Efficacy of various biocides against L. pneumophila depending on the contact time

Abstract number: 1134_01_78

Soler-Membrives A., García-Nuñez M., Ragull S., Sopena N., Pedro-Botet M.L., Sabrià M.

Background:  

Cooling towers have been related to community-acquired legionellosis outbreaks. Thus, the proper maintenance and choice of biocides is important to maintain Legionella counts at undetectable levels.

Aims:  

The aim of this study was to compare the antimicrobial activity of four commercial cooling tower microbicides against Legionella pneumophila, and to determine whether the contact time was an important factor in the application of the correct biocidal concentration.

Material and methods:  

Four cooling tower biocides were tested against Legionella pneumophila (ATCC 33152): 2,2-dibromo-3-nitrilepropionamide (biocide 1), a dilution of biocide 1 (biocide 2), bromo-chloro-5,5-dimethylhydantoin (biodide 3) and 1,3,5-trichloro-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (biocide 4). And in vitro test was performed following the prEN 13623 test method (Chemical desinfectants and antiseptics- Bactericidal activity of products against L.pneumophila- Test method requirements (phase2/step1)).

Results:  

Biocide 1 was found to be the best disinfectant (effective at lower concentrations, 0.005 ppm). Biocide 2 was active at concentrations 20-fold higher than biocide 1 (0.1 ppm), followed by biocide 3, which was active at 5–10 ppm. The data obtained indicated that biocide 4 also had bactericidal activity at concentrations higher than 5ppm. No differences were found when L. pneumophila was exposed to biocide 1, 2 and 4, at different contact times (60 minutes or 24 hours). However, biocide 3 presented a slow bactericidal activity; at least twice the concentration of biocide 3 was needed at short contact times (60 minutes), in contrast to prolonged contact times (24 hours).

Conclusions:  

Of the biocides tested we found 2,2-dibromo-3-nitrilepropionamide (biocide 1) to be the most effective, because it is active at very low concentrations, and its activity is long-lasting.