LEGIONELLA - World Health Organization

above. Fluid flow around the microcolonies (represented by curved arrows in Figure 2.1)
carries nutrients, and the surface is grazed by protozoa (if present), which releases nutrients
and clears surfaces, thus aiding growth.
Biofilms, which may include legionellae and protozoa, can form on the surfaces of poorly
managed buildings or cooling towers (see Figure 2.1). The biofilm facilitates nutrient and
gaseous exchange, and protects microorganisms not only from biocides but also from periodic
increases in temperature and attempts at physical removal, especially in areas where surfaces
are scaled or corroded. Biofilms can form at interfaces, particularly at those between water
and solid surfaces, but have also been found on oil–water interfaces (e.g. in metal-working
fluids). Biofilms are more likely to form where there are areas of low water flow and where
water is allowed to stagnate.
Studies aimed at characterizing bacterial interaction within biofilm ecosystems have evaluated
the effects of parameters such as temperature and surface materials on the growth of L. pneumophila,
and have investigated the effect of biocides on planktonic and sessile legionellae (those
attached to the surface material) (Green & Pirrie, 1993; Walker et al., 1993, 1999; Rogers et al.,
1994; Moorer, 1996; Atlas, 1999; Surman, Morton & Keevil, 1999; Murga et al., 2001; Keevil,
2003). Most studies of Legionella and biofilms use naturally occurring microbial communities,
and therefore give a true picture of such communities (Colbourne et al., 1984; Colbourne &
Dennis, 1985; Verissimo et al., 1990; Storey, Ashbolt & Stenstrom, 2004b). However, some
of the organisms present in biofilms have yet to be identified, and their contribution to the
survival and multiplication of legionellae remains unknown.
Within a biofilm, microorganisms are embedded in an extracellular matrix that provides structure,
stability, nutrients and protection from possible toxic effects of the substrate upon which the
biofilm grows (e.g. copper pipes in water distribution systems). Gradients of nutrients, pH
and oxygen within the matrix support the varying needs of different microorganisms in the
heterogeneous population (Wimpenny, Manz & Szewzyk, 2000; Allison, 2003). Legionellae
grown in biofilms are more resistant than the same bacterial species in the water phase of the
system (Barker et al., 1992; Cargill et al., 1992; Surman, Morton & Keevil, 1993; Santegoeds,
Schramm & de Beer, 1998).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS