Lactobacillus fermentum, a bacterium of human microbiota, acts as an electron donor for the electrochromic [P2Mo(VI)18O62](6-). Since the reductive capacity of L. fermentum correlates with its metabolic activity, the reaction with [P2Mo(VI)18O62](6-) affords a means of evaluating its activity. Following this logic, we have concluded that vancomycin severely affects the activity of L. fermentum whereas omeprazole does not.