Acinetobacter baumannii is an opportunistic nosocomial pathogen with high morbidity and mortality rates. Current treatment options for this pathogen are limited due to its increasing resistance to last-resort antibiotics. Despite A . baumannii’s leading position in the World Health Organisations priority pathogens list, little is known about its virulence regulation. Through a high-throughput screening approach to identify novel biofilm regulators, we identified a previously uncharacterised predicted adenylate cyclase (AC), CavA, as a central regulator of this phenotype. cAMP is a crucial mediator of various aspects of bacterial physiology in other species but information about its role in A . baumannii is limited. We confirm that CavA AC is functional and synthesizes cAMP in A . baumannii . Using dRNA-seq, we verify that CavA is a negative biofilm formation regulator affecting Csu pili and exopolysaccharide production. We demonstrate for the first time that in A . baumannii , cAMP is atop of a hierarchical signalling cascade controlling inter- and intrabacterial signalling by modulating quorum sensing and cyclic di-GMP systems, ultimately governing virulence in vivo and adaptive antibiotic resistance. In contrast to the well-established paradigm in other bacteria where cAMP and cyclic di-GMP levels are inversely regulated, we uncover that the levels of these second messengers are directly proportional in A . baumannii . Overall, this study uncovers the central role of CavA and cAMP in the pathogenic success of A . baumannii and highlights this signalling cascade as a high potential target for novel therapeutic development.

Cyclic AMP is a global virulence regulator governing inter and intrabacterial signalling in Acinetobacter baumannii