Promising Preventative Treatment for Tomato Speck Disease to Combat Antibiotic Resistance

Pseudomonas syringae pv. tomato, the causative agent of tomato bacterial speck, is an agriculturally relevant bacterium that reduces tomato crop yield upon infection. Resistance to broad-spectrum preventative antimicrobial sprays, namely streptomycin and copper sulfate, have been documented since 1990. Current focus lies on identifying alternative prevention methods, one highly attractive option being bacterially produced antimicrobial peptides, bacteriocins, often with narrow killing spectra. Specifically, the bacteriocin-like inhibitory compounds produced by plant-growth-promoting-rhizobacteria (PGPR) are of interest to defend against P. syringae. However, of the rhizosphere and intraspecies co-culture screens performed, no single bacterial isolate was more inhibitory than when the bacteriocin, nisin (19.7 μg), was applied directly on each isolate of the pathogenic lawn. This revealed nisin as a potent inhibitor to P. syringae, prompting its further study. A series of detached tomato leaf-dipping assays were developed and employed to assess the relative decrease in P. syringae colony forming units between the nisin/ streptomycin treatment and control. Both the highly pure nisin (93%) and food-grade nisin (2.5%) were more effective than the 200 PPM streptomycin sulfate after 24 hours on the tomato leaf (91.3%, 96.8%, and 40.0% reduction, respectively). These results reveal that nisin remains highly effective when allowed to dry on a leaf surface. Future directions should continue to assess nisin’s activity in-planta in both field and greenhouse settings. This work brings us closer toward implementing nisin as a sustainable agriculture treatment effective at preventing P. syringae infection, protecting tomato crop yield, and reducing associated economic loss.