Anticandidal activity of major human salivary histatins

We have previously shown that histatins 1, 3, and 5 are homologous, histidine-rich proteins present in human parotid and submandibular secretions which contain 38, 32, and 24 amino acids, respectively. Interest in these proteins stems from the fact that histatins exhibit candidacidal and candidastatic activities. The goal of the present investigation was a detailed functional characterization of these anticandidal activities of histatins at the levels of killing of blastoconidia, killing of germinated cells, and inhibition of germination by using three bioassays. Candidacidal activities were evaluated at several ionic strengths, in the presence of different mono- and divalent ions, and at multiple pH values. In addition, the susceptibility of Candida albicans in different growth phases to histatins was investigated. While all three major human histatins demonstrated candidacidal activities, they differed in their abilities to kill blastoconidia and germinated cells, with histatin 5 being the most active, histatin 3 showing moderate activity, and histatin 1 exhibiting the lowest level of activity. For the inhibition of germination, however, histatin 3 exhibited more activity than either histatin 1 or histatin 5. The candidacidal activity of histatins was inversely proportional to both the ionic strength and the divalent cation concentration in the medium. Stepwise reduction of the pH of the assay medium enhanced the candidacidal activities of histatins 1 and 3, while the activity of histatin 5 was pH independent over the range of pHs 4 to 8. C. albicans in log-phase growth was more susceptible to histatins 1 and 3 than cells in stationary phase. Cells in either growth phase were still more vulnerable to histatin 5 than to histatins 1 and 3. The results obtained establish the functional relationship of the major histatins with respect to both their fungicidal and fungistatic activities and provide insights into their activities under ionic and pH conditions likely to be encountered in vivo in the oral cavity. Moreover, the data point towards possible mechanisms responsible for the anticandidal activities of histatins.