3 plates containing 5% blood agar are placed on a table in a distance of maximum 1 m from the knees. These plates will be uncovered and open to inoculation by the microorganisms in the air of the operating theatre at the start of the intervention time of 75 minutes following draping. The plates are covered at the end of the intervention, following final knee sampling. An unused sample of the sampling fluid is plated in duplicate and will serve as a negative control.