The disclosure provides lantibiotics geobacillin I and geobacillin II from Geobacillus thermodenitrificans, compositions comprising the lantibiotics, and methods of use of the lantibiotics. Further disclosed are the sequences of lantibiotics geobacillin I and geobacillin I, and alternative structures of geobacillin I and geobacillin II comprising amino acid substitutions. Antimicrobial compositions comprising one or more isolated geobacillins and a pharmaceutically acceptable carrier, and methods of reducing reproduction of bacteria comprising administering to a subject a therapeutically effective amount of the antimicrobial composition are also provided.