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 lacto nes (AHLs) and a transcription factor, activated by the AHLs. In this study , a possible involvement of intracellular Ca2+ as second messenger and/or p rotein kinase activity during signal transduction is analyzed. When N-hexan oyl-L-homoserine lactone was added to a suspension of Fura-2-loaded Serrati a 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 mol ecule N-3-oxohexanoyl-L-homoserine lactone, which is not produced by S. liq uefaciens, did not induce changes in [Ca2+](i). Using a protein kinase acti vity assay on AHL- stimulated cells, an increase in kinase activity after N -butanoyl-L-homoserine lactone stimulation of S. liquefaciens cells was det ected, whereas the kinase activity induced by N-3-oxohexanoyl-L-homoserine lactone was not statistically significant. The conclusion from this study i s that changes in [Ca2+](i) are involved in quorum sensing signal transduct ion in the Gram-negative bacteria S. liquefaciens. We also conclude that ki nase activity is induced in S. liquefaciens upon AHL stimulation. We sugges t that the transient intracellular [Ca2+] changes and kinase activity, acti vated by the AHL signal, are critical for the quorum-sensing signal transdu ction.