A study of the growth kinetics of two pseudomonads from pasteurized milk and the possible role of quorum sensing

Two strains of Pseudomonas fluorescens viz. LC56 and HC69 isolated from pas teurized milk which gave high and low counts, respectively, when subjected to the pre-incubated count (92/46 EEC). The cultures were subjected to a co ld stress (-18 degrees C for 20 h) before the thawed cultures were decimall y diluted to give cell concentrations ranging from approximately 10(6) to 1 0(0) cfu/ml and their conductivity measured as an index of growth at 25 deg rees C. Under these conditions isolate LC56 exhibited a significantly (p <0 .05) longer lag phase duration (LPD) but a significantly (p <0.05) faster e xponential growth rate (EGR). An inverse relationship was observed between inoculum level and LPD for both isolates. However isolate LC56 alone exhibi ted a significantly (p<0.05) longer LPD at the lowest inoculum level compar ed to higher cell concentrations. The addition of L-homoserine lactone and alpha-amino-gamma-butyrolactone, 2 agents involved in the bacterial quorum sensing phenomenon, was found to cause a significant (p < 0.001) reduction in both LPD and EGR for both isolates. This latter observation provides cir cumstantial evidence that strains of Ps. fluorescens are susceptible to thi s phenomenon.