The Superiority of Slime

Slime is an underappreciated facet of bacterial biology. Bacteriologists have worked for decades with pure laboratory cultures, but bacteria generally don't produce slime in laboratory conditions. In nature, all bacteria secrete slime. The real bacterial cell membrane may secrete over itself either a rigid outer coating (sometimes wrongly called the outer cell wall) or an extracellular coating, the glycocalyx, a feltlike mass of tangled polysaccharide fibers (slime, in other words) that can surround a single cell, a group of cells, or both.

Slime is useful to bacteria for several reasons. It allows limited movement of a cell so that it can adjust a little to the microenvironment even without obvious locomotory devices. Slime protects against radiation, so cells that depend on photosynthesis in very shallow water are less exposed to harm from UV radiation. Gas diffusion is perhaps 10,000 times slower through slime than through water, so it is easier for a slimy bacterium to survive periods of fluctuation in external chemistry. Gas bubbles can be trapped in slime and used for good adaptive reasons, as in stromatolites. Slime is sticky and chemically active; it allows bacteria to attach tightly to surfaces that range from human teeth and lungs to cattle intestines to rocks on the shore.

On a higher level, slime helps to form complex structures. Bacterial cells can divide inside their slime coat, so microcolonies and then large colonies form as increasingly large units, well glued together. Slime helps to trap sediment and nutrients. Stable communities of bacteria can resist chemical, physical, or pharmacological disturbance, whether they form photosynthetic mats on the shoreline of salty lagoons or masses of plaque on teeth.

Sometimes bacteria of different kinds are attracted to one another to form a consortium inside a common slime coat. Different bacteria can act efficiently together. For example, one species will break down organic molecules, releasing hydrogen to methanogenic archaebacteria, which use the hydrogen to produce methane. Consortia of bacteria help to digest food in the stomachs of ruminants and help to form stromatolites. Bacteria usually grow faster in consortia than they do alone. Consortia of gut bacteria may even act on their host animal to produce sugars for them!

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. "Staph" infections of the bacterium Staphylococcus occur when millions of bacteria cover infected surfaces in a slime mat. Most serious medically, slime insulates bacteria from contact with antibiotics.

Some cyanobacteria today store oxygen in bubbles in their slime coating. The trapped bubbles are very useful: they act as an oxygen store during darkness so the bacterium can respire at night; they detoxify any poisonous sulfide that diffuses into the microenvironment; and they may inhibit the growth of nearby anaerobic bacteria. 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. How old is it? 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 bacteria existed for a long time before they evolved a rigid outer coat; perhaps until then they used the simpler alternative of slime.

Could slime be as old as life itself? 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.

Last updated March 5, 2006.

Links checked September 30, 2005.

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