Antimicrobial peptides are components of the innate immune systems in animals and plants as natural defense against pathogens. Critical issues like manufacturing costs have to be addressed before mass production of these peptides for agriculture or community sterilizations. Here, we report a cost-effective chemical synthesis method to produce antimicrobial cocktails, which was based on the heat conjugation of amino acids in the presence of phosphoric acid and plant oil at 150 °C. The conjugates showed potent biological activities against all tested bacteria including a multi-drug resistant Staphylococcus aureus strain Y5 and ampicillin resistant Pseudomonas aerugenosa ATCC9027 strain, demonstrating potential in agriculture, and prophylactic applications in hospital and community settings.