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* Abstract The mechanisms responsible for bacterial gliding motility have been a mystery for almost 200 years. Gliding bacteria move actively over surfaces by a process that does not involve flagella. Gliding bacteria are phylogenetically diverse and are abundant in many environments. Recent results indicate that more than one mechanism is needed to explain all forms of bacterial gliding motility. Myxococcus xanthus "social gliding motility" and Synechocystis gliding are similar to bacterial "twitching motility" and rely on type IV pilus extension and retraction for cell movement. In contrast, gliding of filamentous cyanobacteria, mycoplasmas, members of the Cytophaga-Flavobacterium group, and "adventurous gliding" of M. xanthus do not appear to involve pili. The mechanisms of movement employed by these bacteria are still a matter of speculation. Genetic, biochemical, ultrastructural, and behavioral studies are providing insight into the machineries employed by these diverse bacteria that enable them to glide over surfaces.
Key Words twitching, type IV pilus, Myxococcus, Flavobacterium, Mycoplasma
INTRODUCTION
Surfaces are important features of many environments. Nutrients are concentrated at surfaces, making them attractive sites for bacterial colonization, often resulting in the formation of complex biofilms (26,76). Bacteria have evolved efficient strategies to move over surfaces. Proteus mirabilis, Vibrio parahaemolyticus, Serratia marcescens, and many others employ numerous flagella to spread over moist surfaces in a process known as swarming motility (46). Other bacteria, such as Pseudomonas aeruginosa and Neisseria gonorrhoeae, use type IV pili to move in a process called twitching motility (49). Finally, diverse bacteria such as Myxococcus xanthus, Flavobacterium johnsoniae, Phormidium uncinatum, and many others slither over surfaces by a mysterious process known as gliding motility (54,77,103).
Bacterial gliding motility is defined as smooth translocation of cells over a surface by an active process that requires the expenditure of energy. Gliding does not require flagella, and cell movement generally follows the long axis of the cell. As a result of their movements, gliding bacteria often produce colonies that have thin spreading edges (Figure 1A).
Bacteria from many branches of the eubacterial phylogenetic tree exhibit gliding motility (Table 1). It is particularly common in three large groups: the myxobacteria (members of the delta proteobacteria), the cyanobacteria, and the CytophagaFlavobacterium group. Gliding bacteria live in environments as diverse as the human mouth, ocean sediments,...