The aim of this study was to test the hydrolysis product of a quaternary amine-containing organosilicon salt (Si-QAC) for its preventive and antimicrobial activity against the American foulbrood (AFB) pathogen Paenibacillus larvae, a major cause of colony losses worldwide. The effects of Si-QAC were examined on 28 different local strains of P. larvae in the spore, vegetative and spore-vegetative forms, as well as the P. larvae strain ATCC 9545. The experiment was set up for spore and vegetative forms in four different ways. The results show that Si-QAC inhibits bacterial growth significantly in in vitro experiments. The extended studies were completed by cage experiments to identify toxicity levels. It was found that there is no toxic effect on honey bees by direct or indirect contamination with Si-QAC. These studies suggest that the preventive activity of Si-QAC against P. larvae is most effective as a disinfectant.