Selection and In Vitro Assessment of Plant Growth-Promoting Bacteria from Black Soldier Fly (Hermetia illucens) Frass

Frass, the principal byproduct of Black Soldier Fly (BSF) farming, is increasingly valued as a sustainable organic fertilizer, partly due to its potential to harbor plant growth-promoting microorganisms (PGPM). This study investigated the presence and activity of PGPM in frass obtained from 10 rearing substrates and evaluated the effect of mandatory heat treatment (70 °C, 1 h). Using a rhizosphere-mimicking agar medium, 149 bacterial isolates were recovered and screened for PGPM-specific traits. Six promising isolates, belonging to Serratia, Peribacillus, Acinetobacter, Pseudocitrobacter, Bacillus, and Enterobacter, were further tested in vivo on Arabidopsis thaliana. They displayed variable effects on seed germination, root elongation, and root hair development linked to their phytohormone profiles. Several strains were recovered from both untreated and heat-treated frass, highlighting their thermotolerance. These findings demonstrate that BSF frass harbors PGPM with strong potential for biofertilizer development.