Effect of quorum sensing agents on the growth kinetics of Pseudomonas spp. of raw milk origin.

Psychrotrophs, particularly strains of Pseudomonas fluorescens, dominate the microflora of refrigerated raw milk and secrete heat-stable extracellular enzymes (proteases and lipases) which survive pasteurisation and even UHT heat treatments and degrade the casein and fat components of raw milk causing a reduction in cheese yield, gelation of UHT milk and off flavours in many dairy products. These enzymes are usually produced in the late log/early stationary growth phases when the cell density is high. This fact indicated that induction of these enzymes may be a candidate for quorum sensing, a phenomenon by which bacteria can sense and respond to cell population size by means of chemical signals based on the homoserine lactone molecule. It is assumed that the production of these enzymes contributes to the selective advantage of psychrotrophs and hence have a significant effect on their growth kinetics. In the work reported here nine proprietary homoserine lactone compounds were screened, using water as a control, for their effect on the lag phase duration (LPD) and exponential growth rate (EGR) of three strains of Pseudomonas fluorescens isolated from refrigerated raw milk after 1, 3 and 5 days storage. Two compounds viz. N-benzoyloxycarbonyl-L-homoserine lactone and N-3-oxyhexanoyl-DL-homoserine lactone were found to significantly (P < 0.001) reduce the LPD and increase the EGR of the three strains. Further work with these compounds is warranted, monitoring growth in parallel with enzyme production, to determine the extent to which extracellular enzyme production is mediated by the quorum sensing phenomenon in this species.