Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): María López, Celia Mayer, Laura Fernández-García, Lucía Blasco, Andrea Muras, Federico Martín Ruiz, German Bou, Ana Otero, María Tomás, Feng Gao.
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. Microarray analysis of one of these clinical isolates, Ab1 (A. baumannii ST-2_clon_2010), previously cultured in the presence of 3-oxo-C12-HSL (a QS signalling molecule) revealed a putative QQ enzyme (α/ß 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. baumannii 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 (α/ß hydrolase gene, AidA protein) was detected in these strains.
Quorum Sensing (QS) is a general mechanism used by Gram-negative bacteria to regulate many biological processes, including virulence, competence, conjugation, resistance, motility and biofilm formation . The production and detection of bacterial cell-cell signalling molecules by various species have been linked to the enhanced development of single and multi-species biofilms . A variety of structurally different bacterial cell-cell signalling molecules have been shown to mediate cell-cell communication, including acyl homoserine lactones (AHLs) and autoinducer-2 molecules (AI-2). AHLs have been proposed to mediate intra-species bacterial communication; different species typically only recognize AHLs produced from closely related species . On the other hand, AI-2 has been shown to mediate inter-species signalling . The term AI-2 describes a family of inter-convertible molecules derived from the precursor molecule (4,5-dihydroxy-2,3-pentanedione, DPD) . This precursor molecule is produced or detected by many Gram-positive and Gram-negative bacteria .
The QS system enables bacterial populations to live and proliferate in an environment with effective intercellular communication . In clinical isolates of A. baumannii, little is known about the cascade of genes controlled by this system and associated with various mechanisms, including surface motility (phenotypic expression). In Acinetobacter baumannii ATCC 17978, the A1S-0112 to A1S_0118 operon has been associated with activation of the QS system, and the pilT gene has been related to motility . In this study, only pilT expression (surface motility) was controlled by QS in the LB broth used: normal LB and modified (nutrient depleted) LB-LN.