Bacteria have many ways of tolerating stressful circumstances. Some have special genes coding for robust stress-tolerance systems which allow them to keep out toxic substances, pump out toxic substances, or fortify themselves against lethal conditions. Others take common stress-tolerance systems and make them work overtime. But more often than not, bacteria survive stressful situations through forming a community.*
This community is known as a biofilm, which isn’t overly informative, but is certainly shorter to say than “amalgamation of bacteria, viruses, dead cells, and extracellular goo,” which is what a biofilm actually is. Biofilms can be 3D and very dense, or one layer of cells thinly covered in extracellular matrix (goo), as can be seen in the two different images below. These images both show biofilms made up of a single type of bacteria, but in nature, they’re more likely to be made up of many different types. While not strictly speaking a multicellular organism, these communities do communicate with each other (more on that later) and help each other.
So what makes a biofilm helpful to the bacteria?
- Many sanitizers can’t get through the gooey layer covering the biofilm in order to kill the bacteria. Consequently, bacteria in a biofilm are much more likely to survive sanitation procedures.
- The bacteria in the biofilm are often experiencing some level of stress (oxygen stress towards the center, nutrient stress, toxic byproduct building up, etc). This primes them to survive harsher stresses.
- The close contact allows bacteria to exchange DNA easily, which allows antibiotic resistance genes, stress resistance genes, and virulence genes to be passed around between the biofilm’s inhabitants.
- Biofilms are extremely resistant to being scrubbed off or even cooked off. Even if all the bacteria in the biofilm are killed, chances are the biofilm’s extracellular matrix (goo) components will still be left on, which makes it easier for new bacteria to attach to the surface.
So what do bacteria need to form a biofilm?
- A sufficient number of bacteria to make it practical. I’m not sure anyone knows what this number is, but bacteria do take a headcount via a sort of molecular sonar (quorum sensing).
- A surface they can attach to. Bacteria can attach to wood, plastics, glass, and many other surfaces, so this offers a lot of possibility.
Where do biofilms form?
- On the surfaces of fruits and vegetables, especially near the stems.
- In your kitchen sink, most likely
- On your cutting board (especially if it’s wood)
- On food processing equipment
- On meats/poultry
- In drains
- On your teeth (yes, plaque is a biofilm)
- In your gut
- In catheters inside patients
- On contaminated artificial joints/implants in humans
- Virtually anywhere
What can we do about it?
Unfortunately, prevention is the only cure. Once formed, biofilms tend to come back again and again. The takeaway from a household perspective is that bleaching your kitchen sink/cutting board is probably not going to make it sterile for very long. For a food industry, it means that if they find evidence of a biofilm, they have to replace that part of or even the entire piece of equipment. From a medical standpoint, sterility is paramount for catheters, implants, and and other common hosts of biofilms.
*This is not the kind of article where the author moralizes on the importance of communities in human stress tolerance. However, I will note that, anecdotally, communities of people who care about you are very helpful in surviving stress.
This is a nice online, plain-text chapter on biofilms by researchers at the University of Montana: https://www.cs.montana.edu/webworks/projects/stevesbook/contents/contents.html
Also a good resource: https://mpkb.org/home/pathogenesis/microbiota/biofilm