Author: Maoge Zang
Zang, Maoge, 2024 The Acinetobacter baumannii lipid landscape and its impact on antimicrobial resistance, Flinders University, College of Science and Engineering
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The global burden of antimicrobial resistance is contributed in part by the Gram-negative bacteria A. baumannii. This pathogen readily persists in the nosocomial environment and facilitates opportunistic infections in at-risk individuals. In addition, the broad repertoire of antibiotic resistance mechanisms exacerbates the difficulties in clinical A. baumannii treatment. Thus, there is an urgent need to consider alternative strategies to combat its global resistance dissemination.
Fatty acids are a critical constituent of both prokaryotic and eukaryotic membranes, and are a key nutrient at the forefront of the host-pathogen interface. Although bacterial pathogens have the propensity to acquire fatty acids exogenously, many host dietary fatty acid species are antimicrobial. One such antimicrobial fatty acid is the omega-3 docosahexaenoic acid (DHA), which is a species that is essential for human health and is primarily acquired through the consumption of fatty fish. Supplementation of DHA has been demonstrated to exhibit antimicrobial potential against A. baumannii and reduces the prevalence of its antibiotic resistance evolution. Here, genomic analyses of A. baumannii resistant clones revealed the major role of resistance-nodulation-cell division (RND) efflux systems, AdeABC and AdeIJK, in resistance development. Further phenotypic testing of the isogenic RND efflux mutants determined that incorporation of DHA impacts the functionality of AdeABC, but not AdeIJK. In addition, transcriptomic analysis of A. baumannii following DHA treatment revealed the downregulation of genes associated with iron acquisition and biofilm formation. Importantly, this analysis highlighted that the bacterium adapts to antimicrobial fatty acid stress via changes in its lipid homeostasis mechanisms.
To facilitate successful host infection, the Gram-negative cell envelope serves as an initial defence barrier against both host antimicrobials and antibiotic compounds. During envelope assembly, bacterial lipid homeostasis is not only critical in maintaining the phospholipid-enriched membranes, but also interlinks and regulates multiple other biosynthesis pathways. A key step in the synthesis of bacterial phosphatidylglycerol is mediated by phosphatidylglycerol phosphate (PGP) phosphatases. In A. baumannii, two putative PGP phosphatases, PgpA and PgpB, were identified and shown to contribute to envelope biogenesis via distinct pathways. Specifically, the inactivation of pgpA in two A. baumannii backgrounds dramatically increased the relative abundance of phosphatidylethanolamine with double bonds in its fatty acid chains. This was attributed to the enhanced enzymatic activity of the A. baumannii phospholipid desaturase, DesA. In addition, the loss of PgpA facilitated pleiotropic impacts on the envelope. Mutations of pgpA resulted in a minor reduction in lipid A, increased susceptibility to peptidoglycan-degrading lysozyme, and reduced resistance to aminoglycosides. On the other hand, pgpB inactivation led to no impact on the bacterial lipidomic profile, and instead rendered A. baumannii more susceptible to lysozyme and carbapenem treatment.
Maintaining lipid homeostasis via adequate dietary intake is key in the prevention of malnutrition and lowers the risk of infectious diseases. For A. baumannii, utilisation of host lipids is a niche-specific process that contributes to successful host colonisation. As such, the host lipidomic landscape is a critical interface that can dictate the outcomes of infection. To further explore the impact of dietary lipids on A. baumannii infection, a dietary intervention murine model was established in this study. Compared to the low-fat control diet, enrichment of lard in the high fat diet increased the abundance of the omega-6 arachidonic acid in murine blood and liver. Importantly, dietary fish oil supplementation in a high omega-3 fat diet drastically elevated the relative abundance of DHA in murine blood, lung and liver. Following infection, fish oil supplementation minorly reduced A. baumannii colonisation in the lung and rendered bacteria isolated from this niche slightly more susceptible to gentamicin treatment. Further, transcriptional analysis of A. baumannii isolated from the bronchoalveolar lavage revealed an upregulation of fadB, suggesting enhanced fatty acid breakdown via β-oxidation.
Taken together, this study demonstrated the importance of host fatty acids in antimicrobial resistance of A. baumannii. Adaptation to antimicrobial fatty acid treatment and regulation of cell envelope biogenesis further highlighted the metabolic versatility of the A. baumannii lipid homeostasis mechanism. Lastly, this work provided insights into the utilisation of dietary fatty acids as a promising therapeutic avenue aimed to simultaneously improve the wellbeing of a susceptible patient and enhance antibiotic treatment against this pathogen.
Keywords: Acinetobacter baumannii, antibiotic resistance, antimicrobial lipids, cell envelope, DHA, mouse infection model
Subject: Medical Biotechnology thesis
Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Bart Eijkelkamp