Abstract
BackgroundPseudomonas tolaasii is a problematic pathogen of cultured mushrooms, forming dark brown ‘blotches’ on mushroom surfaces and causing spoilage during crop growth and post-harvest . Treating P. tolaasii infection is difficult, as other, commensal bacterial species such as Pseudomonas putida are necessary for mushroom growth, so treatments must be relatively specific.ResultsWe have found that P. tolaasii is susceptible to predation in vitro by the δ-proteobacterium Bdellovibrio bacteriovorus. This effect also occurred in funga, where B. bacteriovorus was administered to post-harvest mushroom caps before and after administration of the P. tolaasii pathogen. A significant, visible improvement in blotch appearance, after incubation, was observed on administration of Bdellovibrio. A significant reduction in viable P. tolaasii cell numbers, recovered from the mushroom tissue, was detected. This was accompanied by a more marked reduction in blotch severity on Bdellovibrio administration. We found that there was in some cases an accompanying overgrowth of presumed-commensal, non-Pseudomonas bacteria on post-harvest mushroom caps after Bdellovibrio-treatment. These bacteria were identified (by 16SrRNA gene sequencing) as Enterobacter species, which were seemingly resistant to predation. We visualised predatory interactions occuring between B. bacteriovorus and P. tolaasii on the post-harvest mushroom cap surface by Scanning Electron Microscopy, seeing predatory invasion of P. tolaasii by B. bacteriovorus in funga. This anti-P. tolaasii effect worked well in post-harvest supermarket mushrooms, thus Bdellovibrio was not affected by any pre-treatment of mushrooms for commercial/consumer purposes.ConclusionsThe soil-dwelling B. bacteriovorus HD100 preys upon and kills P. tolaasii, on mushroom surfaces, and could therefore be applied to prevent spoilage in post-harvest situations where mushrooms are stored and packaged for sale.
Highlights
Pseudomonas tolaasii is a problematic pathogen of cultured mushrooms, forming dark brown ‘blotches’ on mushroom surfaces and causing spoilage during crop growth and post-harvest
In the presence of B. bacteriovorus HD100 at both 4 × cells/well and 1.6 × cells/well, the OD600nm of P. tolaasii 2192T did not increase from the starting value (OD600nm = 0.05, 9.7 × 106 colony forming units (CFU)/well) over 24 hours
We showed that B. bacteriovorus HD100 is a predator of P. tolaasii 2192T in vitro and in vivo, suppressing population growth of the strain over a 24-hour period where 4 × or 1.6 × PFU B. bacteriovorus HD100 were added to pathogen on post-harvest mushrooms (Figures 1 and 4)
Summary
Pseudomonas tolaasii is a problematic pathogen of cultured mushrooms, forming dark brown ‘blotches’ on mushroom surfaces and causing spoilage during crop growth and post-harvest. The disease is characterised by brown lesions on the outer layers (2–3 mm depth) of the mushroom pileus and stipe, which range from small, light brown spots to larger, dark, sunken and wet lesions, depending on disease severity. This brown discolouration results from mushroom production of melanin, which is a defence response induced in this case by P. tolaasii producing the toxin tolaasin. The disease is found worldwide: P. tolaasii mushroom infection has been documented in several countries, including the USA, Spain, Serbia, the Netherlands, Japan and Korea [1,2,10,11,12,13]
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