Like many municipalities in urban and suburban areas, San Bruno, Calif.’s source water comes both from its own groundwater supply and through a...
Water specialists should make Legionella reduction a top priority
Part 1 of this article (January 2002) reviewed legionellosis
(the diseases caused by Legionella infection), the sources and transmission of
Legionella bacteria and conditions that tend to promote colonization. A
discussion of the pros and cons of routine monitoring, as well as various recommended action limits, also was included.
In order for Legionella outbreaks to occur, a series of
events needs to occur.1 These
events are shown in the left column in Table 1. The factors that affect the
likelihood of these events are shown in the center column, and those that
represent opportunities for facilities managers to reduce the risk of
legionellosis are identified in the right column.
If monitoring is appropriate, samples should be collected at
a variety of locations throughout the system, similar to coliform monitoring.
The number and type of samples should be guided by the circumstances and
objectives of monitoring, but for purposes of this discussion it will be
assumed a facility is thought to be at risk for Legionella and a baseline of
data is required. A thorough walk-through documenting the characteristics and
features of the system along with the operational parameters and controls,
should precede sample collection.
Subsequent to the initial site survey, samples should be collected from some or
all of the following sites.2,3,4
• Incoming water supply
storage tank and water heater
• Representative hot and cold water faucets
cooling towers, evaporative condensers, humidifiers, spas, showers, etc.
water entering or leaving other equipment under suspicion (ice machines,
misters, spray bottles, plastic injection molding equipment, etc.)
Legionella samples may be collected as either swabs or water samples or
preferably both. At faucets and showers, swab samples, first-draw water samples
and post-flushing water samples all should be collected. It is critical that
taps not be flushed prior to sampling as stagnated water data may be
particularly revealing. In addition, biofilm research has shown that in most
habitats, bacteria grow preferentially on surfaces rather than in the aqueous
phase.5 Swabs collected from the
inner walls of faucets have demonstrated equivalent sensitivity to bulk water
samples, while resulting in higher recovery of Legionella.6 This is significant because outbreaks
of Legionnaires’ disease have been linked to exposure to elevated levels
of Legionella and suggested remedial actions have been based on Legionella
concentration.7,8 Swab samples can
be collected by removing the faucet or showerhead and thoroughly swabbing the
interior surfaces. Swab samples should be collected using swabs with transport
media or neutralizing buffer or site water to prevent desiccation.
Water samples should be collected in sterile, plastic
bottles, with sodium thiosulfate added to neutralize chlorine or other
oxidants. Typically, 250 milliliter (mL) to one liter (L) samples are collected
for routine monitoring purposes, although up to 10 L samples may be warranted
in some situations. Samples should not be refrigerated but should be protected
from temperature extremes by shipment in an insulated cooler. Samples should be
sent via overnight delivery to a qualified, experienced laboratory and analyzed
within forty-eight hours of collection.9 The ISO method recommends that the time interval
between sample collection and concentration be two days or less and should not
exceed five days.10
When collecting bulk water samples, the initial temperature
of the water should be recorded as the water runs into the sample bottle.
Following collection of the “first draw” water sample, the water
should be run continuously and temperature allowed to stabilize. The time to
stabilize and final temperature also should be recorded, and then a post-flushing
sample should be collected. Water heater samples should include water and
sediment from the bottom drain and also water from the outlet (if possible).
Humidifiers, spas, etc., should be sampled at their water reservoirs as well as
the supply water. Cooling tower samples should include supply water, storage
tanks, reservoirs, tower pond or basin (distant from make-up water entry and
circulation system return) and any standing water in condensate trays or
cooling coils. Samples should be collected from each tower (or each system if
they are interconnected).
A variety of analytical methods is available for the detection of Legionella.
Screening tests include Direct Fluorescent Antibody (DFA) techniques and
Polymerase Chain Reaction (PCR) techniques, although these methods may not
distinguish between viable and non-viable Legionella and DFA is subject to
cross-reactivity with other organisms. Commercial PCR kits for the detection of
Legionella at the genus level have been demonstrated to be inadequate due to
lack of specificity.11
The traditional culture method for Legionella (still the
“gold standard”) involves plating samples on selective agar and
incubation in a carbon dioxide (CO2) enriched atmosphere for up to 10 days at approximately 37° C. Buffered Charcoal Yeast Extract (BCYE) supplemented with antibiotics such as glycine, polymixin, vancomycin and cycloheximide, (referred to as BCYE-DGVP or BYCE-GVPC) to retard other bacteria and fungal growth is used to
selectively recover Legionella from mixed environmental matrices. Suspect
colonies characteristic of Legionella then are harvested from the BCYE-GVPC
plates (Figure 1) and patch plated to straight BCYE and BCYE-Cys plates for
presumptive identification (Figure 2). (Legionella requires the presence of
L-cystine for growth and positive isolates grow on BCYE-Cys but not on straight
BCYE). Legionella speciation can be confirmed by DFA, PCR or cellular fatty
acid (CFA) analysis. Acid or heat pretreatment of samples may be used to help
clean up the sample and eliminate non-target bacteria. However, pretreatment
may also kill some strains of Legionella, especially weakened isolates, and
reduce the overall concentration of Legionella recovered.12 The culture
procedure detects only viable, culturable organisms and is the generally
accepted standard procedure for Legionella testing. The Centers for Disease
Control and Prevention (CDC) method, Standard Methods #9260J, British Standard
(BS) 6068-4.12-1998 and International Organization for Standardization (ISO)
11731 all are variations of this general procedure.
Supplemental Assays. In
addition to Legionella-specific assays, monitoring programs should include
standard heterotrophic (bacteria) plate count (HPC) analyses and microscopic
examination of the water for amoeba and protozoa.
HPCs are frequently performed, but are commonly—and
erroneously—interpreted as less expensive, surrogate tests for Legionella
control. Managers should not regard general bacterial count data as replacements
for Legionella monitoring. Research has demonstrated that HPCs do not
adequately predict the presence or absence of Legionella in water systems or
the risk of disease.13 However, control of Legionella by necessity includes
controlling the populations of other microorganisms and shifts in HPC;
Legionella ratios or a failure to control HPC levels may indicate a system at
risk, even in the presence of relatively low levels of Legionella.14
Follow up Sampling.
If periodic monitoring yields positive Legionella results, the analytical data
and site risk factors should be reviewed by facility managers and/or safety
personnel to determine the appropriate corrective actions. Various
organizations worldwide have developed criteria for guidance or regulation that
may be helpful to facility managers. However, considerable variation exists
between recommended “action levels” (see Part 1 for discussion). If
the results prompt a change in biocide treatment or operational practices,
follow-up sampling should include Legionella-specific culture assays and not be
limited to HPCs.
System Operation and Disinfection
Excellent operational guidance for facility potable and
industrial water systems including cooling towers, humidifiers, spas and
fountains are available from American Society of Heating, Refrigeration and
Air-Conditioning Engineers, Inc. (ASHRAE), Occupational Safety and Health
Administration (OSHA), CDC, Cooling Technology Institute (CTI) and others. The
reader is directed to these sources for more information (Table 2).
A variety of disinfection techniques has been used in
conjunction with Legionella-contaminated water systems including
hyperchlorination, ultraviolet (UV) light, ozone, thermal eradication,
instantaneous superheating systems, copper-silver ionization and other
techniques. For these or other methods to be successful, all Legionella in the
water column, inside commensal microbes and in biofilms must be killed. The
effectiveness and costs (direct, financial and associated labor) can vary in
practice, and information is readily available in the literature.8,15,16 A
thorough review of disinfection techniques is beyond the scope of this article.
Well-designed water distribution and cooling systems,
coupled with sound management and operational procedures, are essential to
control Legionella in industrial facilities—and a monitoring program
should not be considered as a replacement. However, most experts even those
ill-disposed towards routine Legionella monitoring, would agree that monitoring
should be considered if enough legionellosis risk factors apply to the system
in question. Testing decisions should be based on a thorough review of the
system, operations and population including items such as source water quality,
system design, operational parameters and history, biofouling information,
transmission sources, host susceptibility and Legionnaires’ Disease case
history. No management program, regardless of its treatment, maintenance or
monitoring components, can guarantee the absence of future legionellosis, but
prudent operational practices combined with ongoing review of risk factors will
allow facility managers to minimize exposure to Legionella and to its legal consequences.