The third sample is treated in a way that ensures that as far as possible only Legionella are left on a filter. Elbert uses a filter bank from Merck Millipore for this task. This saves him work by enabling him to process three samples at once. Here too, the highest degree of care is necessary. He uses tweezers to pick up a fine-pored filter base (“So that the vacuum is evenly distributed”) from a brown glass beaker full of ethanol, flames it and places it on the support in the filter bank.

He then removes the 47-millimeter-diameter membrane filter from its airtight blister pack and places it on the base. “The filter has a pore size of 0.45 micrometer, so that it retains large bacteria such as Legionella,” he says. He places a sterilized funnel on top and fills it with exactly 100 milliliters of the sample, puts a lid on, and switches on the compressor. This produces a vacuum of around 13 millibar, and the three samples are filtered within seconds.

The filter membrane is then further treated by being placed in an acidic buffer with a pH of 2.2 for five minutes to kill off any accompanying bacterial flora. “The Legionella remain unaffected by the low pH over this short time,” says Elbert. Finally, he lays the membrane on an agar plate. This nutrient plate also contains compounds such as fungicides and antibiotics, which restrict the growth of accompanying bacterial flora. Then the plate goes into the incubator, where the Legionella can multiply for a good week at their optimal temperature of 36 degrees Celsius.

Daniel Schäfer, Head of Microbiology at the laboratory, takes one of the incubated Petri dishes to the window for evaluation and points to some gray-green spots. “Every one of these spots,” explains the biologist, “is a colony of Legionella that has formed from a single bacterium.” Taking the original sample volume into account, the number of these spots gives the number of colony-forming units per 100 milliliters as specified by the legislators.

If the result is positive, the bacteria in the system tested will have to be killed. “The best way to accomplish this is by heating the system with really hot water at above 70 degrees Celsius for a short time,” explains Schäfer. That will take care of the Legionella.