The Gram-negative opportunistic bacterium Acinetobacter baumannii is a significant cause of hospital-borne infections worldwide. Alarmingly, the rapid development of antimicrobial resistance coupled with the remarkable ability of isolates to persist on surfaces for extended periods of time has led to infiltration of A. baumannii into our healthcare environments. A major virulence determinant of A. baumannii is the presence of a capsule that surrounds the bacterial surface. This capsule is comprised of tightly packed repeating polysaccharide units which forms a barrier around the bacterial cell wall, providing protection from environmental pressures including desiccation and disinfection regimes as well as host immune responses such as serum complement. Additionally, capsule has been shown to confer resistance to a range of clinically relevant antimicrobial compounds. Distressingly, treatment options for A. baumannii infections are becoming increasingly limited, and the urgency to develop effective infection control strategies and therapies to combat infections is apparent. An increased understanding of the contribution of capsule to the pathobiology of A. baumannii is required to determine its feasibility as a target for new strategies to combat drug resistant infections. Significant variation in capsular polysaccharide structures between A. baumannii isolates has been identified, with over 100 distinct capsule types, incorporating a vast variety of sugars. This review examines the studies undertaken to elucidate capsule diversity and advance our understanding of the role of capsule in A. baumannii pathogenesis.

In recent years, effective treatment of infections caused by Acinetobacter baumannii has become challenging due to the ability of the bacterium to acquire or up-regulate antimicrobial resistance determinants. Two component signal transduction systems are known to regulate expression of virulence factors including multidrug efflux pumps. Here, we investigated the role of the AdeRS two component signal transduction system in regulating the AdeAB efflux system, determined whether AdeA and/or AdeB can individually confer antimicrobial resistance, and explored the interplay between pentamidine resistance and growth conditions in A. baumannii ATCC 17978. Results identified that deletion of adeRS affected resistance towards chlorhexidine and 4’,6-diamidino-2-phenylindole dihydrochloride, two previously defined AdeABC substrates, and also identified an 8-fold decrease in resistance to pentamidine. Examination of ΔadeA, ΔadeB and ΔadeAB cells augmented results seen for ΔadeRS and identified a set of dicationic AdeAB substrates. RNA-sequencing of ΔadeRS revealed transcription of 290 genes were ≥2-fold altered compared to the wildtype. Pentamidine shock significantly increased adeA expression in the wildtype, but decreased it in ΔadeRS, implying that AdeRS activates adeAB transcription in ATCC 17978. Investigation under multiple growth conditions, including the use of Biolog phenotypic microarrays, revealed resistance to pentamidine in ATCC 17978 and mutants could be altered by bioavailability of iron or utilization of different carbon sources. In conclusion, the results of this study provide evidence that AdeAB in ATCC 17978 can confer intrinsic resistance to a subset of dicationic compounds and in particular, resistance to pentamidine can be significantly altered depending on the growth conditions.

Antimicrobial resistance is an emerging global health crisis. Consequently, we have a critical need to prolong our current arsenal of antibiotics, in addition to the development of novel treatment options.

Acinetobacter baumannii is a ubiquitous Gram-negative bacterium, that is associated with significant disease in immunocompromised individuals. The success of A. baumannii is partly attributable to its high level of antibiotic resistance. Further, A. baumannii expresses a broad arsenal of putative zinc efflux systems that are likely to aid environmental persistence and host colonization, but detailed insights into how the bacterium deals with toxic concentrations of zinc are lacking. In this study we present the transcriptomic responses of A. baumannii to toxic zinc concentrations. Subsequent mutant analyses revealed a primary role for the resistance-nodulation-cell division heavy metal efflux system CzcCBA, and the cation diffusion facilitator transporter CzcD in zinc resistance. To examine the role of zinc at the host–pathogen interface we utilized a murine model of zinc deficiency and challenge with wild-type and czcA mutant strains, which identified highly site-specific roles for zinc during A. baumannii infection. Overall, we provide novel insight into the key zinc resistance mechanisms of A. baumannii and outline the role these systems play in enabling the bacterium to survive in diverse environments.

Acinetobacter baumannii is one of the world’s most problematic superbugs and is associated with significant morbidity and mortality in the hospital environment. The critical need for new antimicrobial strategies is recognized, but our understanding of its behavior and adaptation to a changing environment during infection is limited.

Acinetobacter baumannii is a significant opportunistic pathogen responsible for infections of the lung, blood, skin, urinary tract, and soft tissues, with some strains exhibiting almost complete resistance to commonly used antibiotics. This multidrug resistance, together with a dearth of new antibiotic development, mean novel methods of treatment and prevention are urgently needed. Although many A. baumannii factors required to colonize the host have been identified, little is known about the specific host molecules recognized by these factors. A. baumannii produces a trimeric autotransporter adhesin known as Ata that has been previously demonstrated to bind components of the host cell's extracellular matrix, which are often heavily glycosylated. We hypothesized that Ata would exhibit lectin activity which would play a role in adherence to the host cell surface. Our biophysical analysis using glycan arrays and surface plasmon resonance demonstrated that Ata binds galactose, N-acetylglucosamine, and galactose (β1−3/4) N-acetylglucosamine with high-affinity. These structures are present on many of the proteins which were previously reported to be bound by Ata. We also demonstrated that the recognition of human plasma fibronectin by Ata requires this ability to bind glycans, as the interaction between Ata and fibronectin does not occur when fibronectin is deglycosylated. This strongly suggests a key role for Ata lectin activity during host adherence. This information will assist in directing the development of new and effective treatments to block host interactions using glycans and/or novel compounds in multidrug resistant A. baumannii infections.

The global distribution of multidrug resistance in A. baumannii has necessitated seeking not only alternative therapeutic approaches but also the means to limit the development of resistance in clinical settings. Highly abundant host bioactive compounds, such as polyunsaturated fatty acids, are readily acquired by A. baumannii during infection and have been illustrated to impact the bacterium’s membrane composition and antibiotic resistance.

Bacterial fatty acids are critical components of the cellular membrane. A shift in environmental conditions or in the bacterium’s lifestyle may result in the requirement for a distinct pool of fatty acids with unique biophysical properties. This can be achieved by the modification of existing fatty acids or via de novo synthesis. Furthermore, bacteria have evolved efficient means to acquire these energy-rich molecules from their environment. However, the balance between de novo fatty acid synthesis and exogenous acquisition during pathogenesis is poorly understood. Here we studied the mouse fatty acid landscape prior and post infection with Acinetobacter baumannii, a Gram-negative, opportunistic human pathogen. The lipid fluxes observed following infection revealed fatty acid- and niche-specific changes. Lipidomic profiling of A. baumannii isolated from the pleural cavity of mice identified novel A. baumannii membrane phospholipid species and an overall increased abundance of unsaturated fatty acid species. Importantly, we found that A. baumannii relies largely upon fatty acid acquisition in all but one of the studied niches, the blood, where the pathogen biosynthesises its own fatty acids. This work is the first to reveal the significance of balancing the making and taking of fatty acids in a Gram-negative bacterium during infection, which provides new insights into the validity of targeting fatty acid synthesis as a treatment strategy.ImportanceAcinetobacter baumannii is one of the world’s most problematic superbugs, and is associated with significance morbidity and mortally in the hospital environment. The critical need for new antimicrobial strategies is recognised, but our understanding of its behaviour and adaptation to a changing environment during infection is limited. Here, we investigated the role of fatty acids at the host-pathogen interface using a mouse model of disease. We provide comprehensive insights into the bacterial membrane composition when they colonise the pleural cavity. Further, we show that A. baumannii heavily relies upon making its fatty acids when residing in the blood, whereas the bacterium favours fatty acid acquisition in most other host niches. Our new knowledge aids in understanding the importance of host fatty acids in infectious diseases. Further, fatty acid synthesis is an attractive target for the development of new antimicrobial strategies, but our work emphasizes the critical need to understand the microbial lipid homeostasis before this can be deemed suitable.