Gliding motility revisited: how do the myxobacteria move without flagella?

In bacteria, motility is important for a wide variety of biological functions such as virulence, fruiting body formation, and biofilm formation. While most bacteria move by using specialized appendages, usually external or periplasmic flagella, some bacteria use other mechanisms for their movements that are less well characterized. These mechanisms do not always exhibit obvious motility structures. Myxococcus xanthus is a motile bacterium that does not produce flagella but glides slowly over solid surfaces. How M. xanthus moves has remained a puzzle that has challenged microbiologists for over 50 years. Fortunately, recent advances in the analysis of motility mutants, bioinformatics, and protein localization have revealed likely mechanisms for the two M. xanthus motility systems. These results are summarized in this review.