At concentrations that inhibit bacterial growth, some antibiotics including gentamicin completely inhibited virus multiplication in protoplasts, and other antibiotics partially inhibited virus multiplication. The inhibition caused by each antibiotic was largely prevented by adding a divalent metal; MnCl was more effective than CaCl and other salts of divalent metals when added at 10 m to the incubation medium. When added immediately after infection, 1 g/ml of gentamicin halved the final virus concentration and 3 g/ml completely inhibited virus multiplication, although 10 g/ml was required to stop bacterial growth. Gentamicin inhibited virus multiplication even when added 24 h after virus inoculation. Also, when protoplasts were exposed to gentamicin for only 1 or 2 h, either immediately after inoculation or 2 h later, the virus concentration was considerably decreased.

Gentamicin seemed not to affect virus multiplication in whole plants. Sap from also strongly inhibited virus multiplication in protoplasts but, unlike gentamicin, it acted in the presence of MnCl. By contrast, chelating agents such as 1 m-EDTA or 5 m-potassium citrate were strong inhibitors of virus multiplication that were inactive in the presence of MnCl. It is suggested that gentamicin and other antibiotics may chelate metals from the protoplast membranes, thus disorganizing their function and affecting virus multiplication.


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