Microbiological and sampling criteria established in the present Spanish legislation for the control of Legionella

Abstract

We have retrospectively investigated the relationship between the presence of total aerobic bacteria (TAB) and the presence of Legionella sp. in 576 water samples from industrial cooling towers. A lower percentage of positive samples for Legionella sp. (19.8%) was encountered when the concentration of TAB was above 104 CFU/ml than below this concentration (80.2%). The concentration of 104 CFU/ml TAB is established in Spanish legislation as the action limit, above which a higher risk of Legionella positive samples is expected. Despite that, our results indicate that levels of TAB above 100 or 1000 CFU/ml seem more useful for predicting the presence of Legionella in the cooling towers investigated. We observed that the concentration of TAB was significatively higher in the positive samples for Legionella than in the negative ones, which seems to indicate that TAB is a good tool for evaluating the effectiveness of the Autocontrol Safety Measures of those cooling towers. Our results reinforce the need for controlling the levels of TAB (at 36 ± 1 oC) because it is a good indicator of the presence of Legionella spp. despite the fact that the standard established as the action limit should be lowered. Within the study we also reviewed the sampling procedures and provide new data on the incubation period of the disease and on the genotyping methods classically employed in epidemiological investigation.