[The occurrence of legionella in dental units and the consequences for practice hygiene]

The objective of our investigation is to establish and explain the contamination of dental treatment units on the basis of a theoretical model, as well as to develop solutions to problems. In a first longitudinal study (6), facultatively pathogenic bacteria were identified in the operating water of dental units; the legionellae findings in screening were verified in a second longitudinal study with consideration of possible causal factors of bacteriological and parasitological nature. We took samples from the dental units and the water conduit system (cold and warm water in the dental surgery, cold water in front of and behind house internal accessory installations) in seven practices at ten measurement times over a period of ten weeks. Microbiological diagnostics were carried out with regard to the occurrence of legionellae, amoebae, pseudomonads and coliforms. The correlation between the metal content of the water and the occurrence of legionellae described in the literature caused us to determine the copper, zinc and iron content of the waters (according to standard German method, 1985). In addition, the physical-chemical parameters temperature, pH value, conductivity and oxidizability as well as the total hardness were determined. The findings in the first phase of the study could be confirmed both qualitatively and quantitatively in the second phase. However, a greater diversity of species was shown with regard to the occurrence of legionellae in connection with an enlarged sample. We were able to isolate free-living amoebae from all dental units, the identification of which was carried out in parallel to that of legionellae in the context of the second investigation (26). With a high variance of the contamination spectrum between the individual practices and the various units within each practice, generalization of the results is not possible; the findings are hence illustrated with examples from selected practices with consideration of the initial and interaction conditions.