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Antibiotic produced by potato pathogen offers new hope

Although we cannot see them without the aid of a microscope, bacteria are all around us, occurring at high densities in diverse habitats. Many bacterial species produce substances that kill other microbes as a way of gaining an ecological advantage in environments that support an array of competing bacterial and fungal species. These substances, once identified, often form the basis of our own antibiotic and antifungal medications that help to fight infections in our bodies and in our crop plants and livestock. In fact, most therapeutic antibiotics used today have been derived from soil bacteria belonging to the phylum Actinobacteria.

Unfortunately, our increasing dependence on antibiotics has resulted in widespread resistance to these medications which, according to the World Health Organization, represents one of the biggest threats to global health, food security, and development today. This has lead researchers to seek alternative new compounds with antifungal and antibiotic properties wherever possible. 

In a new discovery, published in mBio, the peer-reviewed, open access scientific journal published by the American Society for Microbiology, a multinational team of researchers from Europe report on one such compound that may have the potential to protect agricultural crops against fungal infections. The new antifungal antibiotic has been named solanimycin, and was initially isolated from a pathogenic bacterium that infects potatoes. 

Solanimycin acts against a wide range of fungi known to infect and devastate agricultural crops, according to the researchers. In lab studies, the compound also acted against Candida albicans, a fungus that occurs naturally in the human body but that can also cause dangerous infections. The results suggest that solanimycin, and related compounds, could be useful in both agricultural and clinical settings.

The new discovery suggests that, instead of focusing on antibiotics produced by bacteria isolated from soil microbes, researchers seeking new compounds with antibiotic properties would do well to look more closely at plant-based microorganisms as potential sources. This is especially relevant as crops develop resistance to existing treatments, says microbiologist Rita Monson, Ph.D., from the University of Cambridge. She co-led the study with molecular microbiologist Miguel Matilla, Ph.D., at the Spanish Research Council’s Estación Experimental del Zaidín, in Granada.

“We have to look more expansively across much more of the microbial populations available to us,” Monson said.

The pathogenic potato bacterium Dickeya solani, which produces solanimycin, is not new to researchers. It was first identified more than 15 years ago in the lab of molecular microbiologist George Salmond, Ph.D., at the University of Cambridge. At that time, it was found to produce an antibiotic called oocydin A, which was highly effective against multiple fungal pathogens in plants. In the current study, Matilla, Monson, Salmond and colleagues followed a hunch that D. solani may be capable of producing other compounds with antifungal properties. 

After genomic analysis of the bacterial DNA, they proceeded to silence those genes responsible for the production of oocydin A. Interestingly, they found that the bacterium continued to show antifungal activity under these conditions. That observation led to the identification of solanimycin and the identification of the gene clusters responsible for the proteins that make the compound.

The researchers found that the bacterium uses the compound sparingly, producing it in response to cell density. An acidic pH environment – as is present in a potato – also activates the solanimycin gene cluster. Monson said it almost looks like a clever protective mechanism.

“It’s an antifungal that we believe that will work by killing fungal competitors, and the bacteria benefit so much from this,” said Monson. “But you don’t turn it on unless you’re in a potato.”

Monson said the researchers have begun collaborating with chemists to learn more about the molecular structure of solanimycin and understand how it works. Then, she and Matilla said, they hope to see continued testing of the compound in plant and animal models.

“Our future steps are focused on trying to use this antibiotic antifungal for plant protection,” Matilla said. The research team see the discovery as an encouraging sign that plant pathogens – like D. solani – could be coaxed to make compounds that may be used against diseases in plants and people.

“We have to be open to the exploration of everything that’s out there to find new antibiotics,” Matilla said.

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By Alison Bosman, Staff Writer

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