Acinetobacter baumannii phenylacetic acid metabolism influences infection outcome through a direct effect on neutrophil chemotaxis

Abstract

Innate cellular immune responses are a critical first-line defense against invading bacterial pathogens. Leukocyte migration from the bloodstream to a site of infection is mediated by chemotactic factors that are often host-derived. More recently, there has been a greater appreciation of the importance of bacterial factors driving neutrophil movement during infection. Here, we describe the development of a zebrafish infection model to study Acinetobacter baumannii pathogenesis. By using isogenic A. baumannii mutants lacking expression of virulence effector proteins, we demonstrated that bacterial drivers of disease severity are conserved between zebrafish and mammals. By using transgenic zebrafish with fluorescent phagocytes, we showed that a mutation of an established A. baumannii global virulence regulator led to marked changes in neutrophil behavior involving rapid neutrophil influx to a localized site of infection, followed by prolonged neutrophil dwelling. This neutrophilic response augmented bacterial clearance and was secondary to an impaired A. baumannii phenylacetic acid catabolism pathway, which led to accumulation of phenylacetate. Purified phenylacetate was confirmed to be a neutrophil chemoattractant. These data identify a previously unknown mechanism of bacterial-guided neutrophil chemotaxis in vivo, providing insight into the role of bacterial metabolism in host innate immune evasion. Furthermore, the work provides a potentially new therapeutic paradigm of targeting a bacterial metabolic pathway to augment host innate immune responses and attenuate disease.

Light Modulates Metabolic Pathways and Other Novel Physiological Traits in the Human Pathogen Acinetobacter baumannii

“…Most interestingly, we show that light directly modulates metabolic pathways, such as the PAA catabolic pathway, which is repressed by blue light in strain ATCC 19606 in a BlsA-dependent manner. The paa genes encode proteins responsible for the metabolism of aromatic compounds and have been linked to virulence in A. baumannii (11,25) and other human pathogens (26). It would therefore be expected that repression of the PAA pathway by light would 7).…”

Clinical and Pathophysiological Overview of Acinetobacter Infections: a Century of Challenges

“…Again, caution may be warranted in that the A. baumannii strain background studied was ATCC 17978, a lab-adapted strain which was highly virulent in zebrafish but is essentially avirulent in healthy mice with a normal immune system, suggesting differences between the zebrafish and murine models. The authors also found that bacterial phenylalanine production was critical to triggering neutrophil chemoattraction to the site of infection (81). The phenylalanine catabolic pathway degrades phenylalanine, and a key step in that pathway is the paaA gene.…”

“…In at least one study, however, virulence outcomes in C. elegans did not correlate with outcomes in mice, so caution is warranted in interpreting translatability of results in this model (80). Finally, a recent study described a model of Acinetobacter virulence in zebrafish larvae (81). In this model, mutant strains that had been shown to have attenuated virulence in mice also had attenuated virulence, and host defenses depended on neutrophil and macrophage uptake, suggesting commonalities between this invertebrate model and mice.…”

Microstructured Devices for Optimized Microinjection and Imaging of Zebrafish Larvae

“…The otic vesicle (Figure 4D, white dashed outline) provides a useful, immune-privileged and isolated site for testing neutrophil recruitment in zebrafish 9 . To test the ability of zebrafish neutrophils to respond to standard chemoattractants, 100 nM of fMLP and LTB4 were microinjected into the otic vesicles of 2 dpf zebrafish embryos (Figure 4F, J), using the microstructured channel device (Figure 1G) to stabilize embryos in the lateral orientation.…”