Jointed threads: Joseph Leidy was the first to describe symbiotic bacteria growing together in long strings in animal intestines. Microbiological analyses now link the bacteria with anthrax

Natural History, June, 2005 by Lynn Margulis

Leidy recognized from the outset that the spores of jointed-thread entophytes that occur in healthy organisms might be related to contagion. As he observed,

Contagious diseases and some others might have their origin and reproductive character through the agency of cryptogamic spores, which from their minuteness and lightness are so easily conveyed from place to place through the atmosphere by means of the gentlest zephyr.

The word cryptogamic (from the Greek crypto-, "hidden," plus 2amein, "to marry") in Leidy's time referred to an archaic grouping of seedless plants (such as ferns and mosses), as well as to algae and bacteria (regarded as plants that lack flowers). Only two great groups, or "kingdoms," of living beings were recognized. If an organism was not an animal, it had to be a plant.

Given that dichotomous classification, it should be no surprise that Leidy described his jointed threads in the language of botany. The organisms tended to be "rooted" to the epithelium, or inside surface layer of cells, of the animal's intestine. Less frequently, they were rooted to one of the other intestinal inhabitants. They did not swim. They developed shiny spheres that he suspected were "plant spores." Clearly, then, the jointed thread was a plant. (Such a classification does not mean that Leidy failed to recognize his jointed threads as bacteria. It was just that they--and for that matter, all bacteria--were clearly not animals, and so they had to be plants.)

From various termite species, Leidy identified a series of related but distinguishable jointed threads, which he named Arthromitus cristatus. He classified the jointed threads from the common cockroach as a second species, A. intestinalis. After Leidy's detailed scientific articles and drawings were published, such jointed threads were also discovered in the intestines of many other animals, including ducks and dogs. Jointed threads, it turns out, are "plants" inside intestines everywhere.

A century and a half after Leidy, my colleagues and I have often followed in his footsteps in our studies of the microbial communities in termites, wood-eating cockroaches, and a few other anthropods. On occasion, I have collected termites in Arizona south of Tucson, and was fortunate to examine what lives inside the Sonoran desert termite (Pterotermes occidentis). We keep termites in the laboratory, and we have often observed, with variations, just what Leidy depicted in his illustrations. We have even seen the same branching filaments that Leidy drew but did not name.

We were not the first, however, to reconfirm Leidy's findings. In the 1940s, the protozoologist Harold Kirby prepared beautiful, permanent stained slides for the microscopic study of symbionts in the guts of termites. Kirby's interest was in wood nymphs--protists that lack mitochondria, swim vigorously, and digest hefty wood fragments--but his slides include the same straight and branched Arthromitus-like filaments that Leidy's drawings portray. Sixty years after Kirby, we, too, saw the filaments.