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Bacteria can survive the hostile interior of passion fruit seeds

The insides of plants can shelter a large variety of microorganisms, similar to the well-known microbes residing in the human gut. A research team from the Tokyo University of Science is the first to isolate the bacteria contained in the seeds of the passion fruit (Passiflora edulis). 

The endophytic microorganisms residing inside plants’ fruits, leaves, stems, roots, or seeds are not necessarily harmful to plants. Instead, they often develop a synergistic relationship with their host, and are beneficial for processes of germination, growth, and even defense. 

However, most of the plants contain many “secondary metabolites” – natural bioactive compounds with strong antimicrobial properties. This makes such environments typically hostile to endophytic microorganisms.  

Passion fruit seeds are generally full of secondary metabolites, such as resveratrol and, particularly, piceatannol. This is precisely the reason why scientists were curious to investigate whether passion fruit seeds contain any endophytic microorganisms too. 

“The extraordinarily high concentration of piceatannol protects P. edulis seeds from microorganisms. We thought it would be interesting to know if any endophytic microorganism could survive this extreme environment, and if yes, how,” explained study co-author Toshiki Furuya, an associate professor in the Department of Applied Biological Science at Tokyo University of Science.

Remarkably, the researchers discovered 19 bacterial strains in the passion fruit seeds, including three that were previously unknown. Their hypothesis is that the piceatannol from the seeds had bacteriostatic (“bacterial growth-stalling”) rather than bactericidal (“bacteria-killing”) effects on the endophytic bacteria residing in the seeds. 

“Due to the presence of a high concentration of piceatannol, the growth of the bacteria was stagnated inside the seed, but when transmitted to the next-generation seedlings during germination, the bacteria were relieved from the effect of piceatannol and able to grow again,” said study co-author Aoi Ishida.

The scientists also made another surprising discovery. One of the isolated bacteria, Brevibacterium sp. PE28-2, possessed the ability to convert piceatannol and resveratrol to their respective derivatives. This is the first endophytic bacteria shown to exhibit such abilities.

Earlier research demonstrated that endophytic bacteria which are capable to survive in environments rich in secondary metabolites compounds possess biocatalytic activities related to the metabolism of these compounds. This biocatalytic potential can be exploited for therapeutic purposes.

Further research on the mechanisms of endophytic resistance inside of plants is needed to open new paths in medicine and beyond.   

The research is published in the journal MicrobiologyOpen.     

By Andrei Ionescu, Staff Writer

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