Legionella Testing / Biosafety Training
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Dr Richard W Gilpin’s 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