Proteinaceous bioactive compounds from Proteus mirabilis MN12 and DF14N suppress bacterial phytopathogens and prime plant defenses

Abstract

Bacterial phytopathogens cause devastating yield losses in staple crops, yet sustainable alternatives to copper and streptomycin remain scarce. Proteus mirabilis , an environmental member of the Morganellaceae, is known for producing proticines, SOS-inducible, phage-tail-like bacteriocins, but its systematic evaluation as a source of biocontrol agents against plant pathogenic bacteria has not been attempted. Here, we report the isolation of seven P. mirabilis strains from the rhizosphere of tomato and bean in Cameroon, two of which (MN12 and DF14N) produced crude bacteriocin-like compounds (BLCs) with broad-spectrum antibacterial activity. In vitro, BLCs inhibited Xanthomonas campestris pv. campestris , Erwinia amylovora , Ralstonia solanacearum , and Clavibacter michiganensis subsp. michiganensis with minimum inhibitory concentrations (MICs) of 1.0–4.0 mg mL⁻¹ and bactericidal kinetics (minimum bactericidal concentration [MBC]/MIC ≤ 4) against the three Gram-negative targets. The active compounds were proteinaceous, thermostable (≤ 100 °C), and active between pH 5 and 9. In greenhouse assays, seed treatment and foliar application of BLCs reduced disease severity under mixed pathogen challenge to levels comparable to or lower than streptomycin and a commercial Bacillus subtilis biocontrol product, while fully restoring plant biomass. Biochemically, BLC-treated plants exhibited primed defense responses, including elevated catalase, peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase activities, and total phenolic accumulation. Safety profiling indicated low phytotoxicity (EC₅₀ = 35–40 mg mL⁻¹), negligible mammalian cytotoxicity (> 93% viability in Caco-2 and HEK293 cells), rapid soil degradation (half-life ≈ 5 days), and absence of the clinically relevant virulence genes zapA and hpmA . These findings position P. mirabilis -derived BLCs as a previously uncharacterised class of proteinaceous biopesticides and provide an experimentally grounded, safety-inclusive framework for their further development against bacterial phytopathogens.