Where do biofilms grow?

About Section 2

In this section we provide you with a glimpse of the many places in which one can expect to find biofilms. The surprising thing is that they seem to be everywhere. At least everywhere that has the remotest possibility of sustaining life. Recognizing this fact helps us understand the nature of biofilms and may help us learn how to treat harmful biofilms and how to exploit beneficial biofilms for good.

What biofilms need to grow

BIOFILMS grow just about anywhere. All it takes is:

• microorganisms
• moisture
• nutrients
• surfaces

Many, many different organisms, such as bacteria, can form a biofilm colony. As noted in section 1 of this module, over 500 different microorganisms have been found in typical dental plaque. Such microorganisms are all around us, on us, and in us.

What kind of moisture is needed? Biofilms grow in fresh water, salt water, oil pipelines, in the human body, and, well, you name it. Just about any kind of naturally occurring moisture will do.

What biofilms feed on is just as varied. Certain biofilms even thrive on petroleum oil. Interestingly, the capacity of this kind of biofilm to gobble oil has both a bad and a good side. Oil-eating biofilms can grow in and clog an oil pipeline; they can also be used to clean up an oil spill.

As discussed in section 1, to form a biofilm, microorganisms must "glue" themselves to a surface, form colonies, and reproduce. Virtually any surface will serve the purpose: rocks, countertops, human tissue, and so forth.

In the remaining subsections below, we identify some specific environments that support biofilm growth that are of interest to biofilm engineers and scientists.

Natural environments

As we stated above, biofilms grow virtually everywhere, in almost any environment where there is a combination of moisture, nutrients, and a surface.

As one might suspect, biofilms grow in rain forests in many different forms, such as moss, or lichen, on tree trunks. There is, after all, an abundance of moisture, nutrients and surfaces in a rain forest. What one might not suspect is that biofilms also grow in deserts. One common form of desert biofilm is known as "desert varnish," a term descriptive of the fact that the rocks, canyon walls, or sometimes entire mountain ranges on which they grow appear to be varnished with a reddish or other colored stain. Indeed, petroglyphs left on boulders and cave walls by early desert dwellers were often formed by scraping through the coating of desert varnish formations with a hard object.

The wide variety and adaptability of the microorganisms that form biofilms is evident in the extremes of environments—very hot to very cold—in which they are found. Microorganisms that thrive in such extreme environments are known as extremophiles, a self-descriptive term. Such microorganisms (for example, bacteria) can, and do, form biofilm colonies even in such environments.

They have been found at the bottom of the ocean near extremely hot ocean vents. Indeed, they seem to be the first living things to grow around newly formed vents. Other biofilms have been found living on glaciers in the Antarctic.

Yellowstone National Park in the United States offers visitors an amazing display of biofilms that grow as slimy algae in and around the many geysers, hot springs, and warm streams in the Park.

Depending on the water temperature in which they are found, the algae display a dazzling array of beautiful colors that are the subject of many a fine photograph. As stream flows change in the Park, colorful algae formations go dormant and turn a lifeless gray in dry stream beds. But not to worry. New algae growth springs up in the new stream beds, lending a dynamic, ever-changing view of the thermal features.

Biofilms are also an important life link in many ecological communities. The microorganisms that make up biofilms form the basis for food webs that nourish larger organisms such as insect larvae, which are consumed by fish, that are in turn consumed by birds and other animals.

Even plants can benefit from naturally occurring biofilms. One beneficial type of plant-biofilm relationship occurs in the plant roots. Plant roots secrete significant amounts of sugars, amino acids, vitamins and plant hormones that serve as nutrients for biofilms to grow on root hairs. This growth may facilitate the plant's ability to absorb nutrients from the soil. So a mutually beneficial relationship can exist between plants and biofilms.

You may wonder how biofilms get their start in such diverse environments. If there aren't any biofilms in a particular area, but, say, a new thermal feature suddenly develops in an area like Yellowstone National Park, or a new, hot vent opens up on the floor of the ocean, how does a biofilm form there? Here's how. It is known that the entire globe—both above and below ground—is "seeded" with the microorganisms that form biofilm colonies. They flourish, disperse, or become dormant depending on changing environmental conditions. That is, all of the ingredients for biofilm formation—microorganisms, moisture, nutrients, and surfaces—are just a natural part of our natural environment.

These ingredients are indeed all around us, on us, and in us.

Manufactured materials and systems

Whereas biofilms in nature tell a good story that helps us understand the biology of our world, biofilms in industry are slime of a different nature. It seems that biofilms are the culprits in a number of problems faced in industry.

We can relate a few examples of biofilm problems that plague industry. For example, biofilm contamination and fouling (sometimes referred to as biofouling) occur in nearly every industrial water-based process, including water treatment and distribution, pulp and paper manufacturing, and the operation of cooling towers. Biofilms are responsible for billions of dollars in lost industrial productivity, as well as product and capital equipment damage each year. Biofilms are notorious for causing pipe plugging, pipe corrosion and water contamination.

Rather than continue with a long list of examples, we will just present you with a slide show that illustrates instances of biofouling in industry.

Health and the human body

This may come as something of a shock to you, but about 90 percent of the cells in a human body are not human! In fact, the human body is heavily colonized by microbes that have found it a great place to live. We have communities of microbes living on all mucous surfaces and in our digestive tract, as well as on and in layers of our skin. For the most part we all get along; in fact, we depend on some of our gut microbes to help with digestion. Sometimes, however, the microbial load causes problems of infection. Dentists now understand, for instance, that dental caries (cavities) are the result of bacterial infection (and biofilms!). When the normal balance of microbial populations is upset or when our immune system is overwhelmed, we can have a real battle with microbial opportunists.

In the 1990s, as the biofilm concept was being introduced to the medical community, doctors began to make the connection between chronic, low-grade infections and the biofilm mode of growth. Dental professionals made the connection easily, as teeth could readily be scraped for microscopic examination. Internal cases of chronic infection have taken longer to prove, but testing has shown that many troublesome diseases have entrenched microbial populations at their core. Peptic ulcers, once thought to be caused by stress, have been proved to be caused by bacterial communities of Helicobacter pylori. The cyclical flare-up and subsidence of otitis media—the most common cause of children's recurrent earaches—is an example of a typical biofilm-based infection. Click to see Table 1-2, a summary of biofilms in medical contexts, the associated impacts, and journal references.