The "European Technical Guidelines for the Prevention, Control and Investigation, of Infections Caused by Legionella species ", confirmed that, in some investigations, water yielded few legionellae at the time of sampling, although biofilm from inside the pipes contained large quantities of Legionella. This probably reflected the type and positioning of the biocide treatment and zones within the piping where the biocidal effect did not penetrate adequately. Read more about biofilm and Legionella detection, monitoring and prevention.