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To investigate the effect of cinnamaldehyde (CA) on transcription from selected quorum sensing (QS) promoters.The action of CA on QS was assayed using three E. coli green fluorescent protein (GFP) based bioreporters (two inducible and the other constitutive) and two Vibrio harveyi bioluminescent reporter strains. LuxR-mediated transcription from the PluxI promoter, which is induced by 3-oxo-C6-homoserine lactone (HSL), was reduced by 70 per cent following exposure to 200 μmol l−1 CA (26 ppm). The bioluminescence of Vibrio harveyi BB886, which is mediated by 3-hydroxy-C4-HSL, was reduced by 55 per cent after exposure to 60 μmol l−1 CA (8 ppm), and 100 μmol l−1 CA (13 ppm) inhibited the bioluminescence of the autoinducer-2 (AI-2) responsive reporter strain V. harveyi BB170 by nearly 60 per cent. CA did not inhibit the growth of the bioreporter strains at these concentrations. CA had a minimal effect on LasR promoter activity, induced by 3-oxo-C12-HSL.Low concentrations of CA were effective at inhibiting two types of acyl homoserine lactone mediated QS, and also autoinducer-2 mediated QS.Because CA is widely used in the food and flavour industries, its potential to affect bacterial QS regulated processes should be recognized.