Bacteria Are Not Usually Floating Alone. They Build Cities
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Most bacteria in the world live stuck to surfaces inside a slime they build themselves, which changes their behaviour and makes them extraordinarily difficult to kill.
What the structure is
Bacteria attach to a surface, multiply, and secrete a matrix of sugars, proteins and released genetic material that glues them together and to the surface. The result is a structured community rather than a smear, with channels running through it that carry water and nutrients inwards and waste outwards. Cells in different regions behave differently, since those near the outside get oxygen and nutrients while those deep inside are starved and dormant. The whole assembly may contain several species living together.
Why it resists treatment
The community survives doses that would kill the same bacteria floating free:
- •The matrix physically slows antibiotics reaching the interior
- •It also binds and neutralises some of them chemically
- •Dormant cells deep inside are unaffected by drugs targeting growth
- •Those survivors repopulate once treatment stops
- •Genes for resistance pass easily between neighbours in close contact
- •Immune cells struggle to penetrate the matrix at all
Where they cause problems
Anything wet and solid is a candidate surface, which is why the list of affected settings is so long. Dental plaque is the most familiar example and was the first to be studied in these terms. Implanted medical devices including catheters, joint replacements and heart valves are colonised readily, and such infections frequently require the device to be removed rather than treated. Chronic wounds fail to heal partly for this reason. Industrially, the same process fouls pipes, ship hulls, cooling towers and water treatment membranes, and it accelerates corrosion of metal beneath it.
The useful ones
The same arrangement is put to work deliberately in several industries, which is easy to forget given the medical framing. Sewage treatment depends on communities growing on filter media or suspended in flocs, which do the actual work of breaking down waste. Anaerobic digesters producing methane rely on them. Cleaning contaminated ground uses them to degrade fuels and solvents. Fermented foods including vinegar and some cheeses involve them. And the mats of microbes that build layered structures in shallow water produced the earliest fossils of life, which are the same phenomenon three thousand million years earlier.
The talking they do
Cells in these communities coordinate using chemical signals, which is one of the more surprising findings in bacteriology. Each cell releases a small molecule continuously, and each detects the concentration around it, so the concentration indicates how many cells are nearby. Above a threshold, the population switches on genes together, including those for building the matrix, for producing toxins and for dispersing to colonise elsewhere. That arrangement lets a population act only when there are enough of it to matter, and interfering with those signals is an active line of research into treatments that disperse rather than kill.
The takeaway
Bacteria stuck to a surface inside a self-made matrix form a structured community with channels, with starved dormant cells deep inside and active ones at the edge. The matrix slows and binds antibiotics while the dormant cells ignore them, so survivors repopulate after treatment. Chemical signals let the population count itself and switch on genes together.