Cell–cell signalling in bacteria: not simply a matter of quorum

Abstract

Bacterial signalling known as quorum sensing (QS) relies on the synthesis of autoinducing signals throughout growth; when a threshold concentration is reached, these signals interact with a transcriptional regulator, allowing the expression of specific genes at a high cell density. One of the most studied intraspecies signalling is based on the use of N-acyl-homoserine lactones (AHL). Many factors other than cell density were shown to affect AHL accumulation and interfere with the QS signalling process. At the cellular level, the genetic determinants of QS are integrated in a complex regulatory network, including QS cascades and various transcriptional and post-transcriptional regulators that affect the synthesis of the AHL signal. In complex environments where bacteria exist, AHL do not accumulate at a constant rate; the diffusion and perception of the AHL signal outside bacterial cells can be compromised by abiotic environmental factors, by members of the bacterial community such as AHL-degrading bacteria and also by compounds produced by eukaryotes acting as an AHL mimic or inhibitor. This review aims to present all factors interfering with the AHL-mediated signalling process, at the levels of signal production, diffusion and perception.

Rhizobial homologs of the fatty acid transporter FadL facilitate perception of long-chain acyl-homoserine lactone signals

“…This concept probably is satisfactory for the majority of bacterial species studied so far that produce and perceive AHLs with acyl chains of up to 12 carbons; however, longer acyl chains are presumed to impede diffusion of AHLs (reviewed in ref. 28), suggesting an additional route of uptake. This is relevant to several α-proteobacteria, including S. meliloti, which communicate via long-chain AHLs with acyl chains of 14 or more carbons (8,(10)(11)(12).…”

The role of quorum sensing signalling in EPS production and the assembly of a sludge community into aerobic granules

“…Although such studies have focused on populations of bacteria in the laboratory, bacteria in natural and man-made environments mainly exist as phylogenetically highly diverse and complex communities (Kolter and Greenberg, 2006). These communities are unlike those of population derived assemblages, with community members differentially experiencing fluctuations in a wide range of parameters, including pH, temperature, nutrient concentration, oxygen level and diffusion/dilution of signals (Boyer and Wisniewski-Dyé, 2009;Decho et al, 2009;Yeon et al, 2009). Despite these challenges, it is now increasingly clear that QS occurs in and appears to be highly relevant for complex communities, whether they are biofilms on rocks in streams (McLean et al, 1997), stromatolites (Decho et al, 2009) or biofilms formed on hollow fibre membranes in bioreactors (Yeon et al, 2009).…”

Metabolic cross-feeding via intercellular nanotubes among bacteria

“…However, such a diffusion-based transfer of molecules between two cells bears a number of risks: unintended third parties may eavesdrop on the signal or consume the nutrient. Moreover, the exchanged molecule may degrade or be lost by a too fast diffusion 11 .…”