Resistance of Acinetobacter baumannii to bacteriophage and complement-mediated killing

Infections caused by multidrug-resistant Acinetobacter baumannii are an emerging global health threat. Although phages have shown promising results in treating bacterial infections, the mechanisms of the combined effect of phages and innate immunity on clearing A. baumannii remain unclear. Here, we report a synergistic effect of the complement system and phages on clearing multidrug-resistant A. baumannii. We show that A. baumannii rapidly adapts and becomes resistant to phage or serum complement by modifying the expression of capsule and lipooligosaccharides, which can be regulated through reversible transposon mutagenesis in the K locus. Compared to the encapsulated phenotype, the non-encapsulated, phage-resistant A. baumannii showed a higher level of membrane attack complex deposition and were susceptible to killing by complement. In contrast, the encapsulated phenotype escaped the complement system by shedding the membrane attack complex to the environment. Thus, while the complement system targets the non-encapsulated phenotype, the phage infects and eliminates the encapsulated subpopulation. The occurrence of phage-resistant A. baumannii has been observed in patients who have received phage therapy. However, the recognition of these bacteria by the innate immune system and their impact on innate immune cells are not well understood. Herein, in vitro study reveals that the phage-selected, non-encapsulated phenotype is more susceptible to opsonin-independent phagocytosis, whereas the encapsulated bacteria can be phagocytosed more rapidly by macrophages in the presence of serum. Importantly, human macrophages stimulated with non-encapsulated bacteria exhibit lower levels of proinflammatory cytokines and chemokine secretion. These results indicate that although A. baummanii becomes resistant to phages, this bacterial phenotype shows decreased virulence, rapid clearance by macrophages and lower potential to induce a cytokine storm, which can be fatal in patients. Hence, our observations suggest means of combatting antibiotic-resistant A. baumannii by a simultaneous treatment with phages and complement, a combination which can be supplemented further with antibacterial antibodies.