Inhibition of Multiplication of Tobacco Mosaic Virus in Protoplasts by Antibiotics and its Prevention by Divalent Metals

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; MnCl2 was more effective than CaCl2 and other salts of divalent metals when added at 10 mm to the incubation medium. When added immediately after infection, 1 µg/ml of gentamicin halved the final virus concentration and 3 µg/ml completely inhibited virus multiplication, although 10 µg/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 Dianthus barbatus also strongly inhibited virus multiplication in protoplasts but, unlike gentamicin, it acted in the presence of MnCl2. By contrast, chelating agents such as 1 mm-EDTA or 5 mm-potassium citrate were strong inhibitors of virus multiplication that were inactive in the presence of MnCl2. It is suggested that gentamicin and other antibiotics may chelate metals from the protoplast membranes, thus disorganizing their function and affecting virus multiplication.