Density-dependent fitness benefits in quorum-sensing bacterial populations

Abstract

It has been argued that bacteria communicate using small diffusible signal molecules to coordinate, among other things, the production of factors that are secreted outside of the cells in a process known as quorum sensing (QS). The underlying assumption made to explain QS is that the secretion of these extracellular factors is more beneficial at higher cell densities. However, this fundamental assumption has never been tested experimentally. Here, we directly test this by independently manipulating population density and the induction and response to the QS signal, using the opportunistic pathogen Pseudomonas aeruginosa as a model organism. We found that the benefit of QS was relatively greater at higher population densities, and that this was because of more efficient use of QS-dependent extracellular "public goods." In contrast, the benefit of producing "private goods," which are retained within the cell, does not vary with cell density. Overall, these results support the idea that QS is used to coordinate the switching on of social behaviors at high densities when such behaviors are more efficient and will provide the greatest benefit.

Fig. 1.

The hypothesized function of QS. At low cell densities, a large proportion of the extracellular factors (public goods or exoenzymes) disperse before they can be used (by either the cell that produced it or others), and so their production provides little direct or indirect fitness benefit. At high cell densities, a much greater proportion of the extracellular public goods (or the products they produce) can be used by the cell that produced the extracellular factors and their neighbors. Consequently, from the inclusive fitness perspective of an individual cell (33), the production of extracellular public goods is more efficient and beneficial at higher population densities.

Fig. 2.

Manipulating cell density with CAA. (A) The PA01 lasI::Gm^R (signal-negative) mutant grew to a higher cell density when provided with more CAA, both in the presence (■) and absence (□) of the signal 3O-C12-HSL (20 μM). This process demonstrates that cell density and the size of the bacterial population can be tightly controlled in this media. (B) The lasB expression per cell (in relative light units) of the PA01 lasI::Gm^R (signal-negative) mutant was increased by the addition of the signal 3O-C12-HSL (20 μM). All results are shown as means (± SD), eight replicates per treatment.

Fig. 3.

The fitness benefit of responding to QS. The fitness benefit of adding signal (relative final growth), and therefore inducing QS, was greater when CAA was provided at a higher concentration, and hence at higher population densities (◆). Two control treatments showed that when the social aspect of QS was removed, this also removed the effect of population density. Specifically, through: (i) the addition of elastase, which digests the BSA carbon source directly, without the need for QS (○); and (ii) the replacement of the BSA carbon source with adenosine, which is digested intracellularly (■). All results are shown as means (± SD), eight replicates per treatment.

Fig. 4.

Response to signal and cell density. (A) lasB expression per cell is highly induced by 3O-C12-HSL at high density (0.125% CAA) but less so at low density (0.03125% CAA) cultures. A combination of 3O-C12-HSL and C4-HSL induces lasB to a high level at low cell density. (B) Increased fitness (growth) of the population occurs under conditions of high cell density but not low cell density, even with the addition of both signals. All results are shown as means (± SD), six replicates per treatment.