STUDIES ON ENVIRONMENTAL RELEVANCE OF QUORUM SENSING SIGNAL DECAY

Signal degradation impacts all communications. Although acyl-homoserine lactone (acyl-HSL) quorum sensing signals are known to be degraded by defined laboratory cultures, little is known about their stability in nature. Here, we show for the first time that acyl-HSLs are biodegraded in soils sampled from diverse US sites. When amended to soil samples at physiologically relevant concentrations, 14C-labeled acyl-HSLs were mineralized to 14CO2 rapidly and, at most sites examined, without lag. A lag-free turf soil activity was characterized in further detail. Heating or irradiation of the soil prior to the addition of radiolabel abolished mineralization, whereas protein synthesis inhibitors did not. Mineralization exhibited an apparent Km of 1.5 µM acyl-HSL, ca. 1000-fold lower than that reported for a purified acyl-HSL lactonase. Under optimal conditions, acyl-HSL degradation proceeded at a rate of 13.4 nmol . h-1 . g of fresh weight soil-1. An MPN of 4.6 × 105 cells . g of turf soil-1 degraded physiologically relevant amounts of hexanoyl-[1-14C]HSL to 14CO2. The results implicate a real-world challenge for acyl-HSL-producing bacteria to outpace biological acyl-HSL degradation and thus to successfully engage in cell-cell communications in soils and other environments. Furthermore, high-affinity acyl-HSL degraders were enriched in oligotrophic biofilm reactors that were inoculated with the turf soil. When supplied at physiological concentration, acyl-HSL was observed to be >95% consumed through the reactors. Six bacterial strains were isolated belonging to Variovorax, Mesorhizobium, Bradyrhizobium and Labrys genera. Unambiguous growth of the six isolates on physiological amount of acyl-HSLs under oligotrophic condition was demonstrated. A Variovorax strain SOD31 exhibited acyl-HSL-limiting growth kinetics with a half-saturation constant of 1.7 µM, which is in high agreement with what we observed from the turf soil. The results suggest that strain SOD31 may be an active contributor to the soil’s activity of degrading acyl-HSLs. This research reinforces the importance of signal decay as intrinsic to bacterial cell-cell communication, as well as providing bases for further studies towards how signal decay may influence community function and structure in naturally occurring microbial communities.