The lantibiotic nisin has been used for more than 40 years as a food preservative to protect against food-borne pathogens. It has also been approved as a treatment for bovine mastitis. The problem is that nisin isn’t particularly stable at pH 7. Wilfred A. van der Donk and coworkers at the University of Illinois, Urbana-Champaign, have now characterized nisin analogs called geobacillin I and geobacillin II identified in the genome of the thermophilic bacterium Geobacillus thermodenitrificans NG80-2 (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1116815109). The researchers engineered Escherichia coli to produce the geobacillins and determined their structures using tandem mass spectrometry and nuclear magnetic resonance spectroscopy. The structure of geobacillin I contains seven thioether bridges, the most seen in any lantibiotic. The geobacillins inhibited the growth of a variety of gram-positive, but not gram-negative, bacteria. Geobacillin I was three times more active than nisin against one of the main causes of bovine mastitis. In addition, the geobacillins were more stable than nisin at pH 7 and pH 8 and at 37 °C and 60 °C.