[Examination of the water supply with the alkali and alkali-earth supplement optimized DIN luminescent bacteria test, exemplified by the Saar river].

The light intensity emitted by luminescent bacteria is influenced by both the osmolarity and the ion composition of the test medium. The addition of potassium and calcium ions to a sodium chloride solution causes a considerable increase in the light intensity of bacteria. If these elements occur in the proper concentrations in the test material, the luminescence in the test sample will be higher than in the control sample containing only sodium chloride. This physiological dependence did not find the due consideration in the establishment of the German standard method for the luminescent bacteria test using Microtox bacteria (German Institute for Standardization DIN 38,412, Part 34--edition March 1991) where only the osmotic balancing by sodium chloride was taken into account. For testing chemicals, the luminescent bacteria test with Microtox reagent can be run without problems by the procedure that is recommended by Microbics Corporation and which became part of the DIN standard method. The situation is different when analyses of natural waters or effluents from wastewater plants are concerned, where matrix effects have to be expected. Stimulation of light intensity can be a reflection of a nonoptimized test medium and in the worst case may cause false negative results. The new ASW (artificial sea-water) luminescent bacteria medium according to Klein, which besides sodium contains potassium, magnesium, and calcium ions, brings stimulation to both samples and controls, effectively reducing enhancement and increasing the inhibitory effects of samples from the River Saar. However, high light stimulation rates measured in parallel tests with the DIN procedure could not be caused by the low concentrations of alkaline and alkaline-earth ions in the samples. The experiments indicate that there must be additional influencing substances which have not yet been identified. Furthermore, the experiments show that the matrix effects of unknown origin that will exert a positive influence on the light emission may be compensated by addition of alkaline and alkaline-earth ions.