The lantibiotic nisin has been used as an effective food preservative to combat food-borne pathogens for over 40 y. Despite this successful use, nisin's stability at pH 7 is limited. Herein, we describe a nisin analog encoded on the genome of the thermophilic bacterium Geobacillus thermodenitrificans NG80-2. This analog termed geobacillin I was obtained by heterologous expression in Escherichia coli and subsequent purification. Extensive NMR characterization demonstrated that geobacillin I contains seven thioether cross-links, two more than the five cross-links found in nisin and the most cross-links found in any lantibiotic to date. The antimicrobial spectrum of geobacillin I was generally similar to that of nisin A, with increased activity against Streptococcus dysgalactiae, one of the causative agents of bovine mastitis. Geobacillin I demonstrated increased stability compared to nisin A. In addition to geobacillin I, the genome of G. thermodenitrificans NG80-2 also contains a class II lantibiotic biosynthetic gene cluster. The corresponding compound was produced in E. coli, and has a ring topology different than that of any known lantibiotic as determined by tandem mass spectrometry. Interestingly, geobacillin II only demonstrated antimicrobial activity against Bacillus strains. Seven Geobacillus strains were screened for production of the geobacillins using whole-cell MALDI-MS and five were shown to produce geobacillin I, but none produced geobacillin II.