LEGIONELLA - World Health Organization

too low to be detected using culture methods. Such water can potentially introduce legionellae
into storage tanks and systems within the built environment, where the physical and chemical
conditions encourage their growth.
2.2 Factors affecting growth of Legionella
This section discusses the effect of temperature, other microorganisms and virulence factors
on the growth of Legionella.
2.2.1 Influence of temperature
Legionellae have been isolated from hot-water systems up to 66 ºC; however, at temperatures
above 70 ºC they are destroyed almost instantly (Dennis, Green & Jones, 1984; Dennis, 1988b).
Kusnetsov et al. (1996) found that growth of all strains tested decreased at temperatures
above 44–45 ºC, with the growth-limiting temperature being between 48.4 ºC and 50.0 ºC.
The Legionella strains studied produced carbon dioxide up to 51.6 ºC, suggesting that some
respiratory enzymes survive at this temperature. Complex water systems, such as warm-water
plumbing systems, air-conditioners and hot tubs (also known as spa pools), are increasingly
using water in the temperature range that encourages Legionella growth. In addition, these
water systems can potentially produce aerosols, increasing the spread of the bacteria.
Strains of L. pneumophila have been shown to have a decimal reduction time (D) 4 of 80–124 minutes
at 50 ºC, and of 2 minutes at 60 ºC (Dennis, Green & Jones, 1984; Schulze-Robbecke, Rodder
& Exner, 1987). Isolates can be collected easily from many different environmental aquatic
sources with temperatures between 30 ºC and 70 ºC (Fliermans, 1984). For example, legionellae
have been isolated from frozen rivers, thermal ponds and springs, and aquatic sources in the
vicinity of a volcano (Tison & Seidler, 1983). Yee & Wadowsky (1982) showed that naturally
occurring L. pneumophila survived and multiplied in water at temperatures between 25 ºC
and 45 ºC, with an optimal temperature range of 32–42 ºC. The study also found that legionellae
were most commonly isolated at temperatures between 35 ºC and 45 ºC, with the greatest
increase in viable counts occurring between 37 ºC and 42 ºC (Wadowsky & Yee, 1983; Schulze-
Robbecke, Rodder & Exner, 1987). As the temperature falls below 37 ºC, the bacteria’s reproductive
rate decreases and there is little or no increase in numbers of bacteria below 20 ºC.
Therefore, to prevent Legionella infection, the recommended temperature for storage and
distribution of cold water is below 25 °C, and ideally below 20 °C. Recent laboratory studies
of mutant Legionella strains show that the bacteria may grow below 20 ºC under certain conditions
(Soderberg, Rossier & Cianciotto, 2004). Legionella will survive for long periods at low
temperatures and then proliferate when the temperature increases, if other conditions allow.
4 The “decimal reduction time” is a unit of microbial heat resistance, defined as the time required to kill
90% of a population of microorganisms at a constant temperature and under specified conditions.
0 LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS