An extensive screening study for the production of proteolytic inhibitors has been carried out on 75 Streptomyces strains. It was found that 18 of the strains and/or their variants (24%) produced proteinaceous substances, which belonged to the group of typical serine protease inhibitors. 23 samples were tested for inhibitory activity on the replication of influenza virus A/Germany/34, strain Rostock (H7N1) (A/Rostock) in chicken embryonic fibroblast (CEF) cells. Eleven of the tested samples (52.2%) significantly inhibited viral growth. Further the specific inhibitory effect on the replication of influenza virus A/Aichi/2/68 (H3N2) (A/Aichi) in Madin-Darby canine kidney (MDCK) cells and on the growth of herpes simplex virus type 1, strain DA (HSV-1) in Madin-Darby bovine kidney (MDBK) cells was tested. Nine samples significantly inhibited A/Aichi and four - HSV-1. The most effective inhibitors, produced by Streptomyces sp. 225b (SS 225b) and Streptomyces chromofuscus 34-1 (SS 34-1) protected mice from mortality in the experimental influenza A/Aichi virus infection.
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