From the editor:
This is the fifth post in an ongoing series profiling microorganisms of particular significance in water and wastewater systems. In this series we will cover microorganisms of many types including some that are beneficial, inhibitory and pathogenic.
Legionella is a mesophilic, pathogenic organism that can cause Legionnaire’s Disease, a potentially fatal form of pneumonia. While it can be found in natural environments, it starts becoming a concern in man-made systems. It grows optimally at 25-42°C and is transmitted to humans through the inhalation of contaminated water vapours, making things such as cooling towers, hot water tanks, shower heads and hot tubs particularly susceptible. Legionella and Legionnaire’s Disease were first identified in the late 1970’s, when 221 people fell ill from an unknown disease at an American Legion convention in Philadelphia, Pennsylvania. Legionella, which was breeding in the convention hotel’s air conditioning cooling tower, was eventually identified as the cause of the sickness. The outbreak resulted in at least 34 deaths.
Incidents of Legionnaire’s disease has steady risen over the last two decades. While it is likely underdiagnosed, 6,100 cases were identified in the USA in 2016 alone. To help reduce this trend, in 2015 the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) developed Standard 188: Legionellosis: Risk Management for Building Water Systems, which provides guidance on minimum Legionella risk management requirements for building water systems. In the same year, after a series of fatal Legionnaires Disease outbreaks in the Bronx, New York City and New York State became the first jurisdictions in the USA to implement mandatory Legionella monitoring for cooling towers. Monitoring requirements consist of routine (weekly) general microbial monitoring (ie. 2nd Generation ATP, heterotrophic plate counts, dip slides) for risk identification and quarterly Legionella specific testing. Currently, the ‘gold standard’ for Legionella specific testing is done using a culture method. However, due to the long incubation period (7-14 days), recent research has focused on using molecular techniques, such as quantitative polymerase chain reaction (qPCR), to provide faster and more accurate results.
Within a building’s water system, Legionella flourish within biofilms. Biofilms grow on the surface of infrastructure and consist of microorganisms stuck together with extracellular polymeric substances (EPS). They facilitate nutrient exchange, increase biocide resistance, protect against increased temperature and protect against shear stress. For instance, Legionella rely on other microorganisms within the biofilm to provide various required nutrients, such as the amino acid L-cysteine, that they cannot provide for themselves. This highlights the applicability of including general microbial monitoring in a Legionella monitoring plan. Increased microbial risk and increased Legionella risk go hand-in-hand.
Control measures for Legionella typically fall into three categories: temperature control (<20°C, >50°C), disinfection (oxidizing, non-oxidizing or UV) or point-of-use filtration. Which control strategy to use is highly dependent on the type of water system being managed. For instance, non-oxidizing biocides will not be used in a potable water supply.