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Cell Rep. Author manuscript; available in PMC 2013 September 11.
Published in final edited form as:
PMCID: PMC3770465

Convergent evolution of hyperswarming leads to impaired biofilm formation in pathogenic bacteria


Most bacteria in nature live in surface-associated communities rather than planktonic populations. Nonetheless, how surface-associated environments shape bacterial evolutionary adaptation remains poorly understood. Here we show that subjecting Pseudomonas aeruginosa to repeated rounds of swarming, a collective form of surface migration, drives remarkable parallel evolution towards a hyperswarmer phenotype. In all independently evolved hyperswarmers, the reproducible hyperswarming phenotype is caused by parallel point mutations in a flagellar synthesis regulator, FleN, which locks the naturally mono-flagellated bacteria in a multi-flagellated state and confers a growth-rate independent advantage in swarming. Even though hyperswarmers outcompete the ancestral strain in swarming competitions, they are strongly outcompeted in biofilm formation, which is an essential trait for P. aeruginosa in environmental and clinical settings. The finding that evolution in swarming colonies reliably produces evolution of poor biofilm formers supports the existence of an evolutionary tradeoff between motility and biofilm formation.


In nature, bacteria are generally not found in free-swimming, planktonic states but rather living in dense, surface-associated communities called biofilms (Kolter and Greenberg, 2006). Still, much of our knowledge of bacteriology comes from studying bacterial populations in liquid culture, neglecting this very spatial structure (Mitri et al., 2011). When studying bacterial evolution, in particular, most experiments are done in well-mixed liquid cultures (e.g. Blount et al., 2008; Chou et al., 2011; Lenski et al., 1991; Perfeito et al., 2007; Tenaillon et al., 2012; Woods et al., 2006). Bacterial evolution is central to many natural and human activities, from the improvement of bioremediation (Smidt and de Vos, 2004) and the treatment of infectious disease (Ensminger et al., 2012; Lieberman et al., 2011; Taubes, 2008) to the evolution of life on Earth (David and Alm, 2011; Dietrich et al., 2006; Kasting and Siefert, 2002). The structure of natural surface-associated communities likely plays a major role in bacterial evolutionary adaptation (Hibbing et al., 2010; Nadell et al., 2010; Xavier et al., 2009).

Here we investigate evolution in swarming colonies of Pseudomonas aeruginosa. Swarming is a collective form of motility over soft surfaces (Kearns, 2010; Kohler et al., 2000; Rashid and Kornberg, 2000). A versatile environmental microbe, P. aeruginosa is an opportunistic pathogen notorious for causing diverse infections at multiple sites including wounds, the circulatory system, the urinary tract and the lungs of cystic fibrosis patients. As for all disease-causing organisms, evolutionary adaptation is central to P. aeruginosa pathogenesis (Cattoir et al., 2013; Oliver et al., 2000; Smith et al., 2006; Weigand and Sundin, 2012; Yang et al., 2011); however, we have little understanding of selection pressures governing its evolution in surface-associated communities. P. aeruginosa is a well-known biofilm former and its biofilms are notoriously difficult to eradicate by conventional antibiotic treatment (Costerton et al., 1999). Biofilm formation is, therefore, being studied extensively in search of novel therapeutic approaches (reviewed in (Boyle et al., 2013)). Swarming motility raises equally interesting implications. Previous studies have shown that P. aeruginosa cells in swarming colonies can have distinct phenotypes from planktonic cultures, including gene expression (Tremblay and Deziel, 2010) and increased antibiotic resistance (Lai et al., 2009). In addition, the self-produced biosurfactants required for swarming motility (Caiazza et al., 2005; Deziel et al., 2003) are important for biofilm maintenance and dispersion (Boles et al., 2005; Lequette and Greenberg, 2005) as well as to kill immune cells (Jensen et al., 2007). Biofilm formation and swarming motility are inversely regulated (Baraquet et al., 2012; Caiazza et al., 2007; Kuchma et al., 2007; Lee et al., 2007), and the regulation involves the second messenger cyclic diguanylate (c-di-GMP): high levels of c-di-GMP induce biofilm formation and suppress swarming motility (Baraquet et al., 2012). This inverse regulation could be of interest for novel therapeutics against biofilm formation.

Swarming colonies of P. aeruginosa make characteristic branching patterns of striking regularity (see video S1). This phenotype requires coordination of several pathways, including flagellar motility (Kohler et al., 2000), cell-cell signaling/quorum sensing (Kohler et al., 2000; Xavier et al., 2011) and biosurfactant secretion (Caiazza et al., 2005; Deziel et al., 2003). Screening for genes affecting swarming motility yielded over 200 hits in diverse functional categories including motility, transport secretion, metabolism and transcriptional regulation (Yeung et al., 2009). Given the multifactorial nature of swarming motility one would naïvely expect that repeated rounds of swarming in independent lineages could lead to divergent evolution. Such evolutionary diversification has been observed before in bacterial experimental evolution, even with bacteria facing relatively well-defined evolutionary pressures such as new nutrients (Chou et al., 2011; Herring et al., 2006; Woods et al., 2006), a change in temperature (Bennett and Lenski, 2007; Lenski and Bennett, 1993; Tenaillon et al., 2012) or the overexpression of costly genes (Chou and Marx, 2012).

Here we show that repeated passaging of P. aeruginosa on swarming plates leads to striking parallel molecular evolution. After only a few daily passages we see emergence of a hyperswarming phenotype where the colony covers the entire plate. Hyperswarming is caused by single point mutations in the flagellar synthesis regulator FleN, which cause the bacteria to assemble multiple polar flagella and gain a strong, growth-rate independent, advantage in swarming competitions. Importantly, hyperswarmers become poor biofilm formers and are outcompeted by the ancestral strain in biofilm competitions. Experimental evolution in swarming thus provides a unique example of parallel evolution and suggests an evolutionary trade-off between motility and biofilm formation.


Experimental swarming evolution yields hyperswarmers

Three independent lineages initiated from a common ancestor strain, P. aeruginosa PA14, were submitted to experimental evolution through consecutive rounds of swarming. After every 24h period of swarming each colony was harvested in its entirety from the swarming plate and re-suspended in a test tube. A 1/1500 fraction of each of the harvested populations was point-inoculated onto the center of a fresh swarming plate. This procedure was repeated daily for 9 days.

Over time, all three lineages lost the distinctive branching pattern of wild type swarming in favor of a plate-covering morphology (fig. 1A, video S1). We call this phenotype hyperswarming. Lineage #2 started showing hyperswarming at day 5, whereas the other two lineages acquired the phenotype later. All 12 isolated clones showed round hyperswarming colonies (fig. 1B, video S1), confirming heritability and stability of hyperswarming.

Figure 1
Experimental evolution of swarming motility produces a stable and heritable hyperswarmer phenotype

We observed subtle, yet reproducible, differences in colony morphology between different hyperswarmer clones. This suggested that we had obtained an unknown number of distinct clones. We therefore undertook five quantitative phenotypic assays in order to determine the number of distinct clones. The measured phenotypes were swimming motility, twitching motility, biosurfactant production, and the amount of attached and suspended cells in biofilm assays (fig. 2A). The wild type strain provided a reference for all assays. In addition two non-motile clones (flgK and pilB) served as negative controls for the motility and biofilm assays (O’Toole and Kolter, 1998).

Figure 2
Hierarchical clustering of quantitative phenotypic assays suggested the existence of three distinct hyperswarmer clones

In each of the phenotypic assays hyperswarmers showed differences compared to wild type but to varying degrees (fig. 2A). To ascertain the number of distinct clones, the strains were grouped according to their performances in the phenotypic assays using hierarchical clustering (fig. 2B)(Maynard et al., 2010; Xie et al., 2011). Wild type clustered separately from all other strains and the two non-motile mutants clustered together. We determined that there were likely three clusters of hyperswarmers, one cluster consisting of clones 1, 3 and 4, one consisting of clones 5–8 and the last consisting of clones 2 and 9–12. Cell size clearly set apart clone 5 from clones 2 and 10 (figure S1). The fact that the two clones isolated from the same plate (clones 1 and 2 isolated from lineage #2 at day 5) were placed in different groups suggested that population #2 was polyclonal at this early stage. All other clones (clones 3–12), which were isolated from day 9, grouped according to their plate of origin. Based on the clustering, we chose the two early clones (1 and 2) and one clone from each of the late populations (clones 4, 5 and 10, respectively) for further study.

Hyperswarmers outcompete ancestral strain in swarming competitions

The consistent emergence of hyperswarmers from independent lineages suggested a competitive advantage for these clones over the ancestral strain. An obvious way to gain a competitive advantage is an increased growth rate. However, the growth rates measured in liquid cultures with the same nutrient composition as swarming plates through growth curve synchronization (van Ditmarsch and Xavier, 2011) (fig. S2A) revealed that hyperswarmers grow slightly slower than the ancestral strain (fig. 3A). We then tested whether hyperswarmers would lose a direct competition against the ancestral strain in liquid cultures. These competition experiments indeed showed that hyperswarmers have a disadvantage in liquid competition (fig. 3B). Although there are important differences between growth in liquid and the spatially structured environment of a swarming plate, the fact that hyperswarmers (i) have a lower growth rate in liquid and (ii) are outcompeted in liquid co-culture with the ancestral suggests that the process driving the evolution of hyperswarmers is growth independent.

Figure 3
Hyperswarmers lose competitions In liquid culture against the ancestral strain, but prevail in swarming competitions by segregating at the leading edges of expanding swarming colonies

In contrast, similar competitions in swarming plates confirmed that the hyperswarmers have a significant selective advantage in swarming against the ancestral strain (fig. 3C). To evaluate how hyperswarmers manage to win swarming competitions we imaged competition plates and observed that hyperswarmers localized preferentially at the leading edge of swarming colonies (fig. 3D). These competitions further support that growth rate is not the cause of the emergence of hyperswarmers. Rather, they suggest that the hyperswarmers gain an ecological advantage by getting prime access to nutrients as the colony expands, a phenomenon studied in depth with theoretical models (e.g. Xavier and Foster, 2007; Xavier et al., 2009). The loss of branching in hyperswarmer colonies is not caused by a loss of the repulsion effect (Caiazza et al., 2005; Tremblay et al., 2007), as hyperswarmers are still affected in proximity of a surfactant-producing but immotile flgK strain (fig. S2B–C, third segment of video S1).

Hyperswarming is caused by parallel point mutations in fleN

After determining stability and heritability of hyperswarming, we sequenced the whole genome of the five selected hyperswarmer clones and the ancestral strain in search of mutations causing hyperswarming. Using the deposited reference genome of P. aeruginosa UCBPP-PA14 from (Winsor et al., 2011) as the scaffold for read mapping, we compared each of the hyperswarmers to the ancestral strain, so as to specifically call mutations between hyperswarmers and our lab strain. The analysis identified only single nucleotide polymorphisms (SNPs) in one gene. The mutated gene, fleN (PA14_45640), encodes the flagellar synthesis regulator FleN (Dasgupta et al., 2000), suggesting the mutations affected flagellar motility. We found two distinct fleN mutations: clones 1 and 4 harbored mutation FleN(V178G) and clones 2, 5 and 10 harbored FleN(W253C) (fig. 4A). Targeted re-sequencing of fleN in the remaining 7 hyperswarmers revealed that they also harbored single point mutations in fleN (fig. 4A). Interestingly, the fleN mutations agreed with the hierarchical clustering carried out earlier (fig. 2B), confirming that lineage #2 was indeed polyclonal at day 5.

Figure 4
Hyperswarming is caused by point mutations in fleN, which produce polar multi-flagellated cells

Since none of the SNPs found in the hyperswarmers are present in any of the Pseudomonas spp. genomes publically available (Winsor et al., 2011) (fig. 4A), we proceeded to confirm causality of fleN mutations for hyperswarming. The expression level of fleN is vital for its proper functioning, and fleN overexpression yields non-flagellated cells whereas a knockout yields multi-flagellated but non-motile cells (Dasgupta et al., 2000). We thus opted for in cis complementation, to ensure appropriate expression. Allelic replacement of wild-type fleN with mutated fleN produced the hyperswarmer morphology; conversely, the replacement of mutant fleN with the wild-type sequence reverted the swarming morphology to the wild type, branched colony morphology in all clones (fig. 4B). Moreover, all the other phenotypes assessed previously were also complemented by the allelic replacements (fig. S3A). Together, the complementation experiments provide definitive proof that the SNPs identified are necessary and sufficient to cause hyperswarming. In addition, we confirmed that the fleN mutations do not constitute complete loss of function in FleN, since a ΔfleN strain is impaired in swarming motility and behaves different from any hyperswarmer clone in the other phenotypes as well (fig. 4B, S3A). No explanatory SNPs were found for the different cell size in clone 5, but fleN complementation proved that these unknown mutations are not sufficient for the hyperswarming phenotype (fig. 4B).

Parallel evolution of multi-flagellated hyperswarmers

The finding that three independent lineages of experimental evolution led to hyperswarmers through distinct point mutations in fleN suggested that swarming selection was driving parallel molecular evolution. 16 out of 24 additional experimental evolution lineages resulted in hyperswarming (the remaining 9 lineages collapsed due to a phage outburst and those lineages were excluded from further analysis, data not shown). All new hyperswarmer clones (one from each lineage) harbored fleN mutations, 12 harbored FleN(V178G); the remaining four revealed four additional fleN mutations: FleN(F176S), FleN(L179Q), FleN(F203L) and FleN(P254L) (fig. 4A).

At the molecular level FleN has a dual role, influencing flagellar synthesis and chemotaxis as the anti-activator of the flagellar master regulator FleQ, and maintaining flagellar number and polarity due to its interaction with FlhF (Balaban and Hendrixson, 2011; Kazmierczak and Hendrixson, 2013; Kusumoto et al., 2006; Murray and Kazmierczak, 2006). As stated previously, in the complete absence of FleN, P. aeruginosa becomes multi-flagellated, but suffers from a chemotactic defect, impairing motility (Dasgupta et al., 2000)(fig. 4B and S3A). Furthermore, overexpression of FleN leads to non-flagellated cells (Dasgupta et al., 2000; Dasgupta and Ramphal, 2001; Dasgupta et al., 2003). Due to increased swimming and swarming motility of hyperswarmers (Fig. 2A, 3B–C), we decided to investigate whether the fleN mutations affected flagellar synthesis. We did so in two ways. Firstly, we constructed a flagellin promoter-based fluorescence reporter (attB::PfliC-gfp). Flagellin (FliC) is the major extracellular component of the flagellar filament. In liquid culture, hyperswarmers expressed fliC earlier and at higher levels than the ancestral strain (fig. S3C), suggesting up-regulation of flagellar synthesis. Secondly, the ancestral strain, as expected, was mono-flagellated as observed by electron microscopy (EM) and flagella counts (fig. 4C–D, fig. S3B; although there are conditions where P. aeruginosa may have one to two flagella (Kohler et al., 2000; Rashid and Kornberg, 2000)). The hyperswarmers were confirmed to be polar multi-flagellates (fig. 4C–D, fig. S3B). It should be noted that the cells visualized by EM originated from stationary-phase liquid cultures, supporting that fleN mutations lock hyperswarmers in a multi-flagellated state.

Previous studies have shown that both P. aeruginosa PAK and P. putida became polar multi-flagellates when FlhF was overexpressed (Pandza et al., 2000; Schniederberend et al., 2013). Since FlhF and FleN are binding partners (Balaban and Hendrixson, 2011; Kazmierczak and Hendrixson, 2013; Kusumoto et al., 2006; Murray and Kazmierczak, 2006), it is possible that if their interaction is hampered by the fleN mutations, the cells may be experiencing a relatively higher activity of FlhF. However, swarming motility in a constitutive FlhF expression strain was indistinguishable from the ancestral (figure S3D), suggesting that the hyperswarmer phenotype is not due to a relatively higher activity of FlhF.

Hyperswarming comes at the expense of biofilm formation

Since single-nucleotide changes can transform P. aeruginosa into a hyperswarmer and thus produce a pronounced competitive advantage in swarming, we wondered whether hyperswarmers exist in nature. So we investigated swarming motility in 47 isolates of P. aeruginosa (18 environmental isolates from contaminated soils in Canada (Deziel et al., 1996) and 29 clinical isolates from patients at Memorial Sloan-Kettering Cancer Center; fig. 5). All of these strains came from direct isolations and have undergone few generations in a laboratory setting. They should, consequently, represent the natural diversity of P. aeruginosa more appropriately. None of these strains showed hyperswarming (fig. 5). In fact, hyperswarmers clearly occupy a distinct region within the space of colony morphologies when compared to wild type strains (fig. 5). None of the five best swarmers from the clinical strains had coding mutations in fleN compared to our wild type.

Figure 5
Hyperswarming is not found in clinical or environmental isolates

Although all the clinical strains originated all from the same hospital, we observed a large variance in their swarming phenotype (fig. 5) suggesting that the strain library is significantly diverse. We selected one of the best swarmers among these isolates and subjected it to experimental evolution. The selected clinical strain harbored 4 non-coding mutations in its fleN sequence, showing that it is genotypically distinct from PA14. Nonetheless, as with PA14 this isolate evolved a hyperswarmer phenotype, in this case via the mutation FleN(V178G) (fig. S4). The observation that another strain can also evolve hyperswarming through FleN point mutations shows that convergent evolution can occur beyond PA14. Nonetheless, the fact than none of our environmental and clinical isolates showed a hyperswarming phenotype (fig. 5) raises the questions whether there is an opposing selective pressure in the wild. Such an effect could be caused by antagonistic pleiotropy (Taylor and Buckling, 2011) or an evolutionary tradeoff (Shoval et al., 2012), where the improvement in one trait comes at the expense of another.

Biofilm formation is an essential trait for P. aeruginosa, both in environmental and clinical settings (Costerton et al., 1999; Kolter and Greenberg, 2006) and is known to be inversely regulated to swarming motility (Baraquet et al., 2012; Boyle et al., 2013; Caiazza et al., 2007; Kuchma et al., 2007; Lee et al., 2007). An inverse relationship has previously been shown between swarming motility and biofilm formation in a cohort of 237 clinical isolates (Murray et al., 2010). Our compendium of isolates shows a similar trend where good biofilm formers tend to be poor swarmers, whereas poor biofilm formers can be good swarmers (fig. S5E).

We had already observed that hyperswarmer clones are poor biofilm formers in monocultures (fig. 2A), so we proceeded to check their competitive ability against the ancestral strain in mixed biofilms. These competitions revealed that hyperswarmers start losing at the initial attachment phase (fig. 6A) and become severely outnumbered in mature biofilms, both in flow-cell (fig. 6B) and slide biofilm assays (fig. S5A–C). Hyperswarmers were strongly outcompeted even when hyperswarmers were inoculated up to a 100-fold excess compared to the ancestral strain (fig. S5D). The experiments suggest that biofilm formation impairment of hyperswarmers lies in an attachment problem (fig. 6A), whereas wild type cells in the same culture do manage to attach and grow a mature biofilm. Therefore the fleN mutations seem to tip the balance between biofilm and motility (Caiazza et al., 2007; Shrout et al., 2006) in favor of swarming motility in hyperswarmers, but at the expense of biofilm formation (fig. 6C).

Figure 6
Hyperswarmers lose biofilm competitions against the ancestral strain, indicating an evolutionary tradeoff between motility and biofilm formation


Here we show that evolution in surface-associated swarming colonies produces parallel molecular evolution of multi-flagellated hyperswarmers. Swarming motility in P. aeruginosa is a social trait involving many molecular pathways (Caiazza et al., 2005; Deziel et al., 2003; Kohler et al., 2000; Rashid and Kornberg, 2000; Xavier et al., 2011), and a priori prediction of evolutionary trajectories is complicated due to its multifactorial nature. So the consistent emergence of hyperswarmers in independent lineages was surprising, all the more since all hyperswarmers have single point mutations in the same region of the flagellar synthesis regulator FleN. This is a striking example of parallel molecular evolution. In addition, the emergence of the most predominant mutation – FleN(V178G) – in experimental evolution started from a clinical isolate shows that the convergent molecular evolution in strains other than PA14.

These results raise an intriguing question: if P. aeruginosa can be transformed from a mono-flagellated (monotrichous) into a polar multi-flagellated (lophotrichous) bacterium by single point mutations, why is P. aeruginosa mono-flagellated in nature? The answer to this question may lie in the fact that hyperswarmers are poor biofilm formers that lose against wild type in biofilm competitions. As biofilms are an essential part of the P. aeruginosa lifestyle, hyperswarmers likely face a strong counter-selection in the wild, preventing their fixation there. Interestingly, non-flagellated mutants are also poor biofilm formers (O’Toole and Kolter, 1998), suggesting that fine tuning of flagellar motility is essential for biofilm formation.

Swarming is common in P. aeruginosa strains and has been reported by many labs (e.g. (Breidenstein et al., 2012; Kuchma et al., 2010; Lai et al., 2009; Morris et al., 2011; Murray et al., 2010; Shrout et al., 2006; Takahashi et al., 2008; Tremblay and Deziel, 2008; Yeung et al., 2012; Yeung et al., 2009)). Nonetheless, swarming is a conditional phenotype that varies significantly with media composition and agar hardness. For example, round P. aeruginosa swarms without branches have been reported before in specific media conditions (e.g. (Breidenstein et al., 2012; Morris et al., 2011; Takahashi et al., 2008; Yeung et al., 2012)) even in wild-type strains. In contrast, the present study focused on the transition from branched swarms to round swarms while keeping the media composition and hardness constant, in order to determine the underlying genetic mechanism.

Importantly, these hyperswarmer clones could not have been found in traditional liquid culture-based experimental evolution. Hyperswarmers invariably grow slower than the ancestral strain in liquid culture and thus would not get fixed in evolving populations. In contrast, in swarming plates hyperswarmers emerge by virtue of their superior motility, which allows them to segregate at the leading edge of colonies, thereby gaining prime access to free space and unused nutrients. Therefore, hyperswarming, like certain other bacterial traits such as biofilm-building polymer secretion (Nadell and Bassler, 2011; Xavier and Foster, 2007; Xavier et al., 2009), requires spatial structure to be selected.

We also would not expect to find these hyperswarmers in genetic knockout screens. As shown in the present study, a fleN knockout actually swims and swarms worse than the wild type. In contrast to a fleN knockout, the point mutations in hyperswarmers are restricted to the 3′-terminal region of fleN and these mutants swim far better than the wild type. A transposon screen generated superior swarmers (Yeung et al., 2009), but these mutants were not related to flagella. One example is a pqsH knockout, which is a synthase for the extracellular signaling molecule PQS. The pqsH hyperswarmers made larger swarming colonies that still retained the branching pattern of the wild type. Therefore, our plate-covering fleN-mutated hyperswarmers would probably outcompete these transposon mutants.

Interestingly, in that same transposon screen mutants with swarming defects generally showed biofilm overproduction whereas mutants with enhanced swarming showed normal or decreased biofilm ability (Yeung et al., 2009). This adds to the mounting evidence for an inverse regulation between biofilm formation and motility (Caiazza et al., 2007; Yeung et al., 2011), which, as shown here, leads to an evolutionary trade-off.

The molecular mechanism underlying the hyperswarming phenotype is currently unknown. Our experiments suggest, however, that the observed changes are subtle, since neither a fleN deletion, nor constitutive flhF expression (both of which should create multiple polar flagella) recapitulate the hyperswarming morphology.

Placed in a broader perspective, our experiments in spatially structured swarming populations support that adaptive evolutionary paths can be, at least to some extent, predictable. Beyond P. aeruginosa, can we expect parallel evolution to be a general feature? Recent experiments on the adaptation of Escherichia coli to growth in high temperatures provide a counterexample and reinforce the notion that adaptive convergence can occur through diverse molecular changes (Tenaillon et al., 2012). Furthermore, studies in other bacterial pathogens, both in laboratory populations (Ensminger et al., 2012) and epidemics (Lieberman et al., 2011; Snitkin et al., 2012), suggest that adaptations to a host immune system can be divergent. Our evolutionary experiments seem to be uncommon among bacteria and thus constitute a unique experimental model of highly reproducible molecular adaptation. Nevertheless, parallel evolution is important in nature (Christin et al., 2010), as illustrated by a recent finding that herbivorous insects facing the same selective pressure exhibit parallel molecular changes across species (Zhen et al., 2012). Which conditions lead to convergent rather than divergent evolutionary adaptations is therefore a major open question in evolutionary biology. The reproducible evolution of hyperswarmers in P. aeruginosa provides an experimental system to address this question.

Experimental procedures

Swarming motility

Swarming assays and competitions were performed as previously described (Xavier et al., 2011). The selection coefficient is defined as the final ratio of hyperswarmers to ancestral over the initial ratio of hyperswarmers to ancestral. Competitions were performed in both color combinations to exclude a possible selective advantage of either fluorescent marker (GFP or DsRedExpress). The neutral selection coefficient was determined by using wild type in both colors. Repulsion assays were performed in standard swarming conditions.

Experimental evolution

Experimental evolution was carried out in standard swarming conditions. After 24h, the entire swarming colony was collected from the plate and used to inoculate a fresh swarming plate with a population bottleneck of 1/1500. Every cell suspension was glycerol stocked at −80°C.

Phenotypic assays

Swimming motility was performed according to (Murray and Kazmierczak, 2006) with minor adaptations. Twitching motility was also described elsewhere (O’Toole and Kolter, 1998), with minor adaptations. Rhamnolipid production was quantified using the sulfuric acid anthrone assay as described previously (Xavier et al., 2011). Monoculture biofilms were performed using a standard 96-well crystal violet biofilm assay (O’Toole and Kolter, 1998) with some adaptations. Biofilm competitions were performed using the tilted glass slip biofilm method (Xavier et al., 2009), with minor adaptations. Cell sizes where measured from overnight cultures diluted 1/2000 and left to grow for 5 hours at 37°C before gently diluting them out in order to visualize the cells by microscope. GFP labeled were used to facilitate imaging. See extended experimental procedures.

Growth curves, calculation of growth rates and fliC expression

Growth rates were measured using the growth curve synchronization method (van Ditmarsch and Xavier, 2011). The liquid media were of the same composition as swarming media. The fliC promoter was PCR amplified directly from the P. aeruginosa PA14 genome using primers from a previous study (Wyckoff et al., 2002). The promoter (PfliC) was used to replace PrhlAB in pYL122 (Lequette and Greenberg, 2005). The construct was mobilized into P. aeruginosa following four-parental mating and then integrated into the attB site. fliC expression was monitored through GFP.

Whole genome sequencing and SNP calling

P. aeruginosa PA14 whole genome sequencing was done using Applied BioSystems SOLiD sequencing (Bedford, MA) with average 100× coverage. Duplicate reads were removed. Only areas of the genome with at least 10x coverage in high quality score reads were included in the analysis to allow SNP calling ([greater, double equals]94.35% of each sequenced genome). The ancestral and five hyperswarmers were genotyped with the GATK Unified Genotyper (McKenna et al., 2010). Sequence differences between the deposited genome and our lab strain genome were not included in this direct comparison; thus the mutations defining the hyperswarmers were called specifically.

Targeted re-sequencing of fleN

fleN was PCR-amplified from the P. aeruginosa genome using primers fleN_fwd and fleN_rev (see extended methods). Fragments were sequenced in both directions with the Sanger method.

Strain creation through allelic exchange

Site-directed mutagenesis of fleN was performed through homologous recombination (Shanks et al., 2006), with minor adaptations (see extended methods). To create a constitutive FlhF expressing strain, the PA1/04/03 promoter (Lambertsen et al., 2004; Lanzer and Bujard, 1988) was introduced in front of flhF.

Transmission electron microscopy

Transmission electron microscopy was performed directly from overnight cultures. All acquired photos were used to assess the distribution of the number of flagella by manual counts.


  • Experimental evolution of swarming in P. aeruginosa generates hyperswarmers.
  • Parallel evolution in the flagellar regulator FleN is causal for hyperswarming.
  • Point mutations in FleN produce multi-flagellated hyper-swimming bacteria.
  • There is an evolutionary trade-off between motility and biofilm formation.

Supplementary Material




We thank Karina Xavier for comments on the manuscript, Kathleen Gilhuley for supplying the clinical strains, Joe Harrison (University of Calgary) for help with the ΔfleN protocol, Nick Socci (Bioinformatics core), the DNA sequencing core at MSKCC, and Alice Liang and the Microscopy Core at the New York University Langone Medical Center for the transmission electron microscopy. This work was supported by the Lucille Castori Center for Microbes, Inflammation, and Cancer and by the Office Of The Director, National Institutes Of Health of the National Institutes of Health under Award Number DP2OD008440 to JBX.


The authors declare no conflict of interest.

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