Initiation of biofilm formation by Pseudomonas aeruginosa 57RP correlates with emergence of hyperpiliated and highly adherent phenotypic variants deficient in swimming, swarming, and twitching motilities

Pseudomonas aeruginosa is a ubiquitous environmental bacterium capable of f orming biofilms on surfaces as a survival strategy. It exhibits a large var iety of competition/virulence factors, such as three types of motilities: f lagellum-mediated swimming, flagellum-mediated swarming, and type IV pilus- mediated twitching. A strategy frequently used by bacteria to survive chang ing environmental conditions is to create a phenotypically heterogeneous po pulation by a mechanism called phase variation. In this report, we describe the characterization of phenotypic variants farming small, rough colonies that spontaneously emerged when P. aeruginosa 57RP was cultivated as a biof ilm or in static liquid cultures. These small-colony (S) variants produced abundant type IV fimbriae, displayed defective swimming, swarming, and twit ching motilities, and were impaired in chemotaxis. They also autoaggregated in liquid cultures and rapidly initiated the formation of strongly adheren t biofilms. In contrast, the large-colony variant (parent form) was poorly adherent, homogeneously dispersed in liquid cultures, and produced scant po lar fimbriae. Further analysis of the S variants demonstrated differences i n a variety of other phenotypic traits, including increased production of p yocyanin and pyoverdine and reduced elastase activity. Under appropriate gr owth conditions, cells of each phenotype switched to the other phenotype at a fairly high frequency. We conclude that these S variants resulted from p hase variation and were selectively enriched when P. aeruginosa 57RP was gr own as a biofilm or in static liquid cultures. We propose that phase variat ion ensures the prior presence of phenotypic forms well adapted to initiate the formation of a biofilm as soon as environmental conditions are favorab le.