Most bacteria thrive in an aqueous environment. Combine people, heat and a stagnant body of water and you may have the ideal conditions for bacteria to grow. Water provides the medium, people’s skin and bodily secretions provide both bacteria and the nutrients to feed them, and warmth increases the bacteria’s proliferation rate.

To control this biologic process, we usually try to kill bacteria with chemical disinfectants such as chlorine, bromine or ozone. These oxidizers kill bacteria very efficiently, but only if bacteria are suspended in the water — not when they become attached to surfaces and create a bacterial community known as biofilm.

In order to treat biofilm, we must understand what it is and how it reacts with traditional sanitizers.

The bio on biofilm

Recent research shows that many types of bacteria can attach and form complex bacterial communities on any surface that is in contact with water. These communities are known as biofilm. Imagine a coral reef: a colony of billions of tiny sea creatures that make their home by secreting a material that hardens into a rock-like structure. Take this coral reef and make it microscopic, replacing the coral animals with bacteria, and you have biofilm.

Whenever bacteria, water and a surface are combined, the bacteria can migrate to the surface and set up shop. The bacteria adhere to the surface and produce and secrete a sticky exopolysaccharide matrix of complex sugar molecules that also incorporates proteins, nucleic acids and other compounds from the immediate environment.

This complex mileau protects the bacteria from attack by chemicals or other agents that can destroy them. Sheltered by this biofilm, they grow and proliferate, increasing their biomass as long as they have sufficient nutrients and enough fluid flow to remove their waste products. As the biofilm matures, it develops a complex array of channels that bring nutrients closer to the cells and more efficiently remove waste products. Life in the biofilm community also allows the bacteria to communicate and share information among their members to enhance survival in the face of attack, reduced nutrient levels or desiccation of their fluid environment. Small pieces of biofilm or individual bacteria can dissociate from the biofilm mass and float away to establish a new community in a previously unaffected location. In this way, the biofilm can rapidly populate an entire water system.

A spa contains the perfect environment for biofilm formation.

Controlling biofilm formation

To try to control this bacterial growth, we typically use chlorine, bromine or other chemicals that kill bacteria on contact. These chemicals are efficient killers of bacteria that swim free in the water. But we now know that they are not very effective against the bacteria in biofilm.

Recent research from the Center for Biofilm Engineering at Montana State University, and others, has shown that the biofilm matrix absorbs chlorine, bromine and other bactericidal compounds onto the sticky exopolysaccharide matrix that covers the living bacteria. The bactericides may kill the bacteria closest to the surface, but billions of bacteria remain unharmed in the depths of the biofilm. These bacteria can quickly proliferate to replace the ones killed by the chemicals and continue to produce additional biofilm.

The problem is that sanitizing chemicals don’t deeply penetrate the biofilm to kill most of the bacteria within it. Furthermore, the longer the spa is used, the more biofilm forms on it — and the more biofilm that forms, the more chemicals are needed to kill the bacteria suspended in the water. Because a spa is a closed system, as you add more chlorine, bromine and other chemicals, the concentration of those chemicals in the water keeps increasing.

Over time, this creates a sort of chemical soup of odiferous, foaming and turbid water. This is why spa manufacturers recommend completely draining and refilling most spas every three months. But draining a spa does nothing to kill or significantly harm the ever-growing biofilm, which remains largely stuck to all surfaces in contact with water, including pipes, pumps, heaters, filters and the spa shell. Even if the spa is allowed to dry completely, persistor cells — deep within the biofilm — respond by going dormant. They can remain in this suspended animation for years, waiting for a more hospitable environment to return. When the spa is refilled with water, these dormant bacteria become metabolically active, and can resume proliferating and make more biofilm.

Mossy solutions

Current research in our laboratory has found that certain species of sphagnum moss may effectively penetrate biofilm and reduce bacterial proliferation. Sphagnum moss is a soft, leafy plant that grows in bogs.

There is real potential value of sphagnum moss for pool and spa owners. Its antibacterial properties help keep the water clear and clean, and water treated with sphagnum moss may only require 20- to 30 percent of the chlorine or bromine it would otherwise need.

Work is under way to develop additional uses for this plant, which helps maintain the delicate bacterial balance in the water we use. As its potential in water conditioning and purification is realized, we hope to see the cultivation, harvesting and marketing of sphagnum moss become an industry of its own.