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

One of Leidy's pleasures was to take long walks across the Delaware River from Philadelphia to New Jersey, where he would explore what were then isolated and bucolic landscapes along the railroad tracks. Next to the rail beds, Leidy observed with customary curiosity, were discarded ties beset with "white ants," as termites were then called. By contrast, he noted, in sound and sturdy ties the insects were seldom present. Leidy long wondered what the insects ate, because termites also occurred so often in teeming numbers in rotting logs and fences, where no obvious sources of food were present.

Leidy punctured the guts of the insects and examined the contents under his microscope. Amid liquid brown matter in the hindgut, he found the pieces of wood the insects had eaten. But his greatest astonishment came when he looked closely at the brown liquid: "fit was] swarming with myriads of parasites ... wonderful in number, variety, and form." Watching this scene under the microscope, he later wrote, reminded him of "the turning out of a multitude of persons from the door of a crowded meeting-house."

The episode came to be one of Leidy's most abiding microbial discoveries. The myriad citizens in Leidy's jostling crowds turned out to be symbionts that are omnipresent in healthy termites. Leidy later discovered similar microorganisms in wood-eating cockroaches. In 1850 he introduced those intestinal inhabitants to the world of science, in a paper titled "On the existence of entophyta in healthy animals as a natural condition." Among the "entophyta" (literally "plants [living] inside") that Leidy discovered in the termite was a bacterium that, more than a century later, became the subject of our study.

A large teaching chart, completed by Leidy in 1888, shows the life-forms within the intestine of Julus marginatus, a millipede; the arthropod plays host, as Leidy by then had realized, to some of the same microorganisms as the New Jersey termites do [see illustration on preceding page]. In fact, as he recorded on the chart, he discovered bundles of long filaments, which he dubbed "jointed threads," in the intestines of a number of arthropods.

Leidy went on to depict and describe the development and propagation of shiny spherical bodies, or spores, along many of the filaments. He noted that mature spores are released into the digestive tract, and he suspected that from there they move through the intestines by peristalsis. It is now known that they are defecated into the soil, where they survive because of their high resistance to desiccation and heat.

But though the spores from the cells that make up Leidy's jointed threads spend part of their life history in the soil, it is not useful to describe them as soil bacteria, because they do not grow in soil. To reach the next stage in their life cycle, they must be carried, ingested, or blown into some wet environment abundant with food: a clump of decaying vegetation in a farm pond, perhaps, or the intestine of another animal, or a petri dish of nutrient agar in a laboratory. If the spores reach such nutritious surroundings, they germinate; then, seizing the day, they grow and multiply quickly for as long as they are surrounded by enough air, food, and water to continue.