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

Part of the difficulty in making this connection, oddly enough, arose from the fact that, since 1980, practices with laboratory bacteria have bee codified. The professional microbiologist may not describe and name a bacterium simply because it appears in nature--say, within an animal, on a plant or mushroom, or as a coating of hundreds on wood-nymph protists. According to the international rules of bacterial nomenclature, a microorganism must be described and named only in "pure culture" In other words, the microorganism must be described as it lives and grows in a test tube, all by itself. Furthermore, two different pure-culture samples of bacteria must be banked in at least two different international culture collections.

The irony of those rules is that several features of the bacteria in the wild--their habit of assembling into filaments; their shorter, swimming filaments that pursue a docking site on the lining of the animal's intestine; and the spore-attachment fibers [see image on preceding page]--tend not to form outside an animal's intestine. Those accoutrements of symbiotic life are dispensable, and dispensed with, under laboratory conditions, as Arthromitus, the jointed thread that lives "in healthy animals as a natural condition," becomes Bacillus, the test-tube bacterium in the service of microbiology.

The causative agent of anthrax, B. anthracis, differs from B. cereus (and, equivalently, test-tube Arthromitus) in one crucial respect: B. anthracis is armed. B. anthracis is B. cereus with two or three additional plasmids: lengths of DNA formed into tight rings. The relation of B. cereus to B. anthracis is similar to that between a Homo sapiens by himself and a Homo sapiens with an inherited gun. Like the gun, the plasmids may be passed from parent to offspring. Their ammunition is simply the genes that code for a terrible toxin and for a coating that protects the bacterium carrying the toxin from the host's immune system.

David J. Ellar of the University of Cambridge is an expert on the huge genus Bacillus. In addition to B. cereus and B. anthracis, many other members of the genus exist: oxygen-breathing, heat-resistant spore-formers. A large number of those walled, rod-shaped bacteria regularly associate with insects, mammals, or agricultural plants. Ellar heads a Bacillus genome project intent on obtaining a full sequence of bacilli other than B. anthracis to compare with that dangerous one.

After my student colleagues, including Jorgensen, Michael Dolan, Rita Kolchinsky, and Andrew Wier, identified Arthromitus with Bacillus, Ellar asked me to provide him with a sample of Arthromitus newly isolated from an insect intestine. He wanted to examine the genome of a bacillus that had never been tamed by laboratory life. Unfortunately, a lack of research funding and other assistance made it impossible to oblige him, but we had a lively conversation. Ellar remembered once observing a culture of bacilli that had just been acquired for his collection. Examining the culture through a microscope, he had been perplexed by the little fibers he had seen growing on the spores. After he read our Arthromitus papers, Ellar realized that in one new culture the spores had retained their attachment fibers, typical of wild Arthromitus. The culture probably had not been in the laboratory for long. I asked him where he had found the new strain. He looked through his extensive records and called back excitedly a few weeks later. The bacillus with the tiny fibers had been collected in the Pasoh forest of Malaysia. And yes, it came from a termite mound.