Comprehensive genome-wide analysis for the safety assessment of microbial biostimulants in agricultural applications

Microbial biostimulants (MBs) offer a sustainable approach to agriculture by helping to reduce reliance on synthetic fertilizers. However, as MBs are intentionally released into the environment, their safety should be rigorously assessed. While taxa with qualified presumption of safety (QPS) benefit from established safety indications, non-QPS taxa lack such guidance. To address this gap, we propose a pipeline combining whole genome sequencing (WGS) and extensive literature search (ELS) data to evaluate microbial safety. We analysed public genomes of three QPS species (Rhodopseudomonas palustris, Bacillus velezensis, Priestia megaterium) and four non-QPS genera (Arthrobacter, Azotobacter, Azospirillum, Herbaspirillum), screening them for virulence factors (VFs), antimicrobial resistance (AMR) genes and mobile genetic elements (MGEs). Results confirmed the safety of QPS taxa, revealing no VFs and only a few intrinsic and non-clinically relevant AMRs. Among non-QPS taxa, VF hits were more prevalent in Azotobacter and Azospirillum spp., though they were mostly related to beneficial plant interactions rather than pathogenicity. AMR genes in non-QPS taxa were primarily associated with efflux pumps or were sporadically distributed. Notably, the only genus-wide pattern observed was that most Azospirillum and Herbaspirillum genomes harboured chromosomally encoded /3-lactamases sharing similar genetic structures; however, the detected /3-lactamase (bla) genes were distantly related to clinically relevant bla variants, and the absence of MGEs suggests a low risk of horizontal gene transfer, indicating the overall safety of these genera. In general, this WGS-ELS framework provides a robust tool for assessing the safety of non-QPS MBs, supporting regulatory decision-making and ensuring their safe use in sustainable agriculture while safeguarding public health.