LEGIONELLA - World Health Organization

Construction materials — risk factors
In the past, water supply systems were generally constructed of metallic materials such as cast iron,
galvanized iron, brass or copper. Metallic plumbing materials are increasingly being replaced
with synthetic materials such as polyvinyl chloride (PVC) and polybutylene. These different
construction materials vary in their potential to support microbial growth and biofilms. For
example, synthetic materials may leach organic compounds that may provide a source of nutrients
for microorganisms subsequently colonizing them (Colbourne & Ashworth, 1986), and copper
is more resistant to colonization than synthetic materials; however, metallic materials are
more prone to corrosion and this can encourage biofilm formation.
Certain natural materials such as hemp and natural rubber components promote biofilm
formation and thus promote the growth of legionellae more than metallic materials, both in
laboratory conditions and in practice (Niedeveld, Pet & Meenhorst, 1986). Hemp is a traditional
jointing compound, and natural rubber components are often present (together with plastic
materials) in pressure compensating vessels, and in flexible tubes and shower hoses.
Accumulation of sludge, scale, rust, or algae or slime deposits in water distribution systems supports
the growth of Legionella (WHO, 2004).
Despite its natural resistance to biofilm formation, copper pipework can become corroded
through biodeterioration, mediated by microorganisms. This is a particular problem in areas
with soft water (Keevil et al., 1989). In some cases, dosing regimes with chlorine-based biocides
have led to the failure of plumbing systems, requiring costly replacement (Keevil et al., 1989;
Grosserode et al., 1993). The risk of colonization, therefore, should be balanced with other
risks linked to the choice of materials, such as dissolution, corrosion and scaling.
Disinfection — risk factors
Chemical disinfection may not be effective against Legionella that are found in protozoa
(Kilvington & Price, 1990). In addition, the complexity of many piped water systems, particularly
in old buildings, makes effective disinfection difficult; for example, booster disinfection may
not be effective in a complex system.
Presence of biofilms — risk factors
Bacteria in drinking-water systems tend to adhere to surfaces and develop an organic protective
matrix, creating microenvironments known as biofilms (discussed in Chapter 2, Section 2.3).
Legionellae can thrive in biofilms, either directly or as parasites of certain protozoa that graze
on the films.
Temperature — risk factors
Risks from legionellae may be greater in warmer regions (subtropical and tropical), because
temperature is an important factor in the ability of the microorganism to survive and grow.
Published information about Legionella concentrations in drinking-water distribution systems
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS