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When PPLO Became Mycoplasma

The smallest cell has had a long career in the spotlight

Harold J. Morowitz

Now that Craig Venter, Hamilton Smith and Clyde Hutchison, visionaries in the attempt to synthesize a living cell, have created three species of mycoplasma into landmarks on the road to their ambitious goal, mycoplasmas have become a favorite taxon in the popular press and the world of futuristic biology. When I first encountered these organisms they were known as pleuropneumonia-like organisms—PPLOs—and few dreamed of the glory that lay ahead for this enigmatic group. How did these obscure microbes became important, indeed central, in understanding what it takes to be a living organism? The story will be told from an idiosyncratic and personal point of view—the view from my bench over five decades—and will show how mycoplasma instructed—and deceived—us about the nature of life.

2011-03MacroMorowitzFA.jpgClick to Enlarge ImageAs an assistant professor of biophysics at Yale University in the late 1950s, I decided to focus on the origin of life, a subject that had intrigued me since I had read Erwin Schroedinger’s profound little book, What is Life? Agreeing with da Vinci that simplicity is the ultimate sophistication, I decided to look for the smallest and simplest free-living organism, thinking it would be the closest to the earliest organisms. Serendipity struck and Bob Cleverdon, professor of microbiology at the University of Connecticut, appeared in my office and announced that he was looking for a laboratory at Yale to spend a one-year sabbatical. Bob was a broadly trained and experienced bacteriologist, and the search for the smallest and simplest free-living microbe struck his imagination.

Shortly thereafter our collaboration began. Bob wrote letters to a large group of bacteriologists he knew, and I took to Bergey’s Manual of Determinative Bacteriology and the American Type Culture Collection looking for species with words like parvulus in the species name, hinting at tiny cells. Soon cultures were arriving by mail, and the incubator became filled with flasks and petri dishes. The search was complicated by the fact that the smallest bacteria were near the limit of visibility using optical microscopy, and we were not at all sure about what criteria to use as the measure of simplicity. While we were pondering this, the DNA genome became accepted, and the DNA content—total weight and ratio of nucleotides—of several bacterial genomes were appearing in the literature. It seemed obvious to us that simplicity was the same as the smallest amount of DNA in the bacterial chromosome. At that time the genome and the single bacterial chromosome were considered to be the same. Our task changed to seeking the organism with the smallest amount of DNA in the postdivisional cell.

At this juncture serendipity struck again when Cleverdon became aware of the work on pleuropneumonia-like organisms being done by Mark Tourtellotte, a graduate student in the Department of Animal Diseases at the University of Connecticut College of Agriculture. A conversation between Bob and Mark informed us that PPLOs could pass through the fine filters that captured normal bacteria and hence were probably very small, although the fact that they lacked a rigid cell wall might indicate that they were sufficiently deformable to ooze through the filter. In any case it was clear that PPLOs had to be added to our collection of candidate organisms and that a new array of laboratory techniques would have to be learned.

Eventually Bob’s sabbatical was over and he returned to the University of Connecticut, passing the baton to Mark Tourtellotte, who was traveling in the opposite direction from Connecticut to Yale for a post-doctoral fellowship. The smallest-genome project now had two nodes operating cooperatively.

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