Protection against Fungi and Bacteria

Some alkaloids, such as quinine and other Cinchona alkaloids, have been used for centuries as drugs for killing protozoa while having little effect on the mammals ingesting such alkaloids. There have been attempts to extrapolate the poisonous effects of alkaloids to other microorganisms (protists), but it has been difficult to do. It was found that alkaloid-rich plants are affected by parasitic fungi and by pathogenic bacteria as much as alkaloid-free species. Phytophthora will attack, with the same deadly effects, alkaloid-rich Nicotiana species as readily as the less alkaloid-rich 

Since in most cases the resistance is caused by factors other than alkaloids, such as differences in cell wall structure, accumulation of phenolic compounds, or an ability to form suberin layers around the infection point, it seems that alkaloids have never played an important role in protecting plants against fungal and bacterial infections. Most alkaloids are without effect on the development of adapted, as well as foreign, fungi in experiments in vitro. Therefore, it seems justified to assume that in cases in which an alkaloid-rich plant is not infected by a certain parasite, the reason for its immunity is not the presence of alkaloids. 

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