Selection and <i>In Vitro</i> Assessment of Plant Growth-Promoting Bacteria from Black Soldier Fly ( <i>Hermetia illucens</i> ) 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 <i>Serratia</i>, <i>Peribacillus</i>, <i>Acinetobacter</i>, <i>Pseudocitrobacter</i>, <i>Bacillus</i>, and <i>Enterobacter</i>, were further tested <i>in vivo</i> on <i>Arabidopsis thaliana</i>. 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.