Gene expression in Pseudomonas aeruginosa swarming motility

Abstract

Background: The bacterium Pseudomonas aeruginosa is capable of three types of motilities: swimming, twitching and swarming. The latter is characterized by a fast and coordinated group movement over a semi-solid surface resulting from intercellular interactions and morphological differentiation. A striking feature of swarming motility is the complex fractal-like patterns displayed by migrating bacteria while they move away from their inoculation point. This type of group behaviour is still poorly understood and its characterization provides important information on bacterial structured communities such as biofilms. Using GeneChip® Affymetrix microarrays, we obtained the transcriptomic profiles of both bacterial populations located at the tip of migrating tendrils and swarm center of swarming colonies and compared these profiles to that of a bacterial control population grown on the same media but solidified to not allow swarming motility.

Results: Microarray raw data were corrected for background noise with the RMA algorithm and quantile normalized. Differentially expressed genes between the three conditions were selected using a threshold of 1.5 log2-fold, which gave a total of 378 selected genes (6.3% of the predicted open reading frames of strain PA14). Major shifts in gene expression patterns are observed in each growth conditions, highlighting the presence of distinct bacterial subpopulations within a swarming colony (tendril tips vs. swarm center). Unexpectedly, microarrays expression data reveal that a minority of genes are up-regulated in tendril tip populations. Among them, we found energy metabolism, ribosomal protein and transport of small molecules related genes. On the other hand, many well-known virulence factors genes were globally repressed in tendril tip cells. Swarm center cells are distinct and appear to be under oxidative and copper stress responses.

Conclusions: Results reported in this study show that, as opposed to swarm center cells, tendril tip populations of a swarming colony displays general down-regulation of genes associated with virulence and up-regulation of genes involved in energy metabolism. These results allow us to propose a model where tendril tip cells function as «scouts» whose main purpose is to rapidly spread on uncolonized surfaces while swarm center population are in a state allowing a permanent settlement of the colonized area (biofilm-like).

Figure 1

Global gene expression pattern with a change in expression level greater than 1.5 log₂-fold in the three tested conditions. Overrepresentation analysis in functional class percentage for each PseudoCAP function classes according to up- and down-regulated genes showing the differential regulation of all gene classes in tendril tip vs. non-swarming, swarm center vs. non-swarming and tendril tip vs. swarm center.

Figure 2

Microarray results validation by qRT-PCR. Mean log₂ ratios of the qRT-PCR experiments are plotted against the mean log₂ ratios of the microarray experiments. Numbers on the graph refer to genes listed in Table 5.

Figure 3

Proposed model of the transcriptional dynamics displayed in tendril tips and swarm center of a P. aeruginosa swarming colony. Tendril tip cells display an up-regulation of transcripts associated with energy production (ATP synthesis and cytochromes) and ribosomal proteins. At the same time, these cells down-regulate transcription associated with secreted factors (aka virulence factors) and iron acquisition. In contrast, swarm center cells live in a state in which oxidative and copper stress response transcripts are up-regulated.