cyclic analogues of histatin h - 5 , having from about 7 - 20 amino acid units exhibiting substantial homology to histatin h - 5 , and having a cyclic portion of from 5 - 16 of the amino acid units , have been found to exhibit enhanced bio - activity against a variety of different microorganisms . the cyclic structure is imparted by replacement of naturally occurring amino acids in histatin - 5 sequences by , for example , by cysteine units , so as to cause cyclization through formation of disulfide bridges .