Intracellular Ca2+ mobilization and kinase activity during acylated homoserine lactone-dependent quorum sensing in Serratia liquefaciens

Quorum sensing in Gram-negative bacteria involves acylated homoserine lactones (AHLs) and a transcription factor, activated by the AHLs. In this study, a possible involvement of intracellular Ca2+ as second messenger and/or protein kinase activity during signal transduction is analyzed. When N-hexanoyl-L-homoserine lactone was added to a suspension of Fura-2-loaded Serratia liquefaciens, there was a decline in [Ca2+](i), measured as a decrease in the Fura-2 fluorescence ratio. As controls, the addition of the signal molecule N-3-oxohexanoyl-L-homoserine lactone, which is not produced by S. liquefaciens, did not induce changes in [Ca2+](i). Using a protein kinase activity assay on AHL- stimulated cells, an increase in kinase activity after N-butanoyl-L-homoserine lactone stimulation of S. liquefaciens cells was detected, whereas the kinase activity induced by N-3-oxohexanoyl-L-homoserine lactone was not statistically significant. The conclusion from this study is that changes in [Ca2+](i) are involved in quorum sensing signal transduction in the Gram-negative bacteria S. liquefaciens. We also conclude that kinase activity is induced in S. liquefaciens upon AHL stimulation. We suggest that the transient intracellular [Ca2+] changes and kinase activity, activated by the AHL signal, are critical for the quorum-sensing signal transduction.