Sealing constructions are frequently used in reusable instruments to prevent the loss of fluids or gasses in a construction by obstructing flow through the glands in the interface of two or more separate parts, and may be applied to seal patients' body fluids. Patient material contains various types of microorganisms, and with the increase of infections associated with multi-drug resistant bacteria, information about tightness of seals with regards to microorganisms becomes increasingly more important. The bacterial tightness of airtight (at 240 mbar) reproductions of the O-ring sealing construction in the distal tip of Olympus TJF-Q180V duodenoscopes were evaluated. During test runs of this systematic investigation, all product and usage variables expected to influence microbial leakage were closely controlled. The results showed that airtight O-ring seals can leak bacteria, even in static conditions, and moreover, rotation of the axle was found to be a significant bateria leakage promoting factor. With regards to prevention of infections, medical designers, manufacturers and safety controllers should have knowledge of this insight. More systematic research in this manner, for exampe using ISO 3601-2 complying sizes, would provide valuable information.