No, I do not think so, because it is never too late to discover anything and it might lead you to search for something novel. Okay, lets talk about the least benefit you can extract from discovering new antimicrobial compounds from plants is that at the end of the study you will be having very deep understanding of the topic (at least about particular compound or plant or even pathway) and also there will be a number of good research publications. I think you should start the study in this field.