Legionella Testing / Biosafety Training


Richard W Gilpin PhD Blog

Does Legionella survive potable water chlorination? Yes

How does Legionella survive potable water chlorination?

Chlorine residuals in potable water from municipal water supplies usually ranges from 0.1 mg/L to 1.0 mg/L (ppm) with a permissible upper limit of 4.0 mg/L.

We tested the chlorine effects on two Legionella species and two other aquatic bacteria back in 1985 and published the results in Water Research Vol. 19, No. 7, pp. 839-848 1985.

We studied the levels of free available chlorine needed to kill four aquatic bacteria in chlorine demand-free distilled water, pH 7.4.

We found that when we started with 6,000,000/ml of these aquatic bacteria, the surviving colony forming units (CFU) of Flavobacterium aquatile and Legionella pneumophila SG1 were greater than 10 CFU/mI after incubation for 10 minutes at 25°C in water containing 2 mg/L free chlorine.

All cultures were killed after 30 minutes of incubation with 2 mg/L free chlorine.

Pseudomonas aeruginosa and Legionella pneumophila SG1 survived one-minute incubation in water containing 5 mg/L free chlorine.

Means of three separate experiments in which cultures were suspended in chlorine demand-free distilled water adjusted to pH 7.4 and exposed to 0.5 to 12 mg/L free chlorine.

Samples were incubated at 25°C for the indicated times. Lowest free chlorine concentration producing no detectable colonies from 0.1 ml samples plated on BCYE (Legionella) or Nutrient Agar (Flavobacterium and Pseudomonas).

Culture                        Amount (mg/L) free available chlorine needed to kill 6 x 106 CFU/ml

                                                                 1 minute          10 minutes      30 minutes                             

Flavobacterium  aquatile           5.0                       2.5                      2.0

Legionella bozemanae              ND                         2.0                      ND

Legionella pneumophila           12.0                       2.5                      0.5

Pseudomonas aeruginosa       7.0                       2.0                       1.5                                          

ND = not determined