Bananas grow in tropical regions of the world and the humid conditions in the tropics also favour growth of fungi and bacteria. Any insect that eats off a banana leaf will leave it wounded and prone to attacks from these pathogens. Nature is all about balance and therefore the banana leaf will try to defend itself by creating bioactive compounds.
Scientists have evaluated banana leafs from four different species of banana for antibacterial activity against multi-drug resistant pathogens causing infections.
It is thus possible that sometime in the future new types of antibiotics can be developed from banana leafs. Which is a really positive outlook because most antibiotics that are in use today are in danger of becoming obsolete.
 Karuppiah et al: Antibacterial and antioxidant activities of Musa sp. leaf extracts against multidrug resistant clinical pathogens causing nosocomial infection in Asia Pacific Journal of Tropical Biomedicine.