How Biofilms Work


When microbial biofilms bind together sedimentary grains, they can form stromatolites such as these on the coast of Australia.
When microbial biofilms bind together sedimentary grains, they can form stromatolites such as these on the coast of Australia.
iStockphoto/Thinkstock

At first glance, it's not clear what dental plaque, the persistent slime in your shower drain and a slippery submerged rock have in common besides the fact that they can be a headache -- or in some cases a toothache -- to remove. To the naked eye, it's nearly impossible to see what's responsible for these lined surfaces.

If you look closer, with the help of a microscope, you'll realize these slimy aggregations are anything but dull. Instead, they're often diverse, tiny communities of living microorganisms bound in a matrix, or a thick, adhesive substance. Who would've guessed the grimy buildup in your toilet bowl is a complex clump of living, communicating cells?

Though Antoni van Leeuwenhoek, the discoverer of bacteria, described similar formations when he studied his own dental plaque in the 17th century, it wasn't until the 20th century that scientists had the tools they needed to take a closer look at how the structures develop [sources: Montana State University CBE, Costerton and Wilson].

These colonies, also called biofilms, form when single microorganisms attach to a hydrated surface and undergo a "lifestyle switch," giving up life as a single cell to live on a surface in an adhesive cell matrix with other microorganisms [source: Lemon et al.]. Some definitions state that biofilm cells "irreversibly attach" to a surface, which means they can't be removed by gentle rinsing [source: Donlan].

But why should we care about biofilms?

For starters, they can attach to both living and nonliving surfaces (including humans), create problems in the medical field, alter industrial production practices and even contribute to environmental cleanup. In addition, some researchers estimate that biofilms constitute more than half of the world's biomass [sources: Montana State University CBE; Sturman]. Biofilms are so abundant it's surprising that we don't notice them more.

In this article, we'll learn about how biofilms work, how they're both problematic and beneficial, and what researchers are doing to control them. To begin, let's look at the life cycle of a biofilm.

What exactly makes a cell give up flying solo in the first place? We'll find out on the next page.

Life Cycle of Biofilms: Before Attachment

Before we delve into the life cycle of a biofilm, it's important to get a sense of what's usually found in these cell clusters. Microorganisms, or organisms too small to see with the naked eye, are the building blocks for biofilms. Different species of bacteria, protozoans, algae, yeasts and fungi can form biofilms. With most biofilms ranging from a few microns to hundreds of microns (one micron being one-millionth of a meter) in thickness, it's no wonder scientists prefer to use microscopes for the job.

So what's needed to start a biofilm?

Generally, all you need is a hydrated surface submerged in water or some other aqueous solution, microorganisms and favorable conditions. It's not that simple, though. Not all biofilms grow at the same rate or even require similar conditions to survive -- each type of microbe has its own needs. Still, there are some factors that can affect biofilm attachment and growth regardless of species:

  • The availability of nutrients in the hydrated sample
  • The physical and chemical characteristics of the surface, including its polarity
  • The thickness of the conditioning layer, or material already attached to the surface
  • pH levels
  • Temperature
  • The amount of shear, or rate of water flow, in the sample
  • Antimicrobial levels in the sample
  • The number of species in the sample
  • Whether the microorganisms can move on their own
  • The cellular structures of the microbe (appendages)
  • The types of metabolic interactions between cells

Ultimately, it's essential to understand that microorganisms don't necessarily "think" while forming a biofilm; it just happens if the conditions are favorable. If a microbe is pushed by water flow or accidentally bumps into a surface, it may or may not attach the first time, or even at all for that matter. It's unclear what causes a cell to attach to a surface, and some researchers say a combination of factors -- including shear rates, electrostatic forces, conditioning layers (debris already on the surface) and nutrients available to the microorganism -- is more influential than a single factor [source: Sturman].

With microorganisms often at the mercy of their environments, it's amazing how something as small as a bacterium can hold on to a surface to settle in its new home.

Read on to understand why starting a biofilm is a slimy affair.

Biofilm Colonization and Development

Biofilms often grow as algae around hot springs, creating a display of bright colors.
Biofilms often grow as algae around hot springs, creating a display of bright colors.
iStockphoto/Thinkstock

The transition from a free-moving microorganism to an immobile one is what makes biofilms unique in comparison to cells growing in a test tube. But how can microorganisms stick to a surface long term?

First, you'll need to know that once a free-floating cell starts a biofilm or becomes part of an existing one, it uses different genes to create proteins and other substances that will help it adapt to its new lifestyle. Switching genes "off" and "on" can result in the cell changing its behavior. For instance, some genes control whether a microbe such as a bacterium can move on its own, while others can command the cell to go into a dormant state if conditions are harsh. It's similar to the genes that allow us to digest the enzymes in milk as infants, but can be switched "off" after the weaning period, as seen in individuals with degrees of lactose intolerance [source: Bowen].

Regardless of species, all biofilms contain extracellular polysaccharide substances, also called EPSs [source: Lemon et al.]. The term may sound complicated, but just think of EPS as part of a sticky matrix of sugars, proteins and other genetic material released from cells in a biofilm. EPSs not only help hold the cells of a biofilm together, they play a significant role in protecting the colony. In most cases, EPSs make up the majority of a biofilm's mass [source: Christenson and Characklis].

After latching on to a surface, a cell will produce a sticky matrix with EPSs to root itself better and make it easy for other cells to join the colony. Once other cells stick to the matrix and decide to stay, they produce an adhesive matrix as well.

Before you know it, microbes in the biofilm have created elaborate, three-dimensional structures that resemble gluey towers when viewed under a microscope. While some biofilms have only a few cells in them, others can have millions -- and sometimes billions -- of cells intertwined in a single biofilm slime. But as we'll note later, biofilm growth can be slowed or stopped sometimes, mostly by competition among cells and environmental factors [source: Sturman].

Interestingly, communal life also makes it easier for cells to send signals to one another through a method called quorum sensing. This activity helps cells pass information about their neighbors and surrounding environment to one another. Quorum sensing is known to cause changes of behavior in cells and may provide insight into why cells decide to detach from biofilms; however, the meanings of these signals are not fully understood yet by scientists [source: Donlan].

Can the concept of strength in numbers be applied to biofilms? Read about how biofilms are micro-cities of their own on the next page.

Biofilm Interaction

In a sense, biofilms are like cities. Similar to city dwellers, microorganisms pass up solitary life to live communally [source: Watnick and Kolter]. We'll use Watnick and Kolter's analogy describing biofilms as "cities of microbes" to understand how cells in a biofilm interact.

As we discussed earlier, microbes colonize surfaces to build the foundation of a biofilm. Before settling down, some cells move around using flagella or other mobile structures until they find a suitable place to stay -- much like how new residents of a city visit different neighborhoods before choosing a home.

After moving in, new residents may add a room to their new home to create more space for people in a crowded house. In comparison, cells in a biofilm will produce those extracellular polysaccharide substances (EPSs) to include new cells from the outside and other cells created within the community. At a basic level, both cities and biofilms offer their residents protection from outside forces. For bacteria, these forces can be antibiotics or even the human immune system [source: Lemon et al.]. Scientists think a biofilm's overall thickness and density provide some protection [source: Montana State University CBE].

Also, communicating with your neighbors can be easier if you live closer to them. The same principle applies to cells in a biofilm during quorum sensing, when cells are near enough to signal effectively. Researchers hypothesize that this method may be used to establish boundaries between different biofilm colonies as well [source: Watnick and Kolter]. Living in biofilms makes it easier for cells to conjugate, or transfer genetic material, too.

Another important concept to remember is that biofilm structures are flexible. This is why they can be problematic. Most scientists use the term viscoelastic to describe biofilms, meaning they can be stretched like putty when the flow of a liquid pulls or pushes on the colony [source: Montana State University CBE]. These shear forces, or rates of liquid flow, can shape a biofilm colony and cause clumps to disconnect or tumble away.

What if our newcomers to the city grow tired of living in a crowded area? They may move somewhere else. Cells in a biofilm can do the same by detaching from the colony, regaining their mobility and continuing life as floating microorganisms. Detaching may be a harder task for cells embedded beneath other layers of cells and EPSs. After detaching, a microbe may even start its own biofilm or live in another established cell community. We don't know what causes detachment, but scientists say that species type, environmental pressures and competition within the biofilm all play strong roles. Like humans and other animals, microorganisms often move elsewhere to survive when the going gets tough.

Keep reading to learn how these microbe cities can be harmful to people.

Biofilms and Medical Problems

Biofilm formation that occurs in an indwelling catheter, such as this one show on an electron micrograph, may lead to staph infections.
Biofilm formation that occurs in an indwelling catheter, such as this one show on an electron micrograph, may lead to staph infections.
Image Courtesy CDC/Rodney M. Donlan, Ph.D; Janice Carr

Have you ever wondered why getting your teeth cleaned at the dentist is necessary? You already brush your teeth on your own, right?

With more than 500 individual species of bacteria in our mouths, it's safe to say biofilms may pose a problem if we let them [sources: Cromie; Montana State University CBE]. Brushing your teeth removes dental plaque, a type of biofilm on teeth, yet removing it from hard-to-reach areas near our gum lines is often left to the expertise of a dental hygienist. If left on our teeth, dental plaque can lead to cavities and periodontitis -- the medical term for gum infection.

Outside your mouth, biofilm-related health problems are more common than you might think. What's alarming about biofilm infections is the fact that some aren't easy to get rid of and can be tolerant of antimicrobial treatments such as antibiotics. This means some medicines won't work for people who are ill from biofilm infections. Biofilms can cause a variety of health problems, ranging from a common earache to a specific bacterial infection found in people living with a genetic disease called cystic fibrosis.

But biofilms are particularly an area of concern for patients with implanted medical devices. They have been found on some devices more than others, including:

  • Catheters, or tubes inserted in the body to deliver treatment or remove bodily fluids (especially central venous catheters and urinary catheters)
  • Prosthetic joints
  • Mechanical heart valves
  • Pacemakers
  • Contact lenses
  • Endotracheal tubes, used to help breathing or administer anesthesia
  • Intrauterine devices used as contraceptives

In hospital settings, microbes that form biofilms usually enter a patient's body from being transferred on the implant or inside the patient from the patient himself, visitors or hospital staff beforehand. Certain types of biofilms, such as those from the genus Staphylococcus, are more harmful because they release toxins and can be highly resistant to antibiotics, especially if they form biofilms in a patient's body (see How Staph Infections Work to read more).

Getting rid of biofilms, especially staph bacteria, can be a challenge for patients with implants, but there are a few options. Sometimes removing the implant will do the trick, unless the biofilm has formed on live tissue [source: Donlan]. Other techniques include applying stronger doses of antimicrobial drugs to the surface of the implant before it's placed inside a patient or experimenting with implants lined with silver, which has antimicrobial properties.

Unfortunately, there's no universal treatment for medical biofilms in the long run. Preventing biofilms from forming in the first place is the most promising tactic. Patients should always consult their doctors about possible treatments for biofilm infections.

The human body isn't the only arena in our battle against unwanted biofilms. Read on to learn how these persistent clusters negatively affect industry settings and the environment.

Biofilm Damage to the Environment and Industry

As we've learned, these communal microbes can adapt to live on many surfaces, including our teeth and in our bodies, but the vast majority of biofilms are found in nature. For instance, you may feel the presence of biofilms on rocks in a shallow body of water, creating a slippery surface to traverse. Unlike biofilms studied in the lab, these aggregations occur naturally and are one part of a larger ecosystem.

Today, our impact on the environment often results in imbalances in ecosystems. For example, waste runoff can cause an area to have higher levels of certain nutrients than usual. To some microorganisms, this means more food to eat, and their populations may grow out of control as a result. But in order to break down nutrients, some microbes require oxygen, and they will use more than usual to break down a surplus of nutrients. This removal of oxygen from an ecosystem can cause problems for other organisms that share the same habitat, sometimes resulting in dead zones. If given the nutrients to grow out of control, both free-floating microorganisms and sedentary biofilms can flourish and use all of the oxygen in an area, making an environment hard or impossible to live in for other microbes and animals.

In industrial environments, biofilms are a force to be reckoned with. Since most production facilities use water to cool equipment or depend on pipes to transport resources, there's a substantial risk of developing biofilms on these equipment and piping systems. According to one estimate, biofilms cause well over a billion dollars' worth of damage every year in industrial settings, affecting human health and companies' abilities to manufacture their products efficiently [source: Montana State University CBE; Sturman]. Papermaking facilities are especially at risk for biofilm problems, because manufacturing paper requires a lot of water and provides a warm and nutritious environment for microorganisms to grow [source: Sturman].

Biofilms can also negatively affect the quality of drinking water. After waste water is treated, it flows through clean pipes that transport it to our faucets. But in some cases, biofilms can be a nuisance in this process. Scientists at water treatment facilities found that biofilms still form in the pipes that carry clean water, which re-contaminates the water. After studying the issue, they learned that clean drinking water that has been treated contains organic carbon -- a tasty meal for bacteria. Fortunately, removing organic carbon from processed water limits these bacterial biofilms from forming in clean water pipes, granting the water a safe trip to your faucet [source: Sturman].

Biofilms can cause problems, but they can also be beneficial. Continue to the next page to read about how biofilm technology can clean up environmental messes.

Benefits of Biofilms

As we just learned, microorganisms can cause imbalance in an environment if the conditions are right. Ironically, that's why microbes can be beneficial, too. For instance, it turns out that the same nutrient-hungry bacteria that break down carbon in treated water can also restore balance to an area by eating excess carbon when the situation arises.

When oil accidentally winds up in nature (as seen in oil spills), microbes slowly break down oil particles. Since oil is primarily made of carbon, there are a variety of bacteria that break down small oil molecules for food. With this line of thinking, it's feasible to say that biofilms can be potential tools to clean up environmental messes. Using biofilms in this way is an example of bioremediation, or returning an environment from an altered state back to its natural one with the help of microorganisms. Though collecting oil and running it through a biofilm filter of some sort isn't a common method to clean up oil spills today, it may be an interesting option to explore in the future.

Biofilms even have their place in the mining industry. Quite often, valuable ore is separated from normal rock in mining settings. But in the presence of water and oxygen, certain types of leftover crushed rock can create a sulfuric acid solution if left alone. Once the reaction takes place, this acid and other runoff are hard to clean up and can pollute nearby water sources. But if you take out a part of the equation, the rock material won't become acidic and can be disposed of differently. It turns out that placing biofilm-forming bacteria that need oxygen on these rocks will strip the element from its surface and disable this acid runoff from forming [source: Sturman].

In addition to bioremediation, biofilms can be used in biofilm trickling filters to treat waste water [source: Sturman]. In this process, biofilms are grown on rocks or pieces of plastic to clean wastes out of the water slowly trickling through. On a small scale, this process is efficient enough, but most municipal water treatment centers still rely on larger quantities of bacteria to treat wastewater.

Biofilms also benefit other organisms in nature. Underground, microorganisms will form a biofilm around the rhizosphere, or the area between roots and soil, in plants. Chemical interactions in this symbiotic relationship grant both parties access to nutrients that would otherwise not be available. Biofilm formation on plant roots is one of many examples of why biofilms are ecologically important.

Think you know biofilms yet? Find out ways to battle unwanted colonies next.

Tools to Battle Biofilms

The largest problem with biofilms is that they seem to form on areas that are either difficult to access or too delicate to treat. Removing a catheter infested with biofilm bacteria and replacing it with a new one will do the trick; but what about people who depend on their artificial heart valve to survive?

Finding answers isn't easy. Depending on the biofilm, some can be scraped off gently while others may have a strong enough attachment to corrode the surface they call home. And despite your previous success with pressure washing your home or using water flow to rid yourself of dirt, increasing the water flow from your faucet will not peel off biofilms. Sure, you'll remove some cells in the short run, but the biofilm will become accustomed to the higher fluid shear, and will grow a thinner, but more tenaciously attached biofilm in the long run [source: Sturman]. Even if you succeed in removing a biofilm by force, how can you ensure you evict every last cell?

This is why researchers study biofilms at the molecular level. They want to know which conditions will make different species of microbes detach from the biofilm colony and surface. Scientists are coming closer to understanding this relationship in some species already. In one study, researchers found a specific protein that seemed to control detachment in a few types of bacteria [source: Davies et al.]. Similarly, other researchers focus on finding the genes that allow cells to attach to a surface or biofilm in hopes they may reveal how to detach microbes or even weaken their cell matrix to increase the effectiveness of antimicrobials.

Research efforts also concentrate on ways to implant medical devices while reducing biofilm infections. The prevailing idea is that using a combination of techniques may work depending on the species in the biofilm and the condition of the patient. If you want to do your own part in preventing microorganisms from spreading in medical environments, make sure to wash your hands, avoid touching patients unnecessarily and listen to medical staff members about what you can do to help keep the spread of microbes down.

Perhaps the largest weapon against unwanted biofilms is preventing them from colonizing in the first place. Keeping surfaces clean is always a step in the right direction.

Ultimately, it's important to realize that until the appearance of advanced microscopes, very little was known about biofilms. Now, these colonies are being studied in depth by thousands of scientists in different disciplines around the world. We still have a long way to go, but researchers are working hard to answer biofilms' toughest questions. Any discovery enhances our knowledge of these fascinating colonies and brings us closer to controlling biofilms and using them for the better.

For more resources on biofilms, check out the next page.

Related HowStuffWorks Articles

More Great Links

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