Acyl-homoserine lactone quorum sensing in gram-negative bacteria: a signaling mechanism involved in associations with higher organisms

Abstract

Recent advances in studies of bacterial gene expression have brought the realization that cell-to-cell communication and community behavior are critical for successful interactions with higher organisms. Species-specific cell-to-cell communication is involved in successful pathogenic or symbiotic interactions of a variety of bacteria with plant and animal hosts. One type of cell-cell signaling is acyl-homoserine lactone quorum sensing in Gram-negative bacteria. This type of quorum sensing represents a dedicated communication system that enables a given species to sense when it has reached a critical population density in a host, and to respond by activating expression of genes necessary for continued success in the host. Acyl-homoserine lactone signaling in the opportunistic animal and plant pathogen Pseudomonas aeruginosa is a model for the relationships among quorum sensing, pathogenesis, and community behavior. In the P. aeruginosa model, quorum sensing is required for normal biofilm maturation and for virulence. There are multiple quorum-sensing circuits that control the expression of dozens of specific genes that represent potential virulence loci.

Figure 1

Generalized scheme for an acyl-HSL quorum-sensing circuit in a bacterial cell. The orange square indicates an acyl-HSL synthase-LuxI homolog. The diamonds are a LuxR homolog. Yellow diamonds on the bacterial chromosome are the LuxR homolog activated by the acyl-HSL signal. The arrows on the chromosome are qsc genes. The acyl-HSL (AHSL) signal can diffuse in and out of cells. The compound in the box is an acyl-HSL (R₁, H, OH, or O; R₂, (CH₂)_2–14, or CH₂-CH₂-CH⩵CH-CH₂-CH₂). The substrates for the acyl-HSL synthase are an acylated acyl carrier protein (Acyl-ACP) and S-adenosylmethionine (SAM).

Figure 2

Map of qsc genes or operons on the P. aeruginosa chromosome. Arrowheads indicate the direction of transcription. The different colors indicate different regulatory classes. Black, genes that respond primarily to 3OC12-HSL and that can respond to added signal in early logarithmic phase. Red, genes that respond primarily to 3OC12-HSL, but only after cultures have entered stationary phase (late genes). Blue, genes that respond best to both 3OC12-HSL and C4-HSL, and that can respond to added signal in early logarithmic phase. Green, late genes that require both signals for full expression. The lasR, lasI, and rhlR genes are shown in gold. The locations of lux-box-like elements are shown as black dots between the two DNA strands. These elements were identified in putative promoter regions of some but not all of the qsc genes. The numbers are distance in megabases (Mb) on the approximately 6.3-Mb chromosome (numbering starts at oriC). This figure is from ref. .

Figure 3

Diagram of the P. aeruginosa biofilm-maturation pathway. Unattached cells that approach a surface may attach. Attachment involves specific functions. Attached cells will proliferate on a surface and use specific functions to actively move into microcolonies. The high-density microcolonies differentiate into mature biofilms by a 3OC12-HSL-dependent mechanism.

Figure 4

Scanning confocal microscope images of a mature P. aeruginosa wild-type biofilm (Upper) and a quorum-sensing mutant biofilm (Lower). In this case the quorum-sensing mutant was a lasR, rhlR double mutant. The perspective is from above the biofilm on a glass surface. The glass surface is red, and the green is from the green fluorescent protein encoded by the gfp gene in the recombinant P. aeruginosa. The wild-type biofilm consists of thick microcolonies. The immature mutant biofilm appears thinner, and more of the glass surface is exposed. With the lasR, rhlR mutant shown here (but not with lasI, rhlI mutants) zones of clearing around microcolony towers are often observed. Other experiments have shown that these zones are filled with extracellular polysaccharide (M.R.P., unpublished data). The biofilms are in flow-through reaction vessels similar to those described in ref. . The colors were applied to the image by computer enhancement with Adobe photoshop 5.0. The black marker bar is 100 μm in length.