Hypervirulent Klebsiella pneumoniae is an emerging cause of community-acquired pyogenic liver abscess. First described in Asia, it is now increasingly recognized in Western countries, commonly afflicting those with Asian descent. This raises the question of genetic predisposition versus geospecific strain acquisition. We leveraged on the Antibiotics for Klebsiella Liver Abscess Syndrome Study (A-KLASS) clinical trial ongoing in ethnically diverse Singapore, to prospectively examine the profiles of 70 patients together with their isolates’ genotypic and phenotypic characteristics. The majority of isolates belonged to capsule type K1, a genetically homogenous group corresponding to sequence-type 23. The remaining K2, K5, K16, K28, K57 and K63 isolates as well as two novel cps isolates were genetically heterogeneous. K1 isolates carried higher frequencies of virulence-associated genes including rmpA (regulator of mucoid phenotype A), kfu (Klebsiella ferric uptake transporter), iuc (aerobactin), iro (salmochelin) and irp (yersiniabactin) than non-K1 isolates. The Chinese in our patient cohort, mostly non-diabetic, had higher prevalence of K1 infection than the predominantly diabetic non-Chinese (Malays, Indian and Caucasian). This differential susceptibility to different capsule types among the various ethnic groups suggests patterns of transmission (e.g. environmental source, familial transmission) and/or genetic predisposition unique to each race despite being in the same geographical location.

Burkholderia pseudomallei is the causative agent for melioidosis, an infectious disease endemic in South-east Asia and northern Australia. Infection can result in a wide spectrum of clinical outcomes, including asymtomatic, acute or chronic conditions. The ability of the bacteria to survive intracellularly within phagocytes and non-phagocytes is postulated to help this pathogen persist in the body during latent chronic conditions. In some Gram-negative bacteria, such as Shigella and Salmonella, the ability to evade macrophage killing involves inducing rapid macrophage cell death. In several of these instances, these bacteria activate cellular caspase-1 to induce cell death, which is increasingly described to exhibit features more characteristic of oncosis than classical apoptosis. We found that B. pseudomallei is also capable of inducing caspase-1 dependent death in macrophages and this process requires a functional bsa Type III Secretion System (TTSS). Bacterial internalization and pore formation in the cell membrane is necessary for death. Furthermore, cell death is accompanied by the release of IL-1beta and IL-18. We believe that this novel description of macrophage death induced by B. pseudomallei could shed light on the pathogenesis of the bacteria in disease.

HypervirulentKlebsiella pneumoniae(hvKP) causesKlebsiella‐induced liver abscess. Capsule is important for the pathogenesis ofKlebsiellain systemic infection, but its role in gut colonisation is not well understood. By generating ΔwcaJ, Δwzaand Δwzycapsule‐null mutants in a prototypical K1 hypervirulent isolate, we show that inactivation ofwza(capsule exportase) andwzy(capsule polymerase) confer cell envelope defects in addition to capsule loss, making them susceptible to bile salts and detergent stress. Bile salt resistance is restored when the initial glycosyltransferasewcaJwas inactivated together withwzy, indicating that build‐up of capsule intermediates contribute to cell envelope defects. Mouse gut colonisation competition assays show that the capsule and its regulator RmpA were not required for hvKP to persist in the gut, although initial colonisation was decreased in the mutants. Both ΔrmpAand ΔwcaJmutants gradually outcompeted the wild type in the gut, whereas Δwzaand Δwzymutants were less fit than wild type. Together, our results advise caution in using the right capsule‐null mutant for determination of capsule's role in bacterial pathogenesis. With the use of ΔwcaJmutant, we found that although the capsule is important for bacterial survival outside the gut environment, it imposes a fitness cost in the gut.

Low molecular weight (LMW) thiols contain reducing sulfhydryl groups that are important for maintaining antioxidant defense in the cell. Aside from the traditional roles of LMW thiols as redox regulators in bacteria, glutathione (GSH) has been reported to affect virulence and bacterial pathogenesis. The role of GSH in virulence is diverse, including the activation of virulence gene expression and contributing to optimal biofilm formation. GSH can also be converted to hydrogen sulfide (H₂S) which is important for the pathogenesis of certain bacteria. Besides GSH, some bacteria produce other LMW thiols such as mycothiol and bacillithiol that affect bacterial virulence. We discuss these newer reported functions of LMW thiols modulating bacterial pathogenesis either directly or indirectly and via modulation of the host immune system.

Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease with protean clinical manifestations. The major route of infection is thought to be through subcutaneous inoculation of contaminated soil and water, although ingestion and inhalation of contaminated aerosols are also possible. This study examines infection through the intranasal route in a murine model to mimic infection through inhalation. Two strains of mice, C57BL/6 and BALB/c, exhibit differential susceptibilities to the infection, with the C57BL/6 mice being considerably more resistant. To examine host factors that could contribute to this difference, bacterial loads and cytokine profiles in the two strains of mice were compared. We found that infected BALB/c mice exhibited higher bacterial loads in the lung and spleen and that they produced significantly higher levels of gamma interferon (IFN-γ) in the serum than C57BL/6 mice. Although tumor necrosis factor alpha and interleukin-1 could be detected in the nasal washes and sera of both strains of mice, the production in serum was transient and much lower than that of IFN-γ. C57BL/6 mice also exhibited memory responses to bacteria upon reinfection, with the production of serum immunoglobulin G (IgG) and mucosal IgA antibodies. Thus, it is possible that the production of systemic and mucosal antibodies is important for protection against disease in C57BL/6 mice.