Antibiotics and natural plant products must have selective Toxicity for the Microbe. Several hundreds of compounds with antibiotic activity have been isolated from plants which are found antimicrobial. Secondary metabolites in microbes are synthesized after consumption of media and most of these are toxins, similarly, in plants all diffreent types of compounds which are synthesized other than nutrients get modified in their terminal functional groups and show wider toxicity to the micro-organisms, because these are synthesized in response to microbial attack, hence, these are quite selective and toxic to microbes. Mostly plant develop resistance or a primary protection against microbes. 

Prof Ravi tnx alot, but if plants develops resistance its glaring that the modification of the functional groups of the secondary  toxin or genetic modification in microbes may be responsible for their selective resistance against these bioactive compounds in antimicrobials. How?

Plants produce a variety of secondary metabolites to protect themselves from pathogens and herbivores and/or to influence the growth of neighbouring plants. Some of these metabolites are toxic to the producing cells when their target sites are present in the producing organisms. This proves the point that they might be selectively toxic.