From heterogeneity to homogeneity: coordination of siderophore gene expression among clonal cells of the bacteriumPseudomonas aeruginosa

Abstract

There has been great progress in understanding how bacterial groups coordinate social actions, such as biofilm formation, swarming and public-goods secretion. Less clear, however, is whether the seemingly coordinated responses observed at the group level actually mirror what individual cells do. Here, we use a microscopy approach to simultaneously quantify the investment of individual cells of the bacterium Pseudomonas aeruginosa into two public goods, the siderophores pyochelin and pyoverdine. Using gene expression as a proxy for investment, we initially observed no coordination but high heterogeneity and bimodality in siderophore gene expression across cells. With increasing cell density, gene expression became homogenized across cells, accompanied by a shift from pyochelin to pyoverdine expression. We found positive correlations in the expression of pyochelin and pyoverdine genes across cells, and show that cell-to-cell variation is driven by differences in cellular metabolic states. We propose a model explaining how variation in internal iron stocks can spur initial erratic gene expression, while siderophore-mediated signalling and intra-cellular feedbacks later on can induce highly coordinated gene expression and synchronized shifts from pyochelin to pyoverdine. Our work provides new insights into bacterial collective decision-making processes and reveals a three-phase chronobiological siderophore investment cycle in P. aeruginosa.

Enforced specialization fosters mutual cheating and not division of labour in the bacterium Pseudomonas aeruginosa

“…Instead, our results suggest that being a generalist is costly under iron-replete conditions. This interpretation is supported by recent findings at the single cell level showing that the wildtype expresses both pyochelin and pyoverdine synthesis genes at background levels even in iron-rich environments (Mridha & Kümmerli, 2021). Consequently, specialist communities are able to minimize these residual metabolic costs, while the generalist cannot.…”

“…Both pyoverdine and pyochelin can be public goods and shared between cells (Ross-Gillespie et al ., 2015) and although they are simultaneously produced, they take on slightly different functions (Dumas et al ., 2013). Due to its relatively low iron affinity, pyochelin is primarily beneficial and increasingly produced under moderate iron limitation, while pyoverdine has a very high iron affinity and is more relevant and produced in high amounts under severe iron limitation (Dumas et al ., 2013; Ross-Gillespie et al ., 2015; Mridha & Kümmerli, 2021).…”

Enforced specialization fosters mutual cheating and not division of labour in the bacterium Pseudomonas aeruginosa

“…Instead, our results suggest that being a generalist is costly under iron‐replete conditions. This interpretation is supported by recent findings at the single cell level showing that the wildtype expresses both pyochelin and pyoverdine synthesis genes at background levels even in iron‐rich environments (Mridha & Kümmerli, 2021). Consequently, specialist communities are able to minimize these residual metabolic costs, whilst the generalist cannot.…”

“…Both pyoverdine and pyochelin can be public goods and shared between cells (Ross-Gillespie et al, 2015) and although they are simultaneously produced, they take on slightly different functions (Dumas et al, 2013). Due to its relatively low iron affinity, pyochelin is primarily beneficial and increasingly produced under moderate iron limitation, whilst pyoverdine has a very high iron affinity and is more relevant and produced in high amounts under severe iron limitation (Dumas et al, 2013;Mridha & Kümmerli, 2021;Ross-Gillespie et al, 2015). Using this experimental system, we tested whether enforced specialization alleviates the metabolic burden associated with the simultaneous production of both siderophores (intrinsic factor), whilst the benefits of the two molecules can still accrue to all individuals through molecule sharing at the population level.…”

Pseudomonas aeruginosareaches collective decisions via transient segregation of quorum sensing activities across cells

“…While the growth conditions in our experiment were rather uniform (shaken cultures), there is still potential for stochasticity. For example, there might be stochastic inter-cell differences in nutrient uptake, which can spur variation in metabolic states across cells and induce heterogeneity in overall gene expression activity 44,45 . Moreover, the very nature of QS systems operating via diffusible signals can promote extrinsic noise.…”