LEGIONELLA - World Health Organization

2.2.3 Environmental factors and virulence
The virulence mechanisms of Legionella are discussed in Chapter 1. Virulence is influenced by
environmental factors such as temperature, nutrients and sodium concentrations (Edelstein,
Beer & DeBoynton, 1987; Byrne & Swanson, 1998). At the same time, Legionella’s virulence
factors affect the ability of the bacteria to survive adverse environmental influences, such as
temperature extremes (Mauchline et al., 1994), ultraviolet (UV) light, low humidity and biocide
treatments (Rowbotham, 1980; Anand et al., 1983; Rowbotham, 1984; Barbaree et al., 1986).
Chapter 1 (Section 1.5) provides more information on environmental factors and virulence.
2.3 Biofilms
This section discusses the composition and formation of biofilms, their effect on bacterial
growth, and risk factors for the development of biofilms.
2.3.1 Biofilm composition
In 1901, Whipple noted how adherence to surfaces increased the bacterial activity of waterborne
microorganisms. Since then, many studies have recognized the importance of surfaces in
concentrating microorganism activity. Surface-associated microbial activity and colonization,
or “biofilm formation”, occurs worldwide in natural and artificial environments, and on a
range of different surfaces. Microorganisms, including L. pneumophila, form biofilms as a
mechanism to withstand adverse conditions, such as limited nutrients or temperature extremes.
Surface adherence usually occurs by means of an extracellular polysaccharide substance secreted
by the cells. This substance (the glycocalyx, or slime) is a hydrated polyanionic polysaccharide
matrix produced by polymerases affixed to the lipopolysaccharide component of the cell wall
(Morton et al., 1998).
At any stage in a biofilm’s development, portions of the film can be sloughed off by shear stresses
from the movement of water (see Figure 2.1) (Trulear & Characklis, 1982; Taylor Eighmy &
Bishop, 1985). This activity may resuspend the biofilm’s microorganisms within the system’s
water (Rowbotham, 1980), allowing them to colonize other parts of the system if conditions
are appropriate.
Microbial biofilms are extremely complex heterogeneous microbial ecosystems and may consist
of bacteria, algae and grazing protozoa. The latter may display morphological features not
usually associated with microorganisms when grown in pure culture (Cloete et al., 1989).
2.3.2 Biofilm formation
During biofilm formation, the surface to which the film will attach is first conditioned by
nonspecific binding; this process is followed by colonization of pioneering microorganisms,
which multiply to form microcolonies or stacks. The microcolonies are protected by a glycocalyx
layer, but portions can be sheared off and recolonize other parts of the system, as described
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS