Presence of N-acyl homoserine lactones in soil detected by a whole-cell biosensor and flow cytometry

Quorum sensing enables bacteria to regulate expression of certain genes according to population density. N-acyl homoserine lactone (AHL)-based quorum sensing is known to be widespread among gram-negative bacteria. Several bacterial whole-cell biosensors for AHL detection have been developed and some were used in in situ studies of AHL production. From these studies our knowledge of the significance of quorum sensing in various environments has been improved. However, very little is known about production of AHLs in soil environments. In the present study, an approach for detecting AHL production in bulk soil was developed. A whole-cell biosensor based on the regulatory region of the lux-operon from Vibrio fischeri fused to & was constructed, resulting in a luxR-P-luxI-gfpmut3*-fusion in the high copy plasmid, pAHL-GFP. Escherichia coli MC4100 harboring pAHL-GFP responded to the AHL-compound N-octanoyl homoserine lactone (OHL) by expressing green fluorescence. In situ application of E. coli MC4100/pAHL-GFP was tested by adding OHL in different concentrations to sterile soil microcosms. E. coli MC4100/pAHL-GFP were incubated in the soil microcosms and extracted by an improved Nycodenz-extraction method optimized for flow cytometry. The presence of induced cells was then verified by single-cell analysis by flow cytometry. OHL concentrations between 0.5 and 50 nmol per g soil were detected. When introducing the AHL-producing Serratia liquefaciens to soil microcosms, expression of green fluorescent protein was induced in E. coli MC4100/pAHL-GFP. Thereby, the ability of this strain to detect excretion of AHLs by S. liquefaciens in sterile soil was shown. The use of an improved extraction method and a whole-cell biosensor combined with flow cytometry analysis proved to be promising tools in future studies of AHL production by microbial populations in soil environments.