Melioidosis

Abstract

Burkholderia pseudomallei is a Gram-negative environmental bacterium and the aetiological agent of melioidosis, a life-threatening infection that is estimated to account for ∼89,000 deaths per year worldwide. Diabetes mellitus is a major risk factor for melioidosis, and the global diabetes pandemic could increase the number of fatalities caused by melioidosis. Melioidosis is endemic across tropical areas, especially in southeast Asia and northern Australia. Disease manifestations can range from acute septicaemia to chronic infection, as the facultative intracellular lifestyle and virulence factors of B. pseudomallei promote survival and persistence of the pathogen within a broad range of cells, and the bacteria can manipulate the host's immune responses and signalling pathways to escape surveillance. The majority of patients present with sepsis, but specific clinical presentations and their severity vary depending on the route of bacterial entry (skin penetration, inhalation or ingestion), host immune function and bacterial strain and load. Diagnosis is based on clinical and epidemiological features as well as bacterial culture. Treatment requires long-term intravenous and oral antibiotic courses. Delays in treatment due to difficulties in clinical recognition and laboratory diagnosis often lead to poor outcomes and mortality can exceed 40% in some regions. Research into B. pseudomallei is increasing, owing to the biothreat potential of this pathogen and increasing awareness of the disease and its burden; however, better diagnostic tests are needed to improve early confirmation of diagnosis, which would enable better therapeutic efficacy and survival.

Figure 1 |. Milestones in the history of melioidosis.

Melioidosis was first recognised in Rangoon in 1911 by the British doctor Alfred Whitmore and his assistant C. S. Krishnaswami, although the name of the disease was coined by Thomas Stanton and William Fletcher. From the time when the aetiological organism was first identified, it has been renamed many times: Bacterium (or Bacillus) whitmori, Malleomyces pseudomallei, Loefflerella pseudomallei, Pfeifferella whitmori, Pseudomonas pseudomallei and, finally, it was officially named Burkholderia pseudomallei in 1992. CDC, Centers for Disease Control and Prevention.

Figure 2 |. Estimated mortality and reported cases of melioidosis.

Only Australia, Brunei and Singapore have national surveillance data for melioidosis that are comparable to the estimates. Between 2010 and 2015, there were >100 culture-confirmed cases of melioidosis at a single hospital in Lao People’s Democratic Republic yearly, a number that supports the estimated 420 cases per year countrywide. However, ~20,000 cases of melioidosis per year are estimated in Indonesia, but only 64 have been reported in the country since 1921 (REF. 248). A large difference between the numbers of predicted and observed cases is also observed in Bangladesh, Brazil, China, India and Nigeria. This discrepancy could be due to limitations of the model, underuse of clinical microbiology laboratories, lack of awareness of melioidosis and poor disease reporting systems. Based on data from REF. . N/A, not applicable.

Figure 3 |. Schematic model of host-pathogen interactions and pathophysiology of melioidosis.

Burkholderia pseudomallei secretes N-acyl-homoserine lactones (AHL), which are signalling molecules involved in the quorum sensing machinery that is used to coordinate attacks against the host environment and biofilm formation. The type III secretion system (T3SS) effector proteins are necessary for invasion and escape from the endocytic vesicle; cell entry is aided by flagella, lipopolysaccharide (LPS), type IV pili and adhesins BoaA and BoaB. B. pseudomallei then guickly escapes the vesicle by lysing the membrane using T3SS, T6SS and T2SS. Metabolic flexibility (resistance to oxidative stress), resistance to antimicrobial cationic peptides and ecotin production enable bacteria to survive within an acidic endocytic environment. Effector protein BopA and translocator protein BipD further block sequestration in endocytic vesicles and prevent microtubule-associated proteins 1A/1B light chain 3B (LC3)-associated autophagy. Once free in the cytoplasm, B. pseudomallei replicates, induces the formation of actin-based membrane protrusions and can move via continuous polymerization of host cell actin at polar ends (a process regulated by autotransporter BimA), thereby facilitating spread to neighbouring cells, cell fusion and multinuclear giant cell (MNGC) formation. T6SS and the type IV secretion system (VgrG-5) are essential to this process. Toll-like receptors (TLRs) located on cell surfaces recognize pathogen-associated molecular patterns (such as LPS and flagella) and mediating nuclear factor-κB (NF-κB)-induced activation of the immune response, releasing pro-inflammatory cytokines IL-1β and IL-18. Intracellular inflammasome receptors such as NLR family CARD domain-containing protein 4 (NLRC4) and NACHT, LRR and PYD domains-containing protein 3 (NLRP3) recognize bacterial virulence factors and damage-associated molecular patterns (DAMPS), triggering caspase-1-mediated pyroptosis and further release of IL-1β and IL-18. IL-18 further ensures protective interferon-γ (IFNγ) production (mainly from natural killer cells). Neutrophils, dendritic cells, B cells and T cells are recruited towards the site of infection, and the complement and coagulation cascades are activated. AhpC, alkyl hydroperoxide reductase; BLF1, Burkholderia lethal factor 1; CIS, cytokine-inducible SH2-containing protein; DpsA, DNA starvation/stationary phase protection protein; EIF4A, eukaryotic initiation factor 4A; ER, endoplasmic reticulum; iNOS, inducible nitric oxide synthase; IRAK3, interleukin 1 receptor-associated kinase 3; KatG, catalase-peroxidase; MyD88, myeloid differentiation primary response protein; NF-κBIα, NF-κB inhibitor-α; NO, nitric oxide; NOD2, nucleotide-binding oligomerization domain-containing protein 2; ROS, reactive oxygen species; RpoS, RNA polymerase σ-factor RpoS; SOCS3, suppressor of cytokine signalling 3; SodC, copper/zinc superoxide dismutase; TNF, tumour necrosis factor; TRAF6, TNF receptor-associated factor 6; TssM, type VI secretion system.

Figure 4 |. Clinical manifestations of melioidosis.

Examples of possible clinical presentations of melioidosis: an MRI of the brainstem and cervical spinal cord with inflammatory changes consistent with encephalomyelitis (arrow, part 1); a ring-enhancing lesion with surrounding oedema in the MRI image indicating cerebral abscesses (arrow, part 2); a CT image of prostatic abscesses (arrow, part 3); a CT image of a mediastinal mass (arrow, part 4); a child with tense parotitis (arrow, part 5); X-ray image of severe pneumonia (arrow, part 6); photo of a subcutaneous abscess (arrow, part 7); and an MRI image of osteomyelitis of the distal femur with surrounding inflammation (arrow, part 8). Clinical images 1–4, 6–8 courtesy of Bart J. Currie, Menzies School of Health, Australia. Clinical image 5 is reproduced with permission from (REF. 249), Elsevier.

Figure 5 |. Identification of Burkholderia pseudomallei colonies on three common types of agar.

Typical appearance of Burkholderia pseudomallei and Escherichia coli isolated from non-sterile clinical samples. Suspected clinical specimens and suspected bacterial colonies should be processed in a biological safety cabinet. a | B. pseudomallei forms creamy, non-haemolytic colonies that resemble a coliform after 2 days of incubation; by day 4, the colonies are covered by a slight metallic sheen and become dry and wrinkled. b | B. pseudomallei colonies resemble a colourless, non-lactose fermenting coliform after 2 days of incubation; by day 4, the colonies appear dry and wrinkled. c | After 2 days of incubation, the first visible B. pseudomallei colonies are pinpoint with a clear to pale pink colour; by day 4, they become darker pink to purple, flat, slightly dry and wrinkled with a definite metallic sheen. E. coli fails to grow because it is inhibited by gentamicin in the agar. d | Three-disc diffusion antibiotic sensitivity testing: B. pseudomallei is resistant to colistin (or polymyxin) (black arrow) and gentamicin (arrowhead) (although sensitive isolates exist in some areas) and sensitive to co-amoxiclav (red arrow). Parts a–c courtesy of Direk Limmathurotsakul, Premjit Amornchai and Vanaporn Wuthiekanun, Mahidol-Oxford Tropical Medicine Research Unit, Thailand. Part d courtesy of Vanaporn Wuthiekanun, Mahidol-Oxford Tropical Medicine Research Unit, Thailand.