Comparison of Virulence and Quorum Sensing Gene Expression between Planktonic and Biofilm Cells of Legionella pneumophila

Abstract Legionella pneumophila is a waterborne bacterium that has a life cycle consisting of a transmissive phase in which free-living cells spread in the external environment, and a replicative phase with cells multiplying within biofilms. The transition from one phase to another is regulated by the bacterium’s quorum sensing system. As this system also regulates virulence, laboratory cultures of L. pneumophila were used to compare the different expressions of virulence and quorum sensing genes in planktonic and biofilm cells using RT-qPCR assays. The results showed a down regulation of the icmK gene in biofilm cells but only insignificant differences in the expression of 10 other genes associated with filamentation, conjugation, adherence and motility. The two quorum sensing genes examined, lpg1057 and lpg2731 , differed in their expression profiles in planktonic and biofilm cells. While their expression in planktonic cells increased from day 10 until day 15 of incubation, their expression in biofilm cells steadily declined after 10 days of incubation. This decrease in quorum sensing gene expression was shown to be associated with a decrease in cell density in the biofilm, thus suggesting that the trigger for the transition from the replicative phase to the transmissive phase is likely to be nutrient depletion in the biofilm.