Slime is produced by bacteria that infect wounds and diseased tissues in humans. For example, in osteomyelitis and arthritis, or where metal or plastic materials have been medically implanted in humans, bacteria often colonize exposed surfaces that are not protected by normal intact cell walls. Most serious medically, slime insulates bacteria from contact with antibiotics. Some cyanobacteria today store oxygen in bubbles in their slime coating. It s easy to imagine early cyanobacteria using their ability to tolerate oxygen, then evolving to use slime first as a defensive shield and then as an offensive weapon in the competitive environment inside stromatolites.
Slime is thus fundamental to the success of many bacteria, for many different reasons.
Slime was holding together bacterial mats at 3500 Ma in the stromatolites at Warrawoona, so it is as old as the earliest known cells. The Warrawoona cells that we have are preserved because they have their rigid outer bacterial cell coat. The rigid coat is a protective layer that is either covered by slime or has slime as an alternative. Perhaps slime evolved even before the cell membrane among groups of protocells attached to clay particles. A naked gene in polysaccharide slime may have done rather well, especially nestled inside a cleavage crack in a seafloor clay mineral.