[Effect of protamine on the microbicidal efficacy of formaldehyde]

Testing the ability of commercial compounds to provide an effective disinfection of instruments requires test conditions that are close to reality which includes the proper selection of the material used to contaminate the test objects. The adhesion of the material must be strong enough to keep it attached to the test object during and after insertion into the disinfectant solution. Its characteristics should come as close as possible to those of the contaminations encountered in practice. The guideline for instrument disinfectants published by the Robert Koch-Institute recommends the use of coagulated blood. Accordingly, heparinized sheep blood is mixed with the test germs, and protamin is added to initiate coagulation. In the present investigation we compared this contamination procedure with a second one, in which coagulation was achieved by adding a CaCl2 solution to citrate blood. We also included agarose as an almost inert contaminant in our experiments. The results showed that protamine is able to increase the microbicidal efficacy of formaldehyde on staphylococci significantly. When these test germs were embedded either in citrat blood or in agarose, it took about twice the disinfectant concentrations to achieve the same microbicidal effects as with protamine blood (Fig. 1). Remarkably, the results obtained with citrate blood were the same as those with agarose, regardless of the differences in material between the two contaminants. It should also be noted that the microbicidal effect of the formaldehyde proved to be almost independent from the amount of contaminant per test area, hence, from the thickness of the layer. When M. terrae was employed as test germ, the results obtained with protamine blood and citrate blood, respectively, as contaminants were identical (Fig. 2). The same was true for the other test germs investigated, except for E. faecium (Fig. 3). The addition of even very small amounts of protamine to the embedding compound, agarose led to a substantially increased efficacy of the formaldehyde against staphylococci (Fig. 4). This effect was especially distinct in suspension (Fig. 5). Whenever the efficacy of formaldehyde-containing disinfectants is to be tested and evaluated, one should be aware of this synergism between protamine and formaldehyde. In these cases, it is advised to employ other contaminating agents, such as coagulated blood prepared by addition of CaCl2 to citrate blood.