Seed inoculation with antagonistic bacteria limits occurrence of E. coli O157:H7gfp + on baby spinach leaves

Backround

During the last decades, outbreaks of foodborne illnesses have increasingly been linked to fresh and/or minimally processed fruit and vegetables. Enterohemorrhagic Escherichia coli was the causal agent for major outbreaks in Europe with leafy green vegetables and sprouts. To improve food safety, microbial antagonism has received attention during recent years and could be one of the solution to prevent contamination of food borne pathogens on fresh produce. Here we investigate the antagonistic effect of three bacterial strains (Pseudomonas orientalis, P. flavescens and Rhodococcus sp.) isolated from spinach leaves against E. coli O157:H7gfp + under laboratory and greenhouse conditions.

Results

Our results shows that significantly less culturable E.coli O157:H7gfp + were retrieved from the spinach canopy subjected to antagonist seed treatment than canopy inoculation. Seeds inoculated with Rhodococcus sp. significantly reduced growth of E. coli O157:H7gfp + compared with the other antagonists. The result from the in vitro study shows a significant reduction of growth of E. coli O157:H7gfp+, but only after 44 h when E. coli O157:H7gfp + was propagated in a mixture of spent media from all three antagonists.

Conclusions

The antagonistic effect on phyllospheric E.coli O157:H7gfp + observed after seed inoculation with Rhodococcus sp. might be an indication of induced resistance mechanism in the crop. In addition, there was a small reduction of culturable E.coli O157:H7gfp + when propagated in spent media from all three antagonists. Nutritional conditions rather than metabolites formed by the three chosen organisms appear to be critical for controlling E. coli O157:H7gfp+.

Pseudomonas orientalis F9: A Potent Antagonist against Phytopathogens with Phytotoxic Effect in the Apple Flower

In light of public concerns over the use of pesticides and antibiotics in plant protection and the subsequent selection for spread of resistant bacteria in the environment, it is inevitable to broaden our knowledge about viable alternatives, such as natural antagonists and their mode of action. The genus Pseudomonas is known for its metabolic versatility and genetic plasticity, encompassing pathogens as well as antagonists. We characterized strain Pseudomonas orientalis F9, an isolate from apple flowers in a Swiss orchard, and determined its antagonistic activity against several phytopathogenic bacteria, in particular Erwinia amylovora, the causal agent of fire blight. P. orientalis F9 displayed antagonistic activity against a broad suite of phytopathogenic bacteria in the in vitro tests. The promising results from this analysis led to an ex vivo assay with E. amylovora CFBP1430Rif and P. orientalis F9 infected detached apple flowers. F9 diminished the fire blight pathogen in the flowers but also revealed phytotoxic traits. The experimental results were discussed in light of the complete genome sequence of F9, which revealed the strain to carry phenazine genes. Phenazines are known to contribute to antagonistic activity of bacterial strains against soil pathogens. When tested in the cress assay with Pythium ultimum as pathogen, F9 showed results comparable to the known antagonist P. protegens CHA0.