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