Bioluminescent monitoring of a co-culture model of pathogen growth to identify a novel probiotic bacterium

We isolated a novel bacterium from non-pasteurized milk and identified it as a strain of Lactobacillus delbrueckii. Testing in our assay confirmed that this bacterium was able to inhibit the growth of both Staphylococcus aureus and Escherichia coli, but in a delayed fashion, after allowing a period of early growth. This activity was not dependent on living cells but also a property of conditioned medium. We posited that this was because the pathogens stimulated the production of an inhibitory factor from the Lactobacillus, presumably a bacteriocin peptide. Pre-treatment of conditioned medium with a protease abolished this inhibitory activity, indicating that L. delbrueckii is stimulated to produce an antibacterial peptide in the presence of a bacterial pathogen.