Swarming ofPseudomonas aeruginosaIs a Complex Adaptation Leading to Increased Production of Virulence Factors and Antibiotic Resistance

Abstract

In addition to exhibiting swimming and twitching motility, Pseudomonas aeruginosa is able to swarm on semisolid (viscous) surfaces. Recent studies have indicated that swarming is a more complex type of motility influenced by a large number of different genes. To investigate the adaptation process involved in swarming motility, gene expression profiles were analyzed by performing microarrays on bacteria from the leading edge of a swarm zone compared to bacteria growing in identical medium under swimming conditions. Major shifts in gene expression patterns were observed under swarming conditions, including, among others, the overexpression of a large number of virulence-related genes such as those encoding the type III secretion system and its effectors, those encoding extracellular proteases, and those associated with iron transport. In addition, swarming cells exhibited adaptive antibiotic resistance against polymyxin B, gentamicin, and ciprofloxacin compared to what was seen for their planktonic (swimming) counterparts. By analyzing a large subset of up-regulated genes, we were able to show that two virulence genes, lasB and pvdQ, were required for swarming motility. These results clearly favored the conclusion that swarming of P. aeruginosa is a complex adaptation process in response to a viscous environment resulting in a substantial change in virulence gene expression and antibiotic resistance.Swarming motility is a multicellular phenomenon involving the coordinated and rapid movement of a bacterial population across a semisolid surface (14). It is widespread among flagellated bacteria, including Salmonella, Vibrio, Yersinia, Serratia, and Proteus (9,18). Swarming is highly dependent on bacterial cell density, nutrient growth medium, and surface condition moistness (53). In addition to physical changes such as an increase in the number of flagella or cell elongation, swarmer cell differentiation results in substantial alterations in metabolic bias and gene expression, indicating that swarming represents a complex lifestyle adaptation in response to particular medium conditions rather than merely a form of locomotion (18,45).Swarming of Pseudomonas aeruginosa is often typified by a dendritic colonial appearance. This gram-negative bacterium is a major cause of hospital-acquired bacterial infections and the most significant pulmonary pathogen in cystic fibrosis patients (17,20,44). It is one of the most difficult infections to treat due to its high natural (intrinsic) antibiotic resistance. It possesses three types of movement depending on medium viscosity, namely, swimming in aqueous environments, twitching on solid surfaces or interfaces, and swarming on semisolid, viscous media, such as those containing 0.4 to 0.7% (wt/vol) agar. It has been previously shown that swarming of P. aeruginosa is dependent on both flagella and type IV pili, which mediate actual movement, as well as on rhamnolipids, which are proposed to enable swarming cells to overcome the strong surface tension of the water surrounding swarmi...

Density-dependent adaptive resistance allows swimming bacteria to colonize an antibiotic gradient

“…This type of adaptive resistance is reminiscent of the non-genetic resistance that some bacterial species exhibit when engaging in collective swarming motility (Kim et al, 2003;Overhage et al, 2008;Butler et al, 2010;Roth et al, 2013). Nonresistant swarming Salmonella enterica populations were previously shown to migrate from an antibiotic-free agar surface onto an agar surface containing lethal doses of an antibiotic (Butler et al, 2010).…”

Extracellular enzymes affect biofilm formation of mucoid Pseudomonas aeruginosa

“…In contrast, no significant difference was observed between the biofilms of a mucoid P. aeruginosa strain and those of an aprA deletion mutant, indicating that AprA does not have an important function in the development and/or maintenance of the 3D structure of mucoid P. aeruginosa biofilms (Sarkisova et al, 2005). Recently, transcriptome analysis of P. aeruginosa has shown a 4.9-fold increase in lasB expression under swarming conditions (Overhage et al, 2008), indicating a potential role for LasB in the highly complex swarming process. Similarly, in Bacillus subtilis it has been shown that swarming motility is enhanced by the expression of extracellular protease activity (Connelly et al, 2004).…”

Starvation- and antibiotics-induced formation of persister cells in Pseudomonas aeruginosa

“…Other mechanisms such as the development of swarming colonies, the stringent response, i.e. response to nutrient limitation, and the formation of multiresistant subpopulation of persistent cells contribute to the multidrug-resistant phenotype [21][22][23] . Furthermore, P. aeruginosa is versatile in the development or acquisition of new mechanisms of resistance to antibiotic treatment.…”