The role of lipid A in the virulence of ESKAPE pathogens
The exponential growth of antimicrobial resistance worldwide is one of the most concerning threats to mankind in the modern era. Gram-negative bacterial infections are of particular concern and have been identified as an urgent threat by governmental organisations. The Gram-negative pathogens Acinetobacter baumannii and Klebsiella pneumoniae have been highlighted as critical priority pathogens for research and development of novel therapeutics. Both of these pathogens share notoriety for their readiness to develop resistance to all antibiotics currently in circulation, including those of last resort. Antimicrobial resistance increases mortality and morbidity in addition to increasing the rate of diseases such as sepsis due to limited treatment options. Therefore, life-threatening conditions such as septic shock can be sequalae to previously unthreatening diseases such as urinary tract infections and small wounds. Bloodstream infections are a leading cause of mortality in critically ill patients, of which Gram-negative bacteraemia accounts for a large proportion of all hospitalised cases. Lipopolysaccharide/lipooligosaccharide (LPS/LOS) present on the outer membrane of Gram-negative pathogens is the main sepsis-inducing agent known for its significant inflammatory capabilities during septic shock. Here we investigated the lipid A anchoring moiety of A. baumannii LOS which is the most pronounced pro-inflammatory molecule of A. baumannii. Several studies have elucidated A. baumannii lipid A modifications and their role in virulence. Therefore, we investigated the role of the previously unidentified lipid A-remodelling enzyme in A. baumannii and explored its contribution to virulence and resistance to last-resort antibiotics. These findings have identified a novel target for the development of new, urgent antimicrobial compounds. In agreement with published literature, our results may allow development of a broad-spectrum compound against other Gram-negative pathogens possessing this conserved enzyme. Recently, the extracellular pathogen K. pneumoniae has been reported to survive within host cells. It is currently unknown exactly how K. pneumoniae does this or the factors involved. Following the discovery of K. pneumoniae surviving intracellularly, we sought to determine whether Klebsiella remodels its lipid A as a result of adaption to the intracellular lifestyle. Here we report for the first time, K. pneumoniae lipid A is extensively modified intracellularly to assist in survival within host macrophages of both mice and humans. The canonical membrane-bound lipid A receptor, TLR4, and TRAM-TRIF are responsible for K. pneumoniae lipid A remodelling. PhoPQ plays a prominent role in governing the lipid A modifications found intracellularly. These findings shed insight into the complex intracellular pathogenesis of K. pneumoniae and may contribute to potential therapeutics which may target host-directed responses to intracellular K. pneumoniae. Collectively, our work further characterises the importance of lipid A with regards to virulence and antimicrobial resistance in two major pathogens of critical magnitude.
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