Simultaneous functional disruption of the iron acquisition system and type VI secretion system results in complete suppression of virulence in Acinetobacter baumannii

Acinetobacter baumannii (Ab) is one of the most significant bacterial pathogens inducing hospital-acquired infections worldwide, with a high mortality rate. The continuous emergence of multidrug-resistant (MDR) phenotypes presents a significant challenge in combating Ab infections with antimicrobial drugs. In this study, we found that the type VI secretion system and the iron transportation system synergistically enhance siderophore production and further contribute to the virulence of Ab. The double knockout mutant strain, ΔhcpΔbasE, exhibited further reductions in growth rate, siderophore production under iron-deficient conditions, biofilm formation, serum resistance, cell adhesion and invasion, and cytotoxicity compared to the single knockout strains, knockout of T6SS, Δhcp or iron transportation system, ΔbasE. In vitro experiments demonstrated that these two systems work synergistically to enhance virulence, with their combined effect exceeding the additive contributions of each individual system. Consistently, the ΔhcpΔbasE strain failed to cause mortality in the mouse model, even at very high inoculum levels. Further studies revealed that, compared to ATCC17978, ΔhcpΔbasE strain infection resulted in lower levels of extracellular hepcidin and intracellular iron in host cells, which correlate well with the significantly reduced ability to produce siderophores in the double knockout strain. Due to impaired iron acquisition, ΔhcpΔbasE strain became more susceptible to macrophage phagocytosis and exhibited lower survival rates in the host, leading to an inability to trigger a cytokine storm and subsequent host death. The findings of this study provide insights into the Ab pathogenesis and contribute to the development of intervention measures to control clinical Ab infections and mortality.