Quorum sensing network in clinical strains of A. baumannii: AidA is a new quorum quenching enzyme

Acinetobacter baumannii is an important pathogen that causes nosocomial infections generally associated with high mortality and morbidity in Intensive Care Units (ICUs). Currently, little is known about the Quorum Sensing (QS)/Quorum Quenching (QQ) systems of this pathogen. We analyzed these mechanisms in seven clinical isolates of A. baumannii. Micro array analysis of one of these clinical isolates, Ab1 (A. baumanniiST-2_clon_2010), previously cultured in the presence of 3-oxo-C12-HSL (a QS signalling molecule) revealed a putative QQ enzyme (a/beta hydrolase gene, AidA). This QQ enzyme was present in all non motile clinical isolates (67% of which were isolated from the respiratory tract) cultured in nutrient depleted LB medium. Interestingly, this gene was not located in the genome of the only motile clinical strain growing in this medium (A. baumannii strain Ab421_GEIH-2010 [Ab7], isolated from a blood sample). The AidA protein expressed in E. coli showed QQ activity. Finally, we observed downregulation of the AidA protein (QQ system attenuation) in the presence of H2O2 (ROS stress). In conclusion, most of the A. baumanni clinical strains were not surface motile (84%) and were of respiratory origin (67%). Only the pilT gene was involved in surface motility and related to the QS system. Finally, a new QQ enzyme (a/beta hydrolase gene, AidA protein) was detected in these strains.