Adolf Mayer | Biography

German microbiologist Adolf Mayer was one of the first scientists to study tobacco mosaic disease. Although he incorrectly concluded that it was caused by bacteria, other scientists would draw upon his work as they eventually discovered the disease's true cause--the tobacco mosaic virus (also known as TMV). TMV was the first virus ever identified.

Mayer attended college at the Universities of Heidelberg, Ghent, and Halle. In 1876, he moved to the Netherlands, where he became head of the Agricultural School at Wageningen. He was soon approached by a group of Dutch tobacco farmers whose crops were being ravaged by disease. They hoped Mayer could find a way to prevent the disease from spreading. (Tobacco had been brought to Europe from the New World, and by the middle of the 1800s, it was a major Dutch crop.)

Because it caused a mottled pattern to form on the plants' leaves, Mayer named it the tobacco mosaic disease. He began a series of experiments to determine its cause. He found that when he took sap from a diseased plant and applied it to a healthy plant, the healthy plant became diseased. Therefore, he concluded that the disease was infectious rather than hereditary. (Mayer may have been the first person to infect plants with a virus as part of an experiment.) Next, he took infected sap and passed it through a single layer of filter paper. The filtrate (the liquid that passed through the filter) remained infectious. From this evidence, Mayer concluded that the cause of the disease was not a fungus because any fungi would have been too large to pass through the filter.

However, when Mayer passed infected sap through two layers of filter paper, he found that the filtrate was no longer infectious. This evidence suggested to him that the cause of the disease was bacterial, because bacteria would normally pass through a single filter, but not two. However, when Mayer looked at infected sap under a light microscope, he could see no bacteria. In addition, he was unable to grow the infectious agent in artificial cultures like he could with other types of bacteria. In an attempt to account for these seemingly conflicting results, Mayer concluded that the cause of tobacco mosaic disease was an unusually small species of bacteria.

In 1892, however, Russian botanist Dmitri Ivanovski (1864-1920) found that sap infected with tobacco mosaic disease did in fact remain infectious even after passing through two layers of filter paper. Six years later, Dutch microbiologist Martinus Beijerinck (1851-1931) confirmed Ivanovski's results. He went on to conclude that a previously undiscovered type of infectious agent (rather than bacteria) caused the disease. As a result, these two men, rather than Mayer, are generally credited with the discovery of viruses.

Scientists were eventually able to offer explanations for the results of some of Mayer's experiments. For instance, Mayer was unable to see an infectious agent under a light microscope because viruses are much too small to be seen with such an instrument. In addition, he was incapable of growing the infectious agent artificially because viruses can only reproduce inside living cells.