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When PPLO Became Mycoplasma

The smallest cell has had a long career in the spotlight

Harold J. Morowitz

Name Change

There was some confusion in the 1950s as to the name of the taxon that contained PPLOs. In 1958 Eyvind A. Freundt of the Statens Seruminstitut in Copenhagen, one of the world authorities on this group of organisms, authored a monograph, The Mycoplasmataceae (The Pleuropneumonia Group Of Organisms). Freundt’s paper a year later in the 1959 New York Academy Symposium was entitled “Morphology and classification of the PPLO.” I stress the confusion in the early days as to the morphology, physiology, phylogeny and nomenclature of the organisms that looked like the best bet in our search for smallness and simplicity. What was most difficult is that we had no idea of how this group of organisms related to other microorganisms, by phylogeny or biochemistry. Another enigmatic feature of the PPLO was its requirement for cholesterol, which was generally considered to be exclusively a constituent of animals and not seen in bacteria.

In 1962 Bob Cleverdon arranged an extensive workshop on PPLOs at the University of Connecticut. It brought us in contact with Shmuel Razin, who was building a center for PPLO research at the Hebrew University Medical School in Jerusalem. Razin, who came to Connecticut for a sabbatical the following year, was for the next 40 years a central figure in PPLO studies, presiding over his own center and traveling and collaborating with researchers around the world. There is hardly an area in mycoplasmology in which his influence is not felt. His insights contributed to our growing confidence in working with these strange microbes. By 1962 Mark Tourtellotte and I felt confident enough to write an article for Scientific American entitled “The smallest living cells.”

The period from 1962 until 1966 was characterized by a worldwide effort to promote the PPLO from veterinary pathogens to microbes that were understood in terms of biochemistry, morphology and growth characteristics. This was also a period when molecular biology and the genetic code were coming of age. In 1966, The New York Academy of Sciences decided to revisit this group of organisms in a conference entitled “Biology of the Mycoplasma.” The linguistic transition from Pleuropneumonia-like Organisms to Mycoplasma was now complete. Mycoplasma were cellular microorganisms lacking a cell wall but bounded by a cell membrane. The membranes contained cholesterol, which was not synthesized by the mycoplasma but was obtained from the host or from the growth medium. The conference had over a thousand attendees.

At this time our laboratory focused on the size and replication pattern of the mycoplasma genome. The problem was elegantly solved in the dissertation work of Hans Bode, now professor at the University of California, Irvine. He spent the rest of his career after getting his Ph.D. as one of the world leaders in hydra morphogenesis. (Given the current focus on the mycoplasma genome, Bode’s important early work has been improperly overlooked.) In the 1961-to-1964 period, extensive work was being done in several laboratories on a number of bacteria using radioautography and a protein-film electron-microscopic technique developed by Alfred Kleinschmidt and Rudolf Zahn. Using cultures of Mycoplasma hominis sp. H39, cells were lysed on an air-water interface while spreading in a protein film. Samples were then transferred to Formvar grids, dried and shadowed with platinum and palladium. They were then examined in the electron microscope and lengths determined. From the lengths and calibration with phage DNA, the genome size was shown to be 510 million daltons. Work on a number of other strains gave genome values in the range of 450 to 1,000 million daltons. Bode’s work was published in 1967 in “Size and structure of the Mycoplasma hominis H39 chromosome.” Mycoplasma had the smallest genome of any microorganism known at that time.

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