Calcium signals natural transformation in Acinetobacter baumannii

The acquisition of resistance to antibiotics in the opportunistic pathogen Acinetobacter baumannii may be linked to its capacity to undergo natural transformation. This mode of horizontal gene transfer relies on the import of extracellular DNA and its chromosomal integration by homologous recombination. Type IV pilus activity initiates the capture of extracellular DNA, which is then transported to the cytoplasm for recombination in the chromosome. While most Acinetobacter baumannii strains are transformable, the conditions allowing the expression of type IV pilus and other transformation genes remain largely unexplored. By investigating transformation-permissive conditions, we uncovered that calcium is a potent inducer of natural transformation. Type IV pilus genes and other transformation-specific genes (comEA, dprA) are upregulated by submillimolar concentrations of calcium ions, in a growth phase-dependent manner. In contrast, sodium chloride represses expression of pilA, counteracting the calcium-dependent induction, explaining the reported absence of transformation in NaCl-containing medium (such as LB). Independently of transcriptional induction, calcium ions also directly bind the type IV pilus through the calcium-dependent adhesin PilY1. Our data support a model in which calcium strengthens the interaction of PilY1 with the minor pilin complex, increasing pilus dynamics and subsequent pilus-dependent DNA capture. Hence, calcium signals natural transformation through both transcriptional and structural activation of type IV pilus. In addition to providing new insights into the regulation of natural transformation in A. baumannii this work led us to establish a protocol for genetic engineering of A. baumannii by natural transformation. ImportanceAcinetobacter baumannii is a nosocomial pathogen considered a critical research priority due to its resistance to last resort antibiotics. Understanding how A. baumannii evolves and acquires resistance to antibiotics is thus of prime importance. This species is capable of natural transformation, a means to acquire and spread genetic information, including antibiotic resistance genes. However, the conditions under which this process is active in this species remain elusive. We identify calcium ions as potent inducers of natural transformation and propose a model of the signaling of natural transformation by calcium ions. This opens the way for further investigations into the contribution of natural transformation to acquisition of antibiotic resistance. In addition, it provides an efficient way to genetically manipulate most A. baumannii strains.