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

To control this biological 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 the bacteria is suspended in the water — not when it becomes attached to surfaces and creates a bacterial community known as biofilm.

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

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, producing and secreting a sticky exopolysaccharide matrix of complex sugar molecules that also incorporates proteins, nucleic acids and other compounds from the immediate environment.

This complex milieu protects the bacterium from attack against 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 bacterium to communicate and share information among members to enhance survival in the face of attack, reducing 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. But they can form in almost any aqueous environment.

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 bacterium remain unharmed in the depths of the biofilm.

These bacterium 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 an aquatic vessel 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 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 and filters. Even if the vessel is allowed to dry completely, persister 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 vessel is refilled with water, these dormant bacteria become metabolically active, and can resume proliferating and make more biofilm.

Bottom line? Watch out for biofilm and if you see it, clean, clean, clean.