LEGIONELLA - World Health Organization

Figure 2.1 Biofilm formation
Biofilm
Multiplication
Colonisation
Conditioning layer
planktonic
Grazing protozoa
Source: Kindly supplied by Susanne Surman-Lee
Shear stresses
(some species more susceptible)
2.3.3 Effect of biofilms on bacteria growth
Bacteria attached to surfaces and particulate matter within a system are more resistant to
biocide treatments (Ridgway & Olson, 1982; Kuchta et al., 1985; King et al., 1988), making
biocides less effective and allowing the proliferation of potential pathogens (LeChevallier
et al., 1988; Wright et al., 1991).
The presence of biofilms is therefore an important factor for Legionella survival and growth
in water systems (Kramer & Ford, 1994; Rogers et al., 1994; Williams, Molinari & Andrews,
1996; Martinelli et al., 2000; Goossens, 2001). Small numbers of legionellae are found in sources
such as distributed drinking-water supplies, which then feed into water systems within buildings
and cooling towers. This provides a logical explanation for the presence and subsequent growth
of legionellae in these artificial aquatic environments (ASHRAE, 2000; WHO, 2004).
The availability of complex nutrients in biofilms has led some researchers to propose that
biofilms support the survival and multiplication of legionellae outside a host cell. Growth
within a biofilm composed of naturally occurring waterborne microorganisms, in the absence
of protozoa, has been shown in a model system study. Cycloheximide — which inhibits protein
synthesis in all eukaryotic cells, and affects initiation, elongation and termination, (Oleinick,
1977) — was added in high doses to the system. Growth increased in the absence of protozoa,
with both the heterotrophic count (the number of all microorganisms) and the Legionella
count increasing (Surman, Morton & Keevil, 1999; Surman et al., 2002). Rogers & Keevil (1992)
used immunogold labelling of Legionella to show the existence of microcolonies of legionellae
within biofilms. Another study demonstrated that multiplication of Legionella in a biofilm model
was due solely to intracellular multiplication in amoebae (Murga et al., 2001).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS
biofilm
Biofilm sloughing off