Excerpt from:

Pathogens in Natural and Engineered Water Systems: Emerging Issues
Tamim Younos, Valerie J. Harwood, Joseph O. Falkinham III, and Hua Shen

P.12 •
Water Resources IMPACT , Volume 9 • Number 3                                                              May • 2007


Bacterial growth in water distribution systems has been investigated for several decades. For example,
Baylis (1930, cited in van der Kooij, 2003) reported
coliform growth in sediments accumulating in water distribution
pipes. Researchers have found that mycobacterial numbers were substantially higher in the water distribution
systems (on average 25,000-fold) than those collected immediately downstream from the water treatment
facilities, indicating that mycobacteria grow in the distribution system (Falkinham et al. 2001). In recent years,
there has been great concern about the presence of emerging pathogens such as
Legionella spp., Mycobacterium
spp., and Aeromonas spp. and other opportunistic pathogens in water distribution pipes and home plumbing
systems. It should be noted that both Legionnaires’ disease (a serious, life-threatening pneumonia) and Pontiac
fever (a mild, flue like illness) are caused by members of the genus Legionella. Two pathogens of concern in
water systems are discussed below. [Admin note:
Standard test suppresses the colifom aeromonas]

Mycobacterium avium (M. avium) is an environmental opportunistic bacterial pathogen, a normal inhabitant of
natural waters and drinking water distribution systems, and has been listed on the USEPA’s Candidate Contaminant
List (CCL) (Table 1). They are opportunistic pathogens of humans, animals, and fish and are very slow
growing organisms, reproducing, at the most, 1 generation/ day. Slow growth makes them poor competitors for
resources against other microorganisms; however, their die-off rates are lower and they show evidence of adaptation
to harsh conditions. Mycobacteria are very hydrophobic, which makes them resistant to antimicrobial
agents. Their hydrophobicity leads to their attachment to surfaces and proliferation on them, i.e. biofilm formation
(attached populations of microbial cells and polysaccharide that form at interfaces and surfaces such as pipe
walls) because they can grow in waters with relatively low organic nutrient concentrations (> 50 μg assimilable organic
carbon per liter). Biofilm formation increases M. avium persistence in drinking water distribution systems.
Hydrophobicity is also a major contributor to M. avium’s aerosolization and concentration in aerosols above waters.
M. avium numbers increase in recirculating hot water distribution systems in hospitals, office buildings, and apartment
houses. The use of showers and hot tubs (spas) are risk factors for development of M. avium pulmonary disease
because of M. avium aerosolization potential and the entrainment of M. avium- rich biofilms into aerosols upon water

Legionella pneumophila (Legionella), an emerging bacterial pathogen (Table 1), was discovered following an
outbreak of pneumonia amongst attendees at the 1976 American Legion convention in Philadelphia, Pennsylvania.
It is a Gram negative bacterium, commonly found in fresh water environments and replicates in
protozoa as
intracellular parasite. The bacterium Legionella exists in low numbers in natural waters. However, higher water
temperatures in engineered water systems promote rapid growth of Legionella (Fields et al., 2002).

Legionnaires’ disease has been linked to aerosols generated from air conditioners, humidifiers, decorative
fountains, whirlpool spas, and industrial or residential cooling towers. An opportunistic pathogen, the bacterium
causes disease in individuals with weakened immunity. Large outbreaks of disease have been reported in
news media. For example: in Virginia, September 1996, a whirlpool spa display at a home improvement store
caused 23 cases of the disease, and killed 2;
in Spain in 2001, a hospital cooling tower caused 449 cases of the
disease and killed 2;
in September 2005, a Toronto outbreak in an assisted living facility killed 17 people. The
majority of Legionella pneumonia cases, however, are sporadic community infections which are possibly-under
diagnosed and underestimated. In the U.S., it is estimated that up to 20,000 cases occur annually, and mortality
of the disease is up to 26 percent (Benin et al., 2002).  In 2003, the Centers for Disease Control (CDC) reported
increased cases in the Mid-Atlantic region, increasing to 178 cases versus 48 cases in 2002. Disease control and
prevention depends on a rapid, sensitive, and quantitative detection method. Also, since the bacterium is common in
engineered systems, it is important to establish an acceptable level of the bacteria in these water systems.

Maintaining a disinfectant residual in water distribution systems can be an effective way to prevent bacterial
growth of some pathogens in water distribution pipes. However, the challenge is how to balance the need for adequate
disinfection while reducing the potential chronic health effects of chlorination without selecting for other
potential pathogens. Also, it should be noted that some pathogens are resistant to chlorination (M. avium is resistant
to chlorine, chloramines, chlorine dioxide, and ozone). Recently, across the U.S. there is a trend to
use chloramines instead of free chlorine as disinfectant, since there is less potential for generating harmful
byproducts. However, little is known about potential health effects of switching to chloramines that could arise from
increased microbial survival or growth in the drinking water distribution system. Furthermore, a recent study
indicated that a switch from free chlorine to chloramines disinfectant triggered lead release from home plumbing
pipes (Edward and Dudi, 2004). Exposure to high levels of lead in drinking water can pose significant health risk
to society.

Tamim Younos
Research Professor of Water Resources
Virginia Tech, Department of Geography
Blacksburg, VA 24061-0444
(540) 231-8039 / Fax: (540) 231-6673