Louis Pasteur was a godsend to the wine and dairy industries, and every other business dependent on microbes. ©1995-2001 Lucid Interactive

Pasteur eyes bacteria
By conclusively pinning the disease on a living entity rather than "miasmas" or "vapors," de Bary helped advance the science of infectious disease. However, he was soon overshadowed by Louis Pasteur, a French chemist who studied rabies, fermentation of wine, and food spoilage, among other topics.

After several years of study, Pasteur proved that the processes of fermentation (making wine) and putrefaction (making yecchh!) were microbial -- they could not occur in mixtures that had been heated enough to kill microbes. Oddly, after a sample had been heated, Pasteur found that they might decay anyway. Some people considered this proof that decay is "spontaneously generated" in many organic substances.

Pasteur suspected that the decay was caused by new organisms arriving on airborne dust. Like de Bary had before him, he proved his point with a classic high-school-style experiment. He exposed two containers of meat (filet mignon, for all we know) to the air: one directly, and the other through a looping glass tube that allowed dust to settle before reaching the filet. Pasteur controlled for the variable that interested him -- the possibility that dust would carry spoilage organisms into the meat.

Dust kidding
You know as well as we do that if both containers had turned into head cheese, you'd not be reading about the experiment. In fact, Pasteur succeeded -- only the meat exposed to airborne dust spoiled. While de Bary had proven that fungal spores were infectious, Pasteur demonstrated what many scientists had begun to suspect -- that fermentation, decay and infectious disease could be caused by invisible micro-organisms rather than being "spontaneously generated."

Pasteur's apparatus for excluding dust played a critical role in the development of microbiology.
©1996-1997 State Darwin Museum of Natural History, Moscow, Russia

This was the germ theory of disease, and it was a critical advance for basic biology and medicine. According to the germ theory, infectious diseases are caused by itsy-bitsy creatures that evolve, reproduce and feed in ways that are somewhat analogous to other organisms.

In 1877, Pasteur went further by distinguishing two basic categories of microbes: aerobic (which require oxygen) and anaerobic (which kick the bucket if oxygen is present). His proof that heating kills spoilage organisms led to pasteurization -- heat treatment for milk and other foods, and to the canning industry -- a ubiquitous food storage method before refrigeration became so popular. Canning, in fact, uses two of Pasteur's principles: it kills microbes by heat, and the can prevents reinoculation. (Freezing and refrigeration, in contrast, generally inactivate rather than kill spoilage organisms, preventing reproduction.)

Too many Kochs spoil the broth?
If you ever get an infection, you should be grateful Robert Koch got curious about cattle diseases.
© 1997 The Nobel Foundation.

The next major step busted the field of medical microbiology wide open. The advance began with a cattle disease, anthrax, which occasionally afflicts people. Rod-shaped microbes had been spotted in the blood of anthraxed cattle as far back as 1849, but the overall picture was unclear. In 1875, Robert Koch, then an obscure German country doctor, performed a crucial set of experiments on the transmission of anthrax. On December 23, he inoculated a rabbit with blood from the hide of an infected cow. The next day, the rabbit had sickened and died. Under the microscope, Koch saw great numbers of bacteria which, he soon proved, could infect other rabbits. In other words, the pathogen could be passed from animal to animal, and identified at each step as the same pathogen.

Koch's original micrographs of Bacillus anthracis, the bacterium that causes anthrax. Courtesy Ken Todar

By 1878, Koch had built on this process by publishing what we now call "Koch's postulates." These four simple principles remain the gold standard for determining whether an organism causes disease:

Koch didn't stop there: he also identified the tuberculum bacillus, the cause of tuberculosis. His microbial techniques prevented contamination, leading to the growth of pure cultures that could be studied scientifically. But his most critical advance was his simple, elegant and acute procedure for identifying pathogens, which helped (many years later) identify the causes of bacterial diseases such as pneumonia, strep throat, and staphlococcal disease, and virtually every other epidemic since his time. In a sense, however, his postulates are an expansion of the mechanism de Bary had used almost 20 years previously to pin late blight on a living organism.

Where do we categorize all these small things?