Acinetobacter baumannii escape from neutrophil extracellular traps (NETs)

Acinetobacter baumannii and Pseudomonas aeruginosa are the same aerobic gram-negative bacillus and are usually harmless but cause infectious diseases in compromised hosts. Neutrophils play a critical role in infective protection against the extracellular growth of bacteria. Recently, a new biological defense mechanism called neutrophil extracellular traps (NETs) has been attracting attention. In present study, we investigated the responsiveness of neutrophils to A. baumannii and P. aeruginosa, focusing on NET formation. Neutrophils were co-cultured with A. baumannii or P. aeruginosa, and then DNA, histone and neutrophil elastase were stained, and the formation of NETs was evaluated. Neutrophils stimulated with A. baumannii had spread, but their shapes was maintained, and the nucleus was observed as clearly as that in non-stimulated neutrophils. However, neutrophils stimulated with P. aeruginosa did not maintain their cellular morphology, and the nucleus was disrupted with DNA, histones, and neutrophil elastase released into the extracellular space. These results suggest that A. baumannii does not induce NET formation, in contrast to P. aeruginosa. In addition, we measured expression of myeloperoxidase (MPO), reactive oxygen species (ROS) and superoxide in neutrophils, and we found that these expression in P. aeruginosa-stimulated neutrophils was stronger than that in A. baumannii-stimulated neutrophils. Furthermore, A. baumannii was not killed by neutrophils, in contrast to P. aeruginosa. In this study, we show that the reactivity of neutrophils and their biological defense mechanism are different between A. baumannii and P. aeruginosa, which is important for understanding the pathogenicity of these bacteria.