putative portal; however, during melioidosis, lipopolysaccharide (LPS) can drive disruption of the blood-brain barrier (BBB). This study aims to test whether the Trojan horse-like mechanism occurs during endotoxemia. The expression levels of cerebral cytokines, chemokines and cell adhesion molecules; the activation of astrocytes, microglia and endothelial cells; and the increased vascular permeability and brain-infiltrating leukocytes were evaluated using B. pseudomallei, B. thailandensis, B. cenocepacia and B. multivorans LPS-induced brains. Accordingly, different degrees of BBB damage in those brains with endotoxemia were established. The B. multivorans LPS-induced brain exhibited the highest levels of disruptive BBB according to the above mediators/indicators. Into these distinct groups of endotoxemic mice, B. pseudomallei-loaded Ly6C cells or free B. pseudomallei were adoptively transferred at equal bacterial concentrations (10³ CFU). The bacterial load and number of cases of meningeal neutrophil infiltration in the brains of animals treated with B. pseudomallei-loaded Ly6C cells were higher than those in brains induced by free B. pseudomallei in any of the endotoxemic groups. In particular, these results were reproducible in B. multivorans LPS-induced brains. We suggest that B. pseudomallei-loaded cells can act as a Trojan horse and are more effective than free B. pseudomallei in invading the CNS under septic or endotoxemic conditions even when there is a high degree of BBB disruption.

Melioidosis: An Australian Perspective

Burkholderia pseudomallei is endemic in northern Australia, with cases of melioidosis most commonly occurring during the wet season in individuals with diabetes, hazardous alcohol use, and chronic kidney disease. Pneumonia is the most common presentation and the majority of patients are bacteraemic—however, infection may involve almost any organ, with the skin and soft tissues, genitourinary system, visceral organs, and bone and joints affected most commonly. Central nervous system involvement is rarer, but has a high attributable mortality. Increased awareness of the disease amongst healthcare providers, ready access to appropriate antibiotic therapy and high-quality intensive care services has resulted in a sharp decline in the case fatality rate over the last 20 years. Further improvement in clinical outcomes will require a greater understanding of the disease′s pathophysiology, its optimal management, and more effective strategies for its prevention.

Clinical Epidemiology of Septic Arthritis Caused by Burkholderia pseudomallei and Other Bacterial Pathogens in Northeast Thailand

Septic arthritis is a medical emergency, and if not treated appropriately, it can be associated with high morbidity and mortality. Melioidosis, a serious infectious disease caused by the Gram-negative bacillus Burkholderia pseudomallei, is highly endemic in South and Southeast Asia and northern Australia. We reviewed the medical charts of adult patients admitted with bacterial septic arthritis at Sunpasitthiprasong Hospital, Ubon Ratchathani, northeast Thailand from January 2012 to December 2014. Bacterial septic arthritis was defined as one or more hot swollen joints with isolation of a pathogenic organism from an affected joint or from blood. A total of 154 patients with septic arthritis were retrospectively evaluated. The most common causes were B. pseudomallei (48%, N = 74), Streptococcus spp. (29%, N = 44), and Staphylococcus aureus (10%, N = 16). Prevalence of diabetes, bacteremia, and pneumonia was higher in B. pseudomallei septic arthritis than in septic arthritis caused by the other bacteria (all P < 0.01). Seventy three percent (54/74) of patients infected with B. pseudomallei and 69% (55/80) of patients with the other bacteria received effective antimicrobials on the first day of admission (P = 0.60), but in-hospital mortality of the former group was considerably higher (34% versus 14%, P = 0.004). In conclusion, B. pseudomallei septic arthritis is common and associated with high mortality in northeast Thailand. Emergence of Streptococcus arthritis is observed. Difficulty in diagnosing melioidosis and identifying B. pseudomallei in areas where health care workers are not familiar with the disease is discussed. In melioidosis-endemic regions, parenteral ceftazidime could be considered as empirical antimicrobial therapy for patients with septic arthritis and underlying diseases.