[Growth behavior of Enterobacteriaceae in BRILA-MUG-broth and in different modifications of broth]

Both substances, brilliant green and bile, inhibit the growth of gram-positive bacteria in culture media and selectively enrich gram-negative bacteria. Therefore, the brilliant green-lactose-bile broth (BRILA) and the BRLA broth supplemented with tryptophan and methyl-umbelliferyl-beta-D-glucuronide (BRILA-MUG) contain brilliant green as well as bile. Because BRILA-MUG broth as a selective enrichment and differentiating medium of faecal coliform and total coliform bacteria, E. coli and coliforms, respectively, is recommended for testing samples of surface water according to the EC guidelines for bathing waters (no. 76/160 EWG), the question arose of the optimal combination of components in the BRILA-MUG broth. As the described investigations show, the addition of buffer substances did not improve the culture properties of the BRILA-MUG broth. However, the original BRILA broth was improved by supplementing it with buffer substances such as CaCO3 or Na2HPO4. The same effect of culture improvement was obtained by removing brilliant green. This modification of BRILA broth is practically identic with the well-known MacConkey broth. On the other hand, the modification of omitting bile from the original BRILA broth causes a remarkable impairment of the culture properties lowering bacterial counts per ml by 3-5 log. The observations suggest that brilliant green inhibits both, gram-positive bacteria as well as the gram-negative Enterobacteriaceae. Therefore, it is a selective substance of doubtful usefulness.