Nozzles underneath some bacteria eject slime to propel the organism forwards or backwards.

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References

“Gliding motion across surfaces, usually with slime–whether or not by the same scheme–occurs in procaryotic organisms (bacteria and their kin) as well. It’s based on either of two mechanisms. Bacteria are often covered with tiny hairs, pili; retraction of one type (designated IV) through their outer membranes can move them around. Alternatively, they can secrete carbohydrate slime rearward to get a push (Kaiser 2000; Merz and Forest 2002).” (Vogel 2003:450)

Book
Comparative Biomechanics: Life's Physical World, Second EditionPrinceton University PressJune 17, 2013
Steven Vogel

“It has been known for decades that bacteria locomote over surfaces, but the mechanisms that power motility have been unclear. Recent experiments have begun to explain two modes of surface motility. Twitching or social gliding motility is powered by the retraction of type IV pili. Adventurous gliding motility is powered by the rearward secretion of carbohydrate slime. In both cases, cell movement depends on the translocation of enormous volumes of macromolecules through outer membrane pore complexes…Several mechanisms have been proposed to account for gliding motility including treadmill-like motors on the cell surface and secretion of surfactants that draw the cell forward. Recent experiments support another idea: that the gliding of filamentous bacteria — linked chains of dozens to hundreds of cells — is powered by compressive forces arising from the rearward secretion of slime, a polyelectrolyte gel composed of complex carbohydrates.” (Merz et al. 2002:R297)

Journal article
Bacterial surface motility: slime trails, grappling hooks, and nozzlesCurrent Biology, Vol. 12, R297–R303Merz, AJ; Forest, KT

Journal article
Bacterial motility: how do pili pull?Volume 10, Issue 21, Pages R777-R780Kaiser, D

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