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NIPGR scientists identify bacteria that can devour fungi

Scientists have identified an antifungal bacterium that can destroy fungal infections by eating them up. The capability of devouring the fungi, leaving no remnants of the infection could make these bacteria a more effective antifungal agent than the ones currently available. Furthermore, the group has also identified a bacterial protein, which is essential for eating fungi. It opens up various possibilities ranging from using the bacterium or the protein for antifungal sprays over fields or utilizing it as transgene to develop broad spectrum fungal disease resistant plants.

A team led by Dr Gopaljee Jha from the National Institute of Plant Genome Research (NIPGR), an autonomous institute of the Department of Biotechnology, had isolated the bacterium Burkholderia gladioli strain NGJ1 from healthy rice seedling.

In their paper published in Nature Communications the group has shown that the bacteria can eat up broad spectrum of fungi, a property technically called mycophagy.

Fungal pathogens have been a challenge for sustainable agriculture and they have been behind death and disability in humans, wildlife extinctions, population declines, etc. Being eukaryotic in nature, the metabolism of fungi and host (humans/plants/animals) is predominantly similar. There is less difference for any molecules/drugs to specifically target fungi without harming host cells. Hence, it is very difficult to control fungal diseases and globally there have been urgent thrust to devise newer strategies to control them.

The discovery was an accidental one en route to the team’s endeavour to try and understand the molecular intricacies of the bacteria Rhizoctonia solani, which causes sheath blight disease, a significant fungal disease of rice.

While establishing detached bioassay to study the pathogenesis of R. solani on laboratory media plates, the group observed that whenever a particular yellow coloured bacterium ooze out from rice, the growth of R. solani was prevented. Subsequently upon establishing the pure culture, the bacterium was identified as a Burkholderia gladioli strain. The pure culture of the bacterium also exhibited antifungal activity against R. solani on laboratory plates. However, when the confrontation plates were incubated for a week time, the bacterium for aging over entire fungal biomass was observed. It was interesting to observe how a tiny bacterium can feed on fungi, which is many times larger in size.

The scientists observed that the B. gladioli strain NGJ1 treatment not only prevented the growth of R. solani, but also caused reduction in its biomass. Our results suggested that the bacterium releases fungal metabolites by lysing fungal cells and utilizes them for its own growth. Due to such mycophagous property, the bacterium not only suppresses fungal growth but can eradicate fungi by utilizing them as source of nutrients. The bacterium was found to demonstrate mycophagous activity on several other fungi. Thus, considering broad spectrum mycophagous activity, we expected that the NGJ1 would prove to be a better bio-control agent than other antifungal bacteria. This is because, other antifungal bacteria, can suppress the growth of fungi during interaction while the mycophagous bacteria can eat fungi and eradicate fungal biomass. Hence, under favourable condition, the fungi treated with antifungal bacteria can re-grow, but the fungi treated with mycophagous bacteria would not be able to grow and cause disease.

The team consisting of Durga, Sunil, Isha, Rahul, Rajeev, Srayan, Joyati who worked on it for approximately four years, found that treatment with NGJ1 could prevent sheath blight disease of rice.

Through a series of experiments, the group demonstrated that the NGJ1 uses one of its phage proteins to eat fungi. Generally phages are bacterial predators and can lyse bacterial cells. However, during coevolution, the NGJ1 has made its phage inactive (prophage) and deputesone of the prophage tail-like proteins (Bg_9562) to forage over fungi.

It is because of this Bg_9562 protein, the bacterium exhibits broad spectrum antifungal activity. The antifungal activity was observed against several economically important pathogens, such as Rhizoctonia solani (rice sheath blight pathogen), Magnaporthe oryzae (rice blast pathogen), Fusarium oxysporum (pathogenic to various plants), Aschocyta rabiei (chickpea blight pathogen), Venturia inaequalis (apple scab pathogen) and Candida albicans (causes Candidiasis in humans).

The scientists proposed that the protein could be used in different ways to control fungal diseases of plants as well as humans/animals. For example, a formulation using this protein can be sprayed over agricultural fields to control various fungal diseases at one go or an ointment based on this protein can be used to treat fungal infections of animals/humans. Furthermore, the Bg_9562 gene can itself be used as transgene to develop broad spectrum fungal disease resistant plants, which is need of the hour.