LEGIONELLA - World Health Organization

2.3.4 Risk factors for biofilm growth
Biofilm prevention is an important control measure against proliferation of Legionella. Preventing
the growth of biofilms is important because, once established, they are difficult to remove from
complex piping systems (see also Chapter 4).
Various factors increase the likelihood of biofilm formation, including:
• the presence of nutrients, both in the source water and in the materials of the system
• scale and corrosion
• warm water temperatures
• stagnation or low flow as occurs in the deadends of distribution system pipework and in
storage tanks.
The presence of scale and corrosion in a system will increase the available surface area and allow
the formation of microniches that are protected from circulating disinfectants. Scale and corrosion
also increase the concentration of nutrients and growth factors, such as iron, in the water system.
Uncontrolled biofilms can occlude pipework, resulting in areas of poor flow and stagnation
with higher risk of Legionella growth. Furthermore, the presence of both biofilms and protozoa
has a twofold protective effect for the bacteria in the system, because it increases the organic
load and inactivates residual levels of disinfectant. In addition, biofilm and bacteria (including
Legionella spp.) grown inside protozoa are more tolerant of chlorine and other antimicrobial
agents at concentrations above those commonly used to disinfect water supplies and shown
to be lethal under laboratory conditions (Barker et al., 1992).
The materials of the system also affect the growth of biofilms. Some plumbing materials support
or enhance the proliferation of microorganisms, including Legionella spp. (Rogers et al., 1994).
Natural substances, such as rubber gaskets, provide a nutrient-rich substrate and are preferentially
colonized by microorganisms; some plastics leach nutrients into the system. Microorganisms
will even grow on the surface of systems plumbed with copper, which has an inherent resistance
to colonization, once the surface has been conditioned.
Most engineered aquatic systems — especially those that are complex (e.g. those in health-care
facilities and hotels) — have areas containing biofilms, even when the system is well maintained.
When control measures, such as the disinfection regime, are relaxed, microorganisms will
quickly multiply to detectable levels.
Legionella contamination can originate from small areas of a water system that are not exposed
to temperature fluctuations or circulating disinfectant. An example of this occurred in a large
teaching hospital in the United Kingdom, where legionellae were intermittently detected at
one sentinel outlet, despite the fact that there was a comprehensive control regime in place.
The source was eventually tracked down to a 10-centimetre length of water-filled pipe where
there was little or no flow (a “deadleg”). When this section of pipe was removed, subsequent
sampling remained negative (John Lee, Health Protection Agency, UK, personal communication,
June 2005).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS