Living on a surface: swarming and biofilm formation Natalie Verstraeten 1 , Kristien Braeken 1 , Bachaspatimayum Debkumari 1 , Maarten Fauvart 1 , Jan Fransaer 2 , Jan Vermant 3 and Jan Michiels 1 1 Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium 2 Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium 3 Department of Chemical Engineering, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This com- plex multicellular behavior requires the integration of chemical and physical signals, which leads to the phys- iological and morphological differentiation of the bac- teria into swarmer cells. Here, we provide a review of recent advances in the study of the regulatory pathways that lead to swarming behavior of different model bacteria. It has now become clear that many of these pathways also affect the formation of biofilms, surface- attached bacterial colonies. Decision-making between rapidly colonizing a surface and biofilm formation is central to bacterial survival among competitors. In the second part of this article, we review recent develop- ments in the understanding of the transition between motile and sessile lifestyles of bacteria. Flagella-mediated movement on a surface Bacteria often thrive in surface-associated multicellular communities that have advantages over individual cells, such as protection against unfavorable environmental con- ditions (including predation, the presence of antimicrobials and the host immune response). Biofilms are sessile com- munities with microorganisms embedded within a matrix and attached to a surface. However, motile populations, such as swarming bacteria, can rapidly reach novel niches, which they can colonize; this provides ecological advan- tages to the bacteria [1,2]. The choice between sessile and motile lifestyles is clearly an important decision to be made by microorganisms that live in varying habitats and requires the integration of many environmental cues. Swarming motility is a process by which bacteria can rapidly (several mms 1 ) advance on moist surfaces in a coordinated manner. It requires functional flagella and is coupled to the production of a viscous slime layer. The slime layer is thought to extract water from the agar and keeps the cells in a moist environment. Swarming is a group behavior that requires the cells to reach a certain cell number before the process is initiated. Furthermore, swar- mers are often elongated as a result of the suppression of cell division. Swarming is widespread in many genera of Gram-nega- tive and Gram-positive flagellated bacteria and is typically assayed on a solidified medium, containing 0.5–2% agar, from which the bacteria are thought to extract water and nutrients. Species such as Proteus mirabilis and Vibrio parahaemolyticus, which are capable of vigorous swarming even on high-agar medium, often possess pronounced swarmer cell morphology with high numbers of flagella and prominent elongation (5- to 20-fold for Vibrio [3] and 10- to 40-fold for Proteus [2]). Swarming has been studied extensively in P. mirabilis, in which elongated, multinu- cleated and hyper-flagellated swarmer cells can spread as multicellular rafts across surfaces [2]. In P. mirabilis multicellular rafts, flagellar filaments from adjacent swar- mer cells are interwoven in phase and form helical con- nections between the cells [4]. Periodically, the cells revert to the undifferentiated vegetative state; this reversion to the undifferentiated vegetative state is termed consolida- tion. Repeated alternation between both modes results in the appearance of characteristic terraced colonies. Swarm- ing patterns of concentric zones are also found with V. parahaemolyticus. However, in contrast to the complex swarmers P. mirabilis and V. parahaemolyticus, many other species such as Pseudomonas aeruginosa, Rhizobium etli, Serratia liquefaciens, Salmonella enterica serovar Typhimurium (S. Typhimurium) and Escherichia coli move continuously and do not produce pronounced ter- raced colonies (Figure 1). Complex patterns of swarming have also been reported for Gram-positive bacteria such as Bacillus subtilis [5]. This review covers recent advances in swarming and the link between swarming motility and biofilm formation in several well-studied Gram-negative model systems. For several excellent reviews focused more extensively on swarming, see Refs [2,3,6–9]. Morphological differentiation and stimuli of swarming Flagella enable bacteria to move towards favorable environments during swimming and contribute to the virulence of pathogens through adhesion and biofilm for- mation on host surfaces. The transcription of flagellar genes proceeds in a hierarchical, highly regulated manner with master regulatory genes (such as flhDC in Entero- bacteriaceae, fleQ in P. aeruginosa and flrA in Vibrio spp.) integrating multiple environmental signals. The number of flagella is generally upregulated in swarmer cells. Swim- ming and swarming are similarly controlled at the level of Review Corresponding author: Michiels, J. (jan.michiels@biw.kuleuven.be). 496 0966-842X/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tim.2008.07.004 Available online 3 September 2008