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Bacterial Biofilms, Quorum Sensing, and Antibiotic Resistance

Linda Crampton is a writer and teacher with a first-class honors degree in biology. She often writes about the scientific basis of disease.

This is a biofilm created by Staphylococcus bacteria on a catheter. The fibrous slime connects bacterial cells together.

This is a biofilm created by Staphylococcus bacteria on a catheter. The fibrous slime connects bacterial cells together.

Bacterial Communities

Bacteria are interesting and abundant microorganisms that sometimes exist as separate cells. Many bacteria grow in colonies called biofilms, however. In a biofilm, bacteria are attached to each other and to a solid surface, They are also surrounded by a protective material that they secrete. The bacteria in the colony communicate and cooperate with each other. In fact, a biofilm acts almost like a multicellular organism instead of a colony of single cells.

Biofilms form in our bodies. Unfortunately, the bacteria in these films are often harder to attack with antibiotics than bacteria living outside the films. This makes some infections difficult to treat. Studies of biofilm bacteria will hopefully lead to improved treatments for some very unpleasant diseases.

Bacterial biofilms are present on human and animal teeth and gums and are known as dental plaque. This yawning animal is a type of monkey known as a drill.

Bacterial biofilms are present on human and animal teeth and gums and are known as dental plaque. This yawning animal is a type of monkey known as a drill.

Where Are Biofilms Found?

Biofilms form in many places. They generally develop on a surface that is surrounded by liquid, such as our teeth and gums, where the film is known as dental plaque. Biofilms may also develop in other areas of the body, such as on mucosal membranes in sinuses and airways and in wounds. In addition, they can form on metal, plastic, and other artificial materials. They may develop on medical devices such as contact lenses, artificial heart valves, joint replacements, urinary catheters, and dental implants. Showers and water pipes may contain biofilms, and so may drains and sewage treatment plants. The films can also form on rocks in streams and aquariums.

A biofilm often has a slimy consistency and can be thick enough for us to see without magnification, as in the photo below. It may contain one species of bacteria, several bacterial species, or a mixture of bacteria, fungi, algae, and other organisms.

A bacterial biofim in the water of a peat bog photographed by a botanist

A bacterial biofim in the water of a peat bog photographed by a botanist

How Does a Biofilm Form?

The first step in biofilm production is the formation of a conditioning film on a surface. Body fluids and liquids such as pond and sewage water contain polysaccharides, proteins, and other substances that are quickly deposited on any solid that is placed in the liquid. The deposit, or conditioning film, provides nutrition and a suitable habitat for bacteria.

Free-floating bacteria (also called planktonic bacteria) land on the conditioning film and stick to it. The bacteria have several means of adhesion. For example, some have many small projections called pilli on their cell wall, which help them stick to other bacteria and to surfaces.

The bacterial cells become surrounded by a protective material that they release, which is sticky and helps attach them to each other and to the solid surface. This material is known as EPS (Extracellular Polymeric Substance). It contains polymers such as polysaccharides, proteins, and sometimes DNA. A “polymer” is a large molecule made of smaller molecules joined together.

The EPS of a biofilm protects the bacteria from attack by the host’s immune system. As new bacterial cells arrive or are made, they attach to the solid surface and/or to other bacteria in the biofilm. This causes the film to enlarge.

DNA, or deoxyribonucleic acid, contains the genetic code of an organism. It's present in bacterial cells, human cells, and the cells of all other living things. The presence of extracellular DNA (DNA that is outside cells) appears to be necessary for the formation of a bacterial biofilm.

Biofilms and Infections in Humans

Numerous human diseases are known to be caused by bacterial biofilms. These diseases include:

  • sinusitis (inflammation of the sinus)
  • endocarditis (inflammation of the inner layer of the heart muscle, especially around the heart valves)
  • Legionnaires’ disease (a type of pneumonia caused by a bacterium named Legionella pneumophila)
  • prostatitis (inflammation of the prostate gland)
  • otitis media (inflammation of the middle ear)
  • a persistent lung infection by Pseudomonas aeruginosa (which occurs in many cystic fibrosis patients)

One problematic factor in biofilm infections is that the film has the ability to release a group of bacteria. These bacteria can then create another biofilm in a different part of the body.

Pseudomonas aeruginosa is an interesting bacterium. It contains a pigment that fluoresces under UV light. It can form biofilms in the lungs and on medical equipment.

Pseudomonas aeruginosa is an interesting bacterium. It contains a pigment that fluoresces under UV light. It can form biofilms in the lungs and on medical equipment.

Signaling Molecules in Bacteria

As strange as it may sound, the bacteria in a biofilm communicate with each other. They do this by releasing chemicals called signaling molecules or autoinducers. The signaling molecules can affect other bacteria in a process called quorum sensing.

In quorum sensing, bacteria change their gene expression based on the concentration of signaling molecules in their environment. "Gene expression" means that a gene becomes active, instructing the cell to make a specific substance.

A signaling molecule released by one bacterium joins to a receptor on the surface of another one, potentially triggering the activity of a specific gene. Whether or not the stimulation happens depends on how many signaling molecules are present. The microbes can not only detect the presence of the molecules but also their concentration.

The Quorum Sensing Process

When there aren't many bacteria in a biofilm, there is a low level of signaling molecules in the extracellular material. As the population grows, the concentration of the molecules becomes more concentrated. A threshold level of signaling molecules must be present before the bacteria respond. When they do respond, they all change their behavior at the same time and in the same way as a specific gene (or genes) is turned on.

Quorum sensing is used by bacteria to coordinate activities that are more useful when done in a group than by a single bacterium. Examples of these activities include the production of substances that aid the attack on the host or help the production of the biofilm. The process was discovered in species of bacteria belonging to the genus Vibrio that produce light when enough of the bacteria are stimulated. One of these species was Vibrio fischeri, a marine organism that lives in the light-producing organs of certain squid and fish and produces light when its cell density is sufficient.

Bonnie Bassler is the microbiologist who discovered quorum sensing in bacteria. She is currently associated with Princeton University in the United States. Professor Bassler discusses quorum sensing in the video below. Understanding the details of the process is very important with respect to controlling the actions of microbes.

Bacteria growing in a biofilm can be up to 1,000-fold more resistant to antibiotics than the same bacteria grown planktonically.

— Karin Sauer, Binghamton University, State University of New York

Antibiotic Resistance

The outer cells of a biofilm protect the inner cells from attack by disinfectants and antibiotics. The outer cells are more active than the inner ones. Antibiotics might be able to kill the active surface cells of the biofilm, but they don't affect the less active, non-dividing inner cells, which are often called persister cells. When the antibiotic treatment is finished, the persister cells can multiply and build up the film again.

Antibiotics are sometimes able to kill planktonic bacterial cells but are much less effective at killing biofilm bacteria. Scientists haven't known about biofilms for very long compared to some other aspects of the life of bacteria. They need time to understand how the organisms in the film function and to create more effective medications to destroy them. Some of the activities performed by the microbes living in biofilms are different from those of their free-living counterparts.

Antibiotics are medications that kill bacteria. Bacteria living in biofilms are much harder to kill with antibiotics than free-living ones.

Antibiotics are medications that kill bacteria. Bacteria living in biofilms are much harder to kill with antibiotics than free-living ones.

Destroying Bacterial Biofilms

The list of bacteria known to form biofilms is increasing. Some researchers estimate that up to 80% of human microbial infections are caused by biofilm bacteria. Up to 90% of the bacteria in all environments may live in films.

There is a great deal of research being done to find ways to stop the formation of biofilms or to remove them once they form. Silver compounds in bandages have shown some success in breaking up biofilms in wounds and making the bacteria inside them more susceptible to antibiotic attack. Silver has long been known to kill planktonic bacteria. However, research is showing that a much higher concentration of silver is needed to affect biofilm bacteria. Silver coatings are sometimes applied to catheters and have reduced infections. There is some concern about the safety of high concentrations of silver inside the human body, however.

We mostly don't get sick. Most often, bacteria are keeping us well.

— Bonnie Bassler

Good and Bad Bacteria

We need a better understanding of quorum sensing in bacteria. We also need to discover efficient ways to disrupt biofilms created by harmful microbes as well as ways to interfere with bacterial communication. It's important to remember that not all bacteria are bad, however. Some species are very helpful for us.

Bonnie Bassler studies communication between bacteria. As she says in the quote above, many bacteria in our bodies help us instead of making us sick. This is especially true for the ones that live in our intestine. Many of our intestinal microbes make and release important chemicals that our body absorbs and uses. In the case of the bacteria that help us, it might be advantageous to enhance quorum sensing.

Although they can sometimes be a nuisance or dangerous to humans, researchers find bacterial biofilms very interesting to study. There is a lot to learn about the behavior of the bacteria in the films. They are certainly more complex organisms than many people realize.


  • "Unlock the secrets of bacterial biofilms - to use against them" from a professor at the Binghamton University, State University of New York via The Conversation
  • Information about a link between antibiotics, bacterial biofilms, and chronic infections from the phys.org news service
  • "What is quorum sensing?" from the University of Nottingham
  • "Biologists Discover Bacteria Communicate Like Neurons in the Brain" from the University of California San Diego.
  • Strategies for combating bacterial biofilm infections from the International Journal of Oral Science
  • Vibrio fischeri biofilm facts from Oxford Academic

© 2011 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on July 10, 2013:

Thanks for the visit, Rob. The ways in which species evolve and develop new features are certainly fascinating!

Rob on July 10, 2013:

Good stuff. I see people watching for improvements in science, but nature has been dealing with this all along, with or without our cognitive awareness. Look around and see. Now that we know what we're looking at, we'll see it. Plants changing whole biochemistries to deal with predator attack, change their taste and nutritional qualities? They aren't so helpless after all either. I expect it will unfold infinitely and science will always be saying, well, would you look at that! Certainly I see no end in sight at this rate. But keep learning. It's good.

Linda Crampton (author) from British Columbia, Canada on May 03, 2012:

Thank you for the comment, ssiddhanta!

ssiddhanta on May 03, 2012:

Solid information. Thank you :)

Linda Crampton (author) from British Columbia, Canada on October 16, 2011:

Hi, Georgia. Most persister cells exist in biofilms, but a low concentration of persister cells seems to be present in some cultures of planktonic bacteria too.

Georgia on October 16, 2011:

I'm a student in secondary education and have been researching persister cells for a while now in relation to E. coli, and I have a question hopefully you could help me with. Is is possible for persister cells to emerge in a colony which has lost it's ability to produce a biofilm, or simply in planktonic bacterium?

Linda Crampton (author) from British Columbia, Canada on July 02, 2011:

Hi, Prasetio! Thank you very much for the comment and the rating.

prasetio30 from malang-indonesia on July 02, 2011:

This was valuable information. Thanks for share with us. I learn much from you. Rate it up!


Linda Crampton (author) from British Columbia, Canada on June 28, 2011:

Thank you very much, Danette. I appreciate your comment. I love biology and want to write about it in a way that people without a science background can understand and hopefully find interesting as well.

Danette Watt from Illinois on June 28, 2011:

Who would have thought the subject of bacteria could be so interesting! Well, I thought it was and you did a great job of putting this in a way we 'non-scientists' could understand. I especially appreciated it when you added definitions to some of the terms such as 'gene expression.' There is a lot about the natural world -- on all levels -- that we have yet to learn.

Thanks for the interesting hub.

Linda Crampton (author) from British Columbia, Canada on June 27, 2011:

Hi, b. Malin. Yes, it is a bit scary that bacteria have more advanced abilities than we realized. However, a lot of scientists are researching the problem of killing bacteria in biofilms, which is a hopeful sign!

b. Malin on June 27, 2011:

I have to agree with CMHypno. This Hub was quite a learning Experience as well as a little scary but there is so much out there and I guess we can't live in fear. Science will hopefully conquer in the future. Thanks for sharing, Alicia. Enjoyed the Video's as well.

Linda Crampton (author) from British Columbia, Canada on June 26, 2011:

Hi, CMHypno. I find the functioning of the human body very interesting! Thank you for the visit and comment.

CMHypno from Other Side of the Sun on June 26, 2011:

Fascinating Alicia. You know so much about the human body! I had never heard of biofilms before, so thanks for all the great information.