Evaluation of Capsaicin as a Selector for Growth Promotional Bacteria Isolated from Capsicum Peppers

Plant growth-promoting bacteria (PGPB) can act as biostimulants, improving the growth of plants in sustainable agriculture systems that seek to reduce synthetic agrochemical input. Bacteria present in seeds are closely associated with vertical transmission and thus represent a potential trove of biostimulants. Capsicum species are notable for producing capsaicin, a compound with antimicrobial activity that may influence microbial communities associated with pepper fruits and seeds. Using Luria–Bertani (LB) media infused with capsaicin, we isolated bacteria from bell peppers, jalapeno peppers, and habanero peppers, which we verified to have different levels of capsaicin through high-performance liquid chromatography with ultraviolet detection (HPLC-UV). Minimum inhibitory concentration (MIC) assays indicated that the capsaicin resistance of isolated bacteria did not correlate with the pungency level of the host pepper variety. Of the total isolated bacteria, four showed promise as plant growth promoters; two belong to the genera Pseudomonas, one Agrobacterium, and one Bacillus. Our isolates tested positively for potassium and phosphate solubilization, urease production, and indole-3-acetic acid (IAA) phytohormone production. Inoculation of these bacteria into surface-sterilized red clover (Trifolium pratense) and Kentucky bluegrass (Poa pratensis) showed significant improvements in germination rate, seedling root length, and seedling shoot height. These results show that the pungency of peppers does not influence the capsaicin resistance of isolated bacteria. Additionally, seedborne PGPB have the potential for plant growth improvement through various mechanisms, reducing the need for synthetic chemicals.