The Elusive PPLO

The elusive little bugs are smaller than typical bacteria but are generally bigger than true viruses. Even with an electron microscope they are so difficult to corner and classify that their very name is vague--PPLO (for pleuropneumonia-like organisms). But these days that name keeps popping up in lab reports from all over the world.* The baffling microbes have already been indicted for complicity in causing diseases ranging from puerperal (childbed) fever to the "viral pneumonia" that afflicts so many recruits in boot camps. Now they are even being suspected as a possible cause of cancer.

Breakout After Stress. Like viruses, PPLO can invade living cells and destroy them from within. Like bacteria, they can grow in a chemical broth independently of living cells. Though PPLO differ from bacteria in having ill-defined shapes (see diagram), some are believed to be variant forms of bacteria. And like many bacteria, some PPLO are natural inhabitants of the human respiratory, intestinal and genital tracts, where they cause no disease until they are activated when the individual has been subjected to unusual stress.

Such a stress, say Dr. Joseph G. Tully and his National Institutes of Health colleagues in the New England Journal of Medicine, may be something as routine as a therapeutic abortion or surgical repair of a woman's genitalia after a difficult delivery. The surgery, they suspect--on the basis of a near-fatal case of PPLO blood poisoning--may help to spill PPLO into the bloodstream. British researchers have also incriminated PPLO in puerperal fevers and fevers following gynecologic surgery.

One of the few things clear about PPLO is that they have been fouling up microbiologists' experiments for years. They sneak in and contaminate cultures of both viruses and tissues, where they confuse investigators with their odd patterns of growth. Confusion between viruses and PPLO, suggests Dr. Jo/rgen Fogh of Manhattan's Sloan-Kettering Institute, may explain why many researchers, who have suspected viruses of causing some forms of human cancer, have assumed that the mysterious particles sometimes found in cancer cells were viruses when actually they were PPLO.

Garbled Message. Dr. Fogh's most provocative finding: when PPLO infect human cells grown in test tubes, they destroy some cells, but--more significantly--alter the genetic material of others. They cause the development of deformed chromosomes, and even of entirely new chromosomes never seen in natural cells. Thus, their presence gives the cell a garbled genetic message so that it will produce abnormal daughter cells--a process sometimes observed in cancer. Neither Dr. Fogh nor anyone else is yet ready to say flatly that PPLO cause cancer, but since researchers have found the organisms in test-tube growths of cancer cells, medical suspicions are running high.

* Microbiologists refer to a single PPLO as a mycoplasma, then have trouble deciding whether the plural should be mycoplasmas or mycoplasmata.

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