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
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
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
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
These bacterium can quickly proliferate to replace the ones
killed by the chemicals and continue to produce additional
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
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.