Marriage of equals
28 September 2002
NewScientist.com news service
Gail Vines

 Everything turned on his scrupulous technique. Over the years, he had established nearly 2000 separate cultures, some of which he had to keep going for months or even years, as he struggled to separate and cultivate each microorganism in a pure state.

To avoid contamination, he first ensured that every flask, beaker tube, funnel, watch glass and microscope slide was absolutely sterile. All apparatus was baked or boiled for several hours. Next, he concocted an extensive menu of nutrient broths to cater for every taste. The fussiest fungi dined on best bouillon made from lean beefsteak, finely chopped and soaked overnight in distilled water, then filtered and boiled.

Even then, some microorganisms failed to thrive or resisted purification, and for these cases Ward perfected a way of isolating a single yeast cell in a "hanging drop" secured to a microscope slide, thus guaranteeing the culture's purity while he tracked down its identity.

His diligence paid off, for when he published his results in 1892 in the Philosophical Transactions of the Royal Society , no one questioned his astonishing announcement. Buried in this scholarly text is a biological bombshell. The ginger-beer plant, Ward proclaimed, was a new kind of organism - a "composite body", consisting of dozens of microorganisms living amicably together in a symbiotic lump. Not all of these microbes helped in making the beer. The majority Ward regarded as opportunistic interlopers. They turned up by chance, and hung around for the free lunch.

But two organisms were present in every plant sampled, and seemed to be vital to the production of ginger beer. One was a fungus, a new species of yeast he called Saccharomyces pyriformis . The other was a bacterium, which he named Bacterium vermiforme , and is now called Brevibacterium vermiforme .

Ward reckoned that these two microbes had developed a symbiotic relationship, to their mutual benefit. He couldn't be sure of the biochemical details, but he guessed that the bacterium consumed the yeast's waste products, while the yeast benefited from their removal. Together, the two produced the essential ingredients of traditional ginger beer: carbon dioxide and alcohol.

The conclusive proof came when Ward made perfect ginger beer in his laboratory, using his own plant, reconstituted from his pure cultures of the right yeast and bacterium.

Read the rest of this article at New Scientist magazine

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From issue 2362 of New Scientist magazine, 28 September 2002, page 50

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