LEGIONELLA - World Health Organization

considered as potential new species. A more detailed identification can be carried out in reference
laboratories.
Both environmental strains and clinical isolates can be successfully subtyped by molecular techniques,
such as ribotyping, macrorestriction analysis by pulsed-field gel electrophoresis (PFGE), or
PCR-based methods (Schoonmaker, Heimberger & Birkhead, 1992; Pruckler et al., 1995; Van
Belkum et al., 1996; Ballard et al., 2000). The most accurate way to compare epidemiologically
linked environmental and clinical isolates is to use two typing methods — genotyping (e.g.
PFGE) and phenotyping (monoclonal subtyping) — in parallel (Drenning et al., 2001). An
internationally recognized typing method using amplified fragment length polymorphism
(AFLP) was tested on Legionella infections by 11 countries within the European working
group (Fry et al., 2000; 2002). This method allows culture media to be compared without
the need for transportation. In future, sequence-based typing methods, such as that described
by Gaia et al. (2003), will be more commonly used. A consensus sequence-based scheme
based on this previous work, using a standard protocol and dedicated web site 27 will greatly
assist in timely epidemiological investigation, particularly of travel-associated cases of infection
caused by L. pneumophila (Gaia et al., 2005).
11.4.3 Identifying appropriate sampling sites
Selection of sampling sites depends on whether the sampling is for routine monitoring or to
investigate an outbreak. The use of PCR for detecting nucleic acids of legionellae in the environment
has been valuable in some investigations of legionellosis outbreaks, and is particularly useful
for eliminating epidemiologically and geographically implicated sources. Quantitative methods
are being developed for determining whether a potential environmental source is above guideline
or mandatory levels contained in national legislation, where available (Ballard et al., 2000).
The use of PCR to detect legionellae in the environment shows that up to 80% of fresh water
is positive; this compares with only 20–40% when using culture to detect Legionella. The
discrepancy could be due to the presence of non-viable or injured organisms, viable but
non-culturable legionellae, a nonspecific reaction with unrelated organisms (although data
suggest this is not the case), or the presence of new species of legionellae.
The number and types of sites that should be tested to detect legionellae must be determined
on an individual system basis. This is because of the diversity of plumbing, heating, ventilation
and air-conditioning systems in the various institutions that may be sampled, which include
industrial facilities, hotels, hospitals, retirement homes, public facilities and domestic environments.
In 1987, an environmental sampling protocol was published, dealing with selection of appropriate
sites to sample within a hospital (Barbaree et al., 1987) (see Table 11.2). This protocol can
serve as a prototype for identifying sites that should be sampled in various institutions.
27 http://www.ewgli.org
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS