We have all seen biofilms, but most people will just dismiss them as 'slime'. Behind this prosaic term lies a fascinating biology, so here is a quick discussion of these ubiquitous and important entities. Biofilms are everywhere - anywhere where there is moisture, in hostile environments, and even in the human body. You need go no further than your teeth to find an example - a bacterial mat resists our best efforts to dislodge it no matter how exotic the technology of the toothbrush, and goes on to form plaque . More seriously, biofilms colonise catheters, drip cannulae, long intravenous lines, and even the fluid maelstrom surrounding damaged heart valves, where they can grow into exophytic structures of bacteria and slime termed vegetations. Bits of these can break off, lodge in small blood vessels in the brain, and erode a hole in the blood vessel with catastrophic results. Medical significance apart, biofilms are found in drains, sewage systems, water pipes, rivers, beaches, aircraft fuel tanks, and nuclear reactors, where they have been found on fuel elements, thriving in spite of enormous radiation doses. They are capable, in some cases, of corroding stainless steel.
So what is a biofilm and why are they special? They generally arise when an organism, commonly a bacterium, adheres to a surface with which it comes into contact. Something very strange then happens. Due to a mechanism called quorum sensing, the first bacteria to arrive communicate with new arrivals ( bacterial communication is via chemical signals) and when sufficient numbers are present, the bacterial colony undergoes fundamental changes. These changes allow much firmer attachment - activating new gene profiles which confer entirely new properties on the colony. Some of these genes allow the bacteria to manufacture new proteins of various types, such as the polymer glues which attach the colony to the underlying surface and provide a protective coating - of slime. Secondary colonisation takes place with a host of other organisms - other bacteria, algae, fungi, and protists such as rotifers. A key component of the biofilm is the production of considerable amounts of polysaccharide, protein and DNA complexes which we refer to as 'slime' - an inadequate word for what are really complex materials, and the properties of the slimes are as diverse as the biofilms themselves. Within the biofilm slime, it has been discovered that fluidic channels exist through which chemical signalling takes place, altering the behaviour of organisms within the biofilm as well as the properties of the biofilm as a whole. If you can imagine a landscape of tiny towers of slime, the tops streaming downstream with the current, with channels of slow flow around the bases, you will have the general idea. Effectively, biofilms produce a 'boundary layer' of slow flow directly above the base surface, whilst above the slime film water can flow much more rapidly. The biofilm shown in fig.1 is growing on a concrete sluice, over which water flows very rapidly and sometimes with much force - a car was washed over it a few years ago. Microscopy (fig 1a) shows that it is heavily populated with delicate diatoms(at least 10 species were found in this example) and other organisms which are protected from being washed away by the protective slime. The slime film thus allows colonisation of otherwise impossible habitats. The biofilm in fig 1 is rich in diatoms, with filamentous algae and cyanobacteria and is brownish in colour. The biofilm in fig 2. is black/green in colour, and the picture shows some debris adhering to the slime on the vertical surface of a rainwater butt. Microscopy (fig 2a) shows it to be composed of tangled masses of cyanobacteria (also fig 2b), with filamentous and non-filamentous algae growing in the mucilagenous matrix, with protists such as bdelloid rotifers and ciliates grazing on the surface.
You will also find biofilms which look like pools of rust, and others which are a blackish goo stinking of sulphur, and yet more which resemble small oil spills.. These reflect the chemical properties of the bacterial colonies of which they are composed.
Biofilms may therefore be thought of as complex evolving communities of organisms with intrinsic cooperative food chains (ie one man's poison is another mans meat) and protection mechanisms (slime shrugs off detergents and the like). What is really interesting is the change in cell signal traffic, new gene activation, and mutation that occurs in the biofilm environment, and which means that the organisms in the biofilm may behave quite differently from the free-living state, and become adapted to the characteristics of the local enviroment. Some biofilms acquire astonishing properties. These include the ability to scavenge and concentrate contaminants, metals, and even radionuclides from their immediate environment. These properties are potentially of commercial and environmental performance, and researchers are working on the engineering of biofilms with desirable properties. Biofilm slime not only provides adhesion, it provides protection from antibiotics and disinfectants (making medical biofilms very hard to eradicate) and provides a special local environment, which may in a sense be self-contained, effectively isolating some of the organisms in the slime from the outside world. Biofilms protect themselves from the likes of man, and the organisms within them may be thousands of times more resistant to antibiotics than the same bacteria growing outside a biofilm. Within this community, self-sufficiency rules, and the different members of the community morph so that the community as a whole can have its viability needs met from the environment in which the biofilm grows, whether it be a pond, a sewage drain, a contaminated lagoon, or the fuel tank of your holiday jet. Within the layers of the biofilm are anaerobic bacterial regions, dark hypoxic regions, and regions where light penetration allows photosynthetic (algal) members of the community to generate oxygen, which allows the protists to graze in the oxic layer, and so on. All in all, a remarkable lesson in cooperative adaptation of many different types of organism
So, they're not pretty to look at, but they have an importance which is inescapable. They may clog your drains, or sludge your jet fuel lines. They make rocks slippery and make you fall over. If you have cystic fibrosis, they are quite literally the bane of your life promoting unrelenting lung infections. However they can also clean up toxic spills, scavenge and perhaps even mine metals, produce an array of potentially valuable proteins, the variety of which is so enormous that we have only just scratched the surface. These include antibiotics and anti-cancer drugs.
And now for a little bit of blue-skies speculation, almost unencumbered by scientific data! It is clear (and this is supported by data) that biofilms are self contained microenvironments. Researchers have simulated a Martian environment in terms of temperature, atmospheric composition and pressure, and other reproducible parameters. In it they placed a variety of extremophile organisms including bacteria and lichens. The bacteria and lichens had no trouble with this novel environment, indeed in another experiment lichens survived exposure to open space when deployed from a spacecraft - for a fortnight! In one sense, perhaps lichens and biofilms have something in common in that lichens are composed of symbiotic fungi and algae, packaged in a leathery wrapping whereby the alga meets the metabolic needs of the fungus and vice versa. So in a sense, the lichen may have its own spacesuit, with home grown internal life support. The same could well be true of biofilms - built to survive in the harshest of conditions within and protected by the ever-present slime. It is possible, and perhaps likely, that biofilms could survive in an extremely hostile atmospheric environment because they are capable of engineering their own, small scale life-friendly internal environment, sealed in and protected by - slime. The ubiquity of biofilms with their self contained, cooperative, adaptable, and sealed environment characteristics makes it plausible that when signs of life are eventually found in an extraterrestrial environment, it may well be as a biofilm. When one is eventually discovered, you will have heard about it first on Micscape!
Comments to the author, Hugo Baillie-Johnson, are welcomed.
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