Implications of Rewiring Bacterial Quorum Sensing

Abstract

Bacteria employ quorum sensing, a form of cell-cell communication, to sense changes in population density and regulate gene expression accordingly. This work investigated the rewiring of one quorum-sensing module, the lux circuit from the marine bacterium Vibrio fischeri. Steady-state experiments demonstrate that rewiring the network architecture of this module can yield graded, threshold, and bistable gene expression as predicted by a mathematical model. The experiments also show that the native lux operon is most consistent with a threshold, as opposed to a bistable, response. Each of the rewired networks yielded functional population sensors at biologically relevant conditions, suggesting that this operon is particularly robust. These findings (i) permit prediction of the behaviors of quorum-sensing operons in bacterial pathogens and (ii) facilitate forward engineering of synthetic gene circuits.In bacteria, broadcasting of metabolite or small peptide signaling molecules enables sensing of changes in population density (18,37,57). This mechanism, known as quorum sensing, has been implicated in regulating the virulence factors in a number of bacterial pathogens, such as Vibrio cholerae (61), the bacterium responsible for the severe diarrheal disease cholera; Pseudomonas aeruginosa (30), the opportunistic pathogen responsible for death in cystic fibrosis patients and high mortality rates in immunocompromised individuals; and Staphylococcus aureus (32), a major culprit of infections in surgical wounds. Consequently, improved understanding of quorum-sensing regulation should provide more insight into combating these pathogens by using either traditional chemotherapy or emerging technologies such as quorum-sensing inhibitors (40) or regulated degradation of quorum-sensing signals (13,14).The lux module of the marine bacterium Vibrio fischeri, a facultative symbiont of luminescent fish or squid, serves as one model system for understanding quorum sensing. V. fischeri employs the lux module to regulate gene expression as a function of the population density. The key element of this system is a regulatory cassette consisting of the genes encoding LuxI and LuxR. LuxI is an acylhomoserine lactone (acyl-HSL) synthase; LuxR is a transcriptional regulator activated by the acyl-HSL. The acyl-HSL signaling molecule is produced inside the cell but can freely diffuse across the cell membrane into the environment. Therefore, the acyl-HSL concentration is low at low cell density. As the cell density increases, the signal accumulates in the environment and inside the cell. The signal can then bind LuxR to stabilize the transcription factor, which then activates gene expression. This process, in which activation of gene expression occurs only after attaining a critical cell density, is known as autoinduction.In this work, we sought to computationally and experimentally address the fundamental question of how bacteria regulate cell-cell communication. Quorum sensing is often described in terms of a discrete switch; at low cell den...

Mathematical Modelling of Bacterial Quorum Sensing: A Review

“…These results are supported by experimental studies showing the appearance of successive competence cycles in pneumococcal batch cultures. Haseltine and Arnold (2008) examined the lux circuit of V. fischeri and how this impacts QS operons in bacterial pathogens (e.g. bistability in the regulation of the plant pathogen Agrobacterium tumefaciens and the human pathogen P. aeruginosa).…”

Core Principles of Bacterial Autoinducer Systems

“…This property can support a synchronous all-ornone behavior of cells in a population. However, the existence of bistability was reported to depend on the stability of receptor-AI complexes and has only rarely been confirmed experimentally (16)(17)(18). For other AI systems, a graded response at the population level has been reported (19).…”

Quorum-sensing Salmonella selectively trigger protein expression within tumors

“…Cultures grown to higher density before dilution, however, had greater GFP expression with time ( Fig. 2B; P < 0.05) because of residual LuxI and LuxR molecules in the bacteria (30).…”

“…6C). Based on previously measured values of C crit (30,32) and D eff (12,(32)(33)(34), m max was 53,000 molecules·s −1 per bacterium. The predicted fraction of induced colonies was greater at high density and low radius (Fig.…”