Both swimming and swarming motilities depend on bacterial flagella, but they differ in many ways. The most noticeable distinction is that swimming is an Buparlisib solubility dmso individual behavior, whereas swarming is a movement of bacterial populations. Moreover, the cells exhibit differentiation during swarming; they are usually elongated and hyperflagellated compared with the vegetative cells grown in liquid media (Allison & Hughes, 1991; Harshey, 2003; Rather, 2005). Swarming also shares features with other surface phenomena, such as biofilm formation and host invasion, and is associated with pathogenesis in some organisms. For example,

swarming of P. mirabilis facilitates ascending colonization of the urinary tract and is conducive to biofilm formation on catheters (Allison et al., 1994; Stickler et al., 1998). Expression of flagella and virulence factors are coordinated in P. mirabilis and Serratia liquefaciens (Allison et al., 1992; Givskov et al., 1995). The flagellar export apparatus of Yersinia enterocolitica XAV-939 mw also functions as a secretion system for the transport of a virulence-associated phospholipase (Young et al., 1999). In many species, swarming bacteria exhibit adaptive resistance to multiple antibiotics (Butler et al., 2010). In recent years, system-screening studies in various species have revealed numerous swarming-related genes. These genes are involved

in flagellar assembly, synthesis of polysaccharides, chemosensors, 4��8C signal regulation, and metabolic pathways, whereas others are hypothetical genes with unknown functions (Kearns et al., 2004; Inoue et al., 2007; Overhage et al.,

2007). However, the genetic determinants for this special process vary among species, indicating different swarming patterns in various swarming bacteria. Therefore, the study of swarming motility in various bacteria would facilitate a thorough understanding of this special bacterial motion. Considering that many types of genes are related to swarming motility, such a study also provides a tractable model to study the function of genes involved in bacterial differentiation, multicellularity, and pathogenesis. Citrobacter freundii is a motile gram-negative bacterium living in soil and aqueous environments; it is often isolated in clinical specimens as an opportunistic pathogen. In this study, we demonstrated that swarming motility could be induced in C. freundii. It was examined in detail because little is known about this motility in C. freundii. To discover the genetic determinants that affect swarming, the mini-Tn5 transposon mutation was used to screen swarming-associated genes by impairing bacterial swarming ability. Our results showed that a number of genes are related to the swarming of C. freundii, among which several have been newly identified. The following strains were used in this study: C. freundii ATCC8090 was a gift from Dr Tomofusa Tsuchiya of Okayama University, Japan; P.