Her dissertation was on the structure of the antibiotic gramicidin. Tyrothricin was later found to be a mixture of the peptides gramicidin and tyrocidine. It is synthesized by gramicidin S synthetase. Later this was shown to be a mix of gramicidin and tyrocidine. The antibiotics gramicidin and tyrocidine were first isolated from it. Parts of its sequence are identical to gramicidin S. Tyrocidine is the major constituent of tyrothricin, which also contains gramicidin. Gause isolated this product and named it Gramicidin S. Bacteria die at lower gramicidin concentrations than human cells. This is in contrast to unrelated gramicidin S, which is a cyclic peptide. The active ingredients are polymyxin B, bacitracin and occasionally garamycin or gramicidin. Gramicidin and alamethicin had been popular starting points for selective modifications. Each has 2 isoforms, so the mix has 6 different types of gramicidin molecules. In some countries bacitracin is replaced with gramicidin. In 1964, the sequence of gramicidin A was determined by Reinhard Sarges and Bernhad Witkop.