A Machine Learning-Based Raman Spectroscopic Assay for the Identification of Burkholderia mallei and Related Species.

Burkholderia (B.) mallei, the causative agent of glanders, and B. pseudomallei, the causative agent of melioidosis in humans and animals, are genetically closely related. The high infectious potential of both organisms, their serological cross-reactivity, and similar clinical symptoms in human and animals make the differentiation from each other and other Burkholderia species challenging. The increased resistance against many antibiotics implies the need for fast and robust identification methods. The use of Raman microspectroscopy in microbial diagnostic has the potential for rapid and reliable identification. Single bacterial cells are directly probed and a broad range of phenotypic information is recorded, which is subsequently analyzed by machine learning methods. Burkholderia were handled under biosafety level 1 (BSL 1) conditions after heat inactivation. The clusters of the spectral phenotypes and the diagnostic relevance of the Burkholderia spp. were considered for an advanced hierarchical machine learning approach. The strain panel for training involved 12 B. mallei, 13 B. pseudomallei and 11 other Burkholderia spp. type strains. The combination of top- and sub-level classifier identified the mallei-complex with high sensitivities (>95%). The reliable identification of unknown B. mallei and B. pseudomallei strains highlighted the robustness of the machine learning-based Raman spectroscopic assay.

Raman spectroscopic detection and identification of Burkholderia mallei and Burkholderia pseudomallei in feedstuff.

Burkholderia mallei (the etiologic agent of glanders in equines and rarely humans) and Burkholderia pseudomallei, causing melioidosis in humans and animals, are designated category B biothreat agents. The intrinsically high resistance of both agents to many antibiotics, their potential use as bioweapons, and their low infectious dose, necessitate the need for rapid and accurate detection methods. Current methods to identify these organisms may require up to 1 week, as they rely on phenotypic characteristics and an extensive set of biochemical reactions. In this study, Raman microspectroscopy, a cultivation-independent typing technique for single bacterial cells with the potential for being a rapid point-of-care analysis system, is evaluated to identify and differentiate B. mallei and B. pseudomallei within hours. Here, not only broth-cultured microbes but also bacteria isolated out of pelleted animal feedstuff were taken into account. A database of Raman spectra allowed a calculation of classification functions, which were trained to differentiate Raman spectra of not only both pathogens but also of five further Burkholderia spp. and four species of the closely related genus Pseudomonas. The developed two-stage classification system comprising two support vector machine (SVM) classifiers was then challenged by a test set of 11 samples to simulate the case of a real-world-scenario, when "unknown samples" are to be identified. In the end, all test set samples were identified correctly, even if the contained bacterial strains were not incorporated in the database before or were isolated out of animal feedstuff. Specifically, the five test samples bearing B. mallei and B. pseudomallei were correctly identified on species level with accuracies between 93.9 and 98.7%. The sample analysis itself requires no biomass enrichment step prior to the analysis and can be performed under biosafety level 1 (BSL 1) conditions after inactivating the bacteria with formaldehyde.

Cross-species comparison of the Burkholderia pseudomallei, Burkholderia thailandensis, and Burkholderia mallei quorum-sensing regulons.

Burkholderia pseudomallei, Burkholderia thailandensis, and Burkholderia mallei (the Bptm group) are close relatives with very different lifestyles: B. pseudomallei is an opportunistic pathogen, B. thailandensis is a nonpathogenic saprophyte, and B. mallei is a host-restricted pathogen. The acyl-homoserine lactone quorum-sensing (QS) systems of these three species show a high level of conservation. We used transcriptome sequencing (RNA-seq) to define the quorum-sensing regulon in each species, and we performed a cross-species analysis of the QS-controlled orthologs. Our analysis revealed a core set of QS-regulated genes in all three species, as well as QS-controlled factors shared by only two species or unique to a given species. This global survey of the QS regulons of B. pseudomallei, B. thailandensis, and B. mallei serves as a platform for predicting which QS-controlled processes might be important in different bacterial niches and contribute to the pathogenesis of B. pseudomallei and B. mallei.

Gas chromatography-mass spectrometry method for rapid identification and differentiation of Burkholderia pseudomallei and Burkholderia mallei from each other, Burkholderia thailandensis and several members of the Burkholderia cepacia complex.

AIMS: To develop a simple gas chromatography-mass spectrometry (GC-MS) method for the detection and differentiation of Burkholderia pseudomallei and Burkholderia mallei from each other, Burkholderia thailandensis and several members of the Burkholderia cepacia complex. METHODS AND RESULTS: Biomarkers were generated by one-step thermochemolysis (TCM) and analysed using a GC-MS system. Fragments of poly-3-hydroxybutyrate-co-hydroxyvalerate [poly(3HBA-co-3HVA)] produced by TCM were useful biomarkers. Several cellular fatty acid methyl esters were important in differentiating the various Burkholderia species. A statistical discrimination algorithm was constructed using a combination of biomarkers. The identities of four B. pseudomallei strains, four B. mallei strains and one strain of each near neighbour were confirmed in a statistically designed test using the algorithm. The detection limit for this method was found to be approximately 4000 cells. CONCLUSIONS: The method is fast, accurate and easy to use. The algorithm is robust against different growth conditions (medium and temperature). SIGNIFICANCE AND IMPACT OF THE STUDY: This assay may prove beneficial in a clinical diagnostic setting, where the rapid identification of B. pseudomallei is essential to effective treatment. This method could also be easily employed after a biological attack to confirm the presence of either B. pseudomallei or B. mallei.

Rapid identification of Burkholderia mallei and Burkholderia pseudomallei by intact cell Matrix-assisted Laser Desorption/Ionisation mass spectrometric typing.

BACKGROUND: Burkholderia (B.) pseudomallei and B. mallei are genetically closely related species. B. pseudomallei causes melioidosis in humans and animals, whereas B. mallei is the causative agent of glanders in equines and rarely also in humans. Both agents have been classified by the CDC as priority category B biological agents. Rapid identification is crucial, because both agents are intrinsically resistant to many antibiotics. Matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-TOF MS) has the potential of rapid and reliable identification of pathogens, but is limited by the availability of a database containing validated reference spectra. The aim of this study was to evaluate the use of MALDI-TOF MS for the rapid and reliable identification and differentiation of B. pseudomallei and B. mallei and to build up a reliable reference database for both organisms. RESULTS: A collection of ten B. pseudomallei and seventeen B. mallei strains was used to generate a library of reference spectra. Samples of both species could be identified by MALDI-TOF MS, if a dedicated subset of the reference spectra library was used. In comparison with samples representing B. mallei, higher genetic diversity among B. pseudomallei was reflected in the higher average Eucledian distances between the mass spectra and a broader range of identification score values obtained with commercial software for the identification of microorganisms. The type strain of B. pseudomallei (ATCC 23343) was isolated decades ago and is outstanding in the spectrum-based dendrograms probably due to massive methylations as indicated by two intensive series of mass increments of 14 Da specifically and reproducibly found in the spectra of this strain. CONCLUSIONS: Handling of pathogens under BSL 3 conditions is dangerous and cumbersome but can be minimized by inactivation of bacteria with ethanol, subsequent protein extraction under BSL 1 conditions and MALDI-TOF MS analysis being faster than nucleic amplification methods. Our spectra demonstrated a higher homogeneity in B. mallei than in B. pseudomallei isolates. As expected for closely related species, the identification process with MALDI Biotyper software (Bruker Daltonik GmbH, Bremen, Germany) requires the careful selection of spectra from reference strains. When a dedicated reference set is used and spectra of high quality are acquired, it is possible to distinguish both species unambiguously. The need for a careful curation of reference spectra databases is stressed.

Development and validation of a method for purification of mallein for the diagnosis of glanders in equines.

BACKGROUND: The allergic test of mallein is one of the most frequently used tests, together with the Complement Fixation Test (CFT), for the diagnosis of glanders in endemic areas. Mallein, a purified protein derivative (PPD), is produced similarly to PPD tuberculin and the end product is a primarily proteic antigen, which is only poorly purified. The immuno-allergic activity of mallein is believed to be due to a high molecular weight group of proteins present in the antigen. To improve the quality of the antigen, in terms of sensitivity and specificity, a new method of mallein production was developed, in which purification was accomplished by ultrafiltration in a Tangential Flow Filtration system (TFF). RESULTS: The TFF methodology efficiently separated the high and low molecular weight protein groups of mallein. The five TFF-purified malleins, produced from Burkholderia mallei strains isolated from clinical cases of glanders in Brazil, proved to be more potent than standard mallein in the induction of an allergic reaction in sensitized animals. Regarding specificity, two of the purified malleins were equivalent to the standard and three were less specific. CONCLUSION: Some of the TFF-purified malleins showed considerable potential to be used as an auxiliary test in the diagnosis of glanders.

Identification of Burkholderia mallei and Burkholderia pseudomallei adhesins for human respiratory epithelial cells.

BACKGROUND: Burkholderia pseudomallei and Burkholderia mallei cause the diseases melioidosis and glanders, respectively. A well-studied aspect of pathogenesis by these closely-related bacteria is their ability to invade and multiply within eukaryotic cells. In contrast, the means by which B. pseudomallei and B. mallei adhere to cells are poorly defined. The purpose of this study was to identify adherence factors expressed by these organisms. RESULTS: Comparative sequence analyses identified a gene product in the published genome of B. mallei strain ATCC23344 (locus # BMAA0649) that resembles the well-characterized Yersinia enterocolitica autotransporter adhesin YadA. The gene encoding this B. mallei protein, designated boaA, was expressed in Escherichia coli and shown to significantly increase adherence to human epithelial cell lines, specifically HEp2 (laryngeal cells) and A549 (type II pneumocytes), as well as to cultures of normal human bronchial epithelium (NHBE). Consistent with these findings, disruption of the boaA gene in B. mallei ATCC23344 reduced adherence to all three cell types by ~50%. The genomes of the B. pseudomallei strains K96243 and DD503 were also found to contain boaA and inactivation of the gene in DD503 considerably decreased binding to monolayers of HEp2 and A549 cells and to NHBE cultures.A second YadA-like gene product highly similar to BoaA (65% identity) was identified in the published genomic sequence of B. pseudomallei strain K96243 (locus # BPSL1705). The gene specifying this protein, termed boaB, appears to be B. pseudomallei-specific. Quantitative attachment assays demonstrated that recombinant E. coli expressing BoaB displayed greater binding to A549 pneumocytes, HEp2 cells and NHBE cultures. Moreover, a boaB mutant of B. pseudomallei DD503 showed decreased adherence to these respiratory cells. Additionally, a B. pseudomallei strain lacking expression of both boaA and boaB was impaired in its ability to thrive inside J774A.1 murine macrophages, suggesting a possible role for these proteins in survival within professional phagocytic cells. CONCLUSIONS: The boaA and boaB genes specify adhesins that mediate adherence to epithelial cells of the human respiratory tract. The boaA gene product is shared by B. pseudomallei and B. mallei whereas BoaB appears to be a B. pseudomallei-specific adherence factor.

Molecular epidemiology of glanders, Pakistan.

We collected epidemiologic and molecular data from Burkholderia mallei isolates from equines in Punjab, Pakistan from 1999 through 2007. We show that recent outbreaks are genetically distinct from available whole genome sequences and that these genotypes are persistent and ubiquitous in Punjab, probably due to human-mediated movement of equines.

DNA microarray-based detection and identification of Burkholderia mallei, Burkholderia pseudomallei and Burkholderia spp.

We developed a rapid oligonucleotide microarray assay based on genetic markers for the accurate identification and differentiation of Burkholderia (B.) mallei and Burkholderia pseudomallei, the agents of glanders and melioidosis, respectively. These two agents were clearly identified using at least 4 independent genetic markers including 16S rRNA gene, fliC, motB and also by novel species-specific target genes, identified by in silico sequence analysis. Specific hybridization signal profiles allowed the detection and differentiation of up to 10 further Burkholderia spp., including the closely related species Burkholderia thailandensis and Burkholderia-like agents, such as Burkholderia cepacia, Burkholderia cenocepacia, Burkholderia vietnamiensis, Burkholderia ambifaria, and Burkholderia gladioli, which are often associated with cystic fibrosis (CF) lung disease. The assay was developed using the easy-to-handle and economical ArrayTube (AT) platform. A representative strain panel comprising 44 B. mallei, 32 B. pseudomallei isolates, and various Burkholderia type strains were examined to validate the test. Assay specificity was determined by examination of 40 non-Burkholderia strains.

High-resolution melting PCR analysis for rapid genotyping of Burkholderia mallei.

Burkholderia (B.) mallei is the causative agent of glanders. A previous work conducted on single-nucleotide polymorphisms (SNP) extracted from the whole genome sequences of 45 B. mallei isolates identified 3 lineages for this species. In this study, we designed a high-resolution melting (HRM) method for the screening of 15 phylogenetically informative SNPs within the genome of B. mallei that subtype the species into 3 lineages and 12 branches/sub-branches/groups. The present results demonstrate that SNP-based genotyping represent an interesting approach for the molecular epidemiology analysis of B. mallei.

First molecular characterisation of a Brazilian Burkholderia mallei strain isolated from a mule in 2016.

We present the first molecular characterisation based on MLVA and SNP analysis of a strain of Burkholderia mallei isolated from a mule found dead in Brazil in 2016.

Genomic fingerprinting of Burkholderia pseudomallei and B. mallei pathogens with DNA array based on interspecies sequence differences obtained by subtractive hybridization.

The ability to rapidly and efficiently identify causative agents of dangerous human and animal diseases is a prerequisite to diagnosis, prophylaxis and therapy. Such identification systems can be developed based on DNA markers enabling differentiation between various bacterial strains. One source of these markers is genetic polymorphism. An efficient method for detecting the most stable polymorphisms without knowledge of genomic sequences is subtractive hybridization. In this work we report an approach to typing of Burkholderia pseudomallei and B. mallei that cause melioidosis and glanders, respectively. Typing is based on hybridization of bacterial genomes with a DNA array of genomic markers obtained using subtractive hybridization. The array comprised 55 DNA fragments which distinguished the genomes of B. pseudomallei C-141 and B. mallei C-5 strains, and it was used to test 28 radioactively labeled B. pseudomallei strains and 8 B. mallei strains. Each strain was characterized by a specific hybridization pattern, and the results were analyzed using cluster analysis. 18 patterns specific to B. pseudomallei and 6 patterns specific to B. mallei were found to be unique. The data allowed us to differentiate most studied B. pseudomallei variants from one another and from B. mallei strains. It was concluded that DNA markers obtained by subtractive hybridization can be potentially useful for molecular typing of B. pseudomallei and B. mallei strains, as well as for their molecular diagnosis. The method reported can be easily adapted for use both with DNA arrays and DNA microarrays with fluorescent probes.

Pulsed-field gel electrophoresis as a discriminatory typing technique for the biothreat agent burkholderia mallei.

Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) was used to type 21 laboratory strains of Burkholderia mallei. We demonstrated good resolution by PFGE together with clustering of some geographically related isolates, and confirmed previous observations that B. mallei is clonal as defined by MLST.

Genome-wide identification and mapping of variable sequences in the genomes of Burkholderia mallei and Burkholderia pseudomallei.

Burkholderia mallei and Burkholderia pseudomallei, closely related Gram-negative bacteria, are the causative agents of such serious infectious diseases of humans and animals as glanders and melioidosis, respectively. Despite numerous studies of these pathogens, the detailed mechanisms of their pathogenesis is still poorly understood. One of the serious obstacles to revealing factors responsible for pathogenicity lies in the considerable natural variability of B. pseudomallei and B. mallei, which is also a challenge to development of rapid and efficient diagnostic tools facilitating unambiguous identification of the infectious agents. To gain a deeper insight into B. mallei and B. pseudomallei interspecies divergence and intraspecies polymorphism, we compared the genomes of B. mallei C-5 and B. pseudomallei C-141 strains using a subtractive hybridization technique. A library of DNA fragments specific for B. mallei C-5 and absent from B. pseudomallei C-141 was obtained and analyzed. Some of the differential sequences detected were also not found in the recently sequenced genome of B. pseudomallei K96243. However, a multitude of B. mallei C-5 sequences absent from the B. pseudomallei C-141 genome were detected in the genome of B. pseudomallei K96243. On the other hand, some sequences identified as constituents of the B. mallei C-5 genome were not found in the genome of B. mallei ATCC 23344. Some of the differential DNA fragments displayed similarity to different mobile elements that have not yet been described for B. mallei, whereas the others matched fragments of various prophages, or, when translated into protein sequences, components of active transport systems and different enzymes. A substantial proportion of the differential clones had no database matches either at the nucleotide or amino acid sequence level. The results suggest great genome-wide intra- and interspecies variability of B. mallei and B. pseudomallei. The differences identified may be useful as molecular signatures for identification of B. mallei strains.

Ribotyping of Burkholderia mallei isolates.

In this study, the subspecies differentiation of 25 isolates of Burkholderia mallei was attempted based on their ribotype polymorphisms. The isolates were from human and equine infections that occurred at various times around the world. DNA samples from each isolate were digested separately with PstI and EcoRI enzymes and probed with an Escherichia coli-derived 18-mer rDNA sequence to identify diagnostic fragments. Seventeen distinct ribotypes were identified from the combined data obtained with the two restriction enzymes. The results demonstrate the general utility of ribotyping for the subspecies identification of B. mallei isolates.

Clear distinction between Burkholderia mallei and Burkholderia pseudomallei using fluorescent motB primers.

BACKGROUND: A frame-shift mutation in the flagellum motor gene motB coding for the chemotaxis MotB protein of Burkholderia mallei has been utilized to design a conventional duplex PCR assay with fluorescent labelled primers. FINDINGS: Species specificity was tested with a panel of 13 Burkholderia type strains. A total of 41 B. mallei field strains, 36 B. pseudomallei field strains, and 1 B. thailandensis field strain from different geographic regions were tested and correctly identified. Testing of 55 non-Burkholderia bacterial species revealed 100% specificity of the assay. The minimum detection limit was 1 pg DNA or 160 GE for B. mallei and 130 GE for B. pseudomallei, respectively. CONCLUSIONS: This assay enables the clear distinction between B. mallei and B. pseudomallei/B. thailandensis.

Genome-wide comparison reveals great inter- and intraspecies variability in B. pseudomallei and B. mallei pathogens.

Burkholderia mallei and B. pseudomallei, closely related Gram-negative bacteria, are causative agents of serious infectious diseases of humans and animals: glanders and melioidosis, respectively. Despite numerous studies of these pathogens, the detailed mechanism of their pathogenesis is still unknown. The problem is even more complicated due to natural variability of B. pseudomallei and B. mallei strains, the understanding of which is a prerequisite for rational design of tools for diagnostics, prophylaxis and therapy of the diseases. Using a subtractive hybridization technique, we compared the genomes of B. pseudomallei C-141 and B. mallei C-5 strains. A subtracted library of DNA fragments specific for B. pseudomallei C-141 and absent from B. mallei C-5 was obtained and analyzed. A variety of differences have been detected and mapped on the recently sequenced genome of B. pseudomallei K96243. A comparative sequence analysis also revealed considerable genomic differences between B. pseudomallei C-141 and B. mallei ATCC 23344 strains sequenced at The Institute for Genomic Research (TIGR). We also observed significant genomic differences between B. pseudomallei C-141 and B. pseudomallei K96243. Some of the differential DNA fragments displayed similarity to different mobile elements which have not yet been described for B. pseudomallei, whereas the others matched various prophage components, components of active transport systems, different enzymes and transcription regulators. A substantial proportion of the differential clones had no database matches either at the nucleotide or protein level. The results provide evidence for great genome-wide variability of B. pseudomallei, further confirmed by Southern blot analysis of various B. pseudomallei strains. The data obtained can be useful for future development of efficient diagnostic tools allowing rapid identification of species, strains and isolates of B. mallei and B. pseudomallei.

Development of 5' nuclease real-time PCR assays for the rapid identification of the burkholderia mallei//burkholderia pseudomallei complex.

Burkholderia pseudomallei is the causative agent of melioidosis and was classified as a biologic agent by the Centers for Disease Control and Prevention (Atlanta, GA). Acute melioidosis has a case fatality rate of >40%, and septicemia is fatal in up to 90%. The aim of the study was to design 5'-nuclease real-time PCR assays for the rapid and reliable identification of the B. mallei/B. pseudomallei complex. Real-time PCR assays using TaqMan probes targeting the 16S rDNA and fliC were developed on an ABI Prism 7000 sequence detection system (Applied Biosystems, Foster City, CA). Specificity was assessed with 64 B. pseudomallei, nine B. mallei, 126 other Burkholderia strains of 29 species, and 45 clinically relevant non-Burkholderia organisms. Sensitivity, specificity, and positive and negative predictive value of the assays were 100%. Discrimination between B. pseudomallei and B. mallei, an organism which can be regarded as a clone of B. pseudomallei, could not be achieved. A probit analysis revealed that 7.5 and 52 genome equivalents (GE) of B. pseudomallei could be detected using the fliC and the 16S rDNA assays (P = .05), respectively. In spiked blood samples, the detection limit was approximately 300 and 3.000 GE for fliC and the 16S rDNA, respectively. In conclusion, we recommend the simultaneous use of the 16S rDNA and fliC real-time PCR assays for the rapid and specific identification of the B. mallei/B. pseudomallei complex in positive blood cultures or from suspicious bacterial colonies allowing the early onset of appropriate antibiotic therapy.

Evaluation of a latex agglutination assay for the identification of Burkholderia pseudomallei and Burkholderia mallei.

Cases of melioidosis and glanders are rare in the United States, but the etiologic agents of each disease (Burkholderia pseudomallei and Burkholderia mallei, respectively) are classified as Tier 1 select agents because of concerns about their potential use as bioterrorism agents. A rapid, highly sensitive, and portable assay for clinical laboratories and field use is required. Our laboratory has further evaluated a latex agglutination assay for its ability to identify B. pseudomallei and B. mallei isolates. This assay uses a monoclonal antibody that specifically recognizes the capsular polysaccharide produced by B. pseudomallei and B. mallei, but is absent in closely related Burkholderia species. A total of 110 B. pseudomallei and B. mallei were tested, and 36 closely related Burkholderia species. The latex agglutination assay was positive for 109 of 110 (99.1% sensitivity) B. pseudomallei and B. mallei isolates tested.