Diverse Gram-negative bacteria communicate with each other by using diffusible 
N-acyl-homoserine lactone (AHL) signaling molecules to coordinate gene 
expression with cell population density. This mechanism termed 'quorum sensing' 
is involved in the regulation of physiological functions as well as multiple 
virulence determinants. It becomes more and more evident, that bacteria 
communicate not only with each other but also with their host. Up to now, little 
is known about this interkingdom communication. The AHL quorum sensing molecule 
N-3-(oxododecanoyl)-L-homoserine lactone (OdDHL) from Pseudomonas aeruginosa has 
been shown to influence the immune system of the host. The role and potential 
influence of other AHL molecules from other bacteria have so far not been 
determined. In this paper, we investigated the role of 7 different AHLs on 
apoptosis of human Jurkat T lymphocytes. We found, that among all homoserine 
lactones tested, only OdDHL rapidly induced apoptosis which was accompanied by 
the breakdown of the mitochondrial transmembrane potential (DeltaPsi(m)). Since 
overexpression of anti-apoptotic Bcl-2 completely abrogated the apoptotic 
effect, we presume that OdDHL induces apoptosis by activation of the intrinsic 
mitochondrial apoptosis pathway. The reason that bacteria induce apoptosis is 
largely unknown. We suspect that through apoptosis an anti-inflammatory response 
is triggered.