The Debate Over Spontaneous Generation

Until the mid-1880s, many people believed in spontaneous generation, the idea that living organisms could arise from nonliving matter.

Francesco Redi demonstrated that maggots appear on decaying meat only when flies are able to lay eggs on the meat (1668). This was the first real example of modern experimentation with both experimental and control groups.

Even though Redi thought he had disproved spontaneous generation, for maggots anyway, he still believed it occurred in some cases. Everybody was aware that you could put hay in water and in a few days you'd have a bunch of those animalcules that van Leeuwenhoek kept talking about, so for years people continued to believe that microorganisms at least arose via spontaneous generation.

In 1745 John Needham claimed to show that microorganisms could arise spontaneously from heated nutrient broth. Everyone was aware that boiling animalcules would kill them, so Needham boiled broth, sealed the flasks, and got growth. He claimed that these results supported the idea of spontaneous generation.

In 1765 Lazzaro Spallanzani repeated Needham’s experiments and suggested that Needham’s results were due to microorganisms in the air entering his broth before he sealed the flasks. Spallanzani sealed the flasks, evacuated the air, and then boiled. When no growth occurred the conventional wisdom was that the "mysterious life force", which was required for spontaneous generation, was excluded.

While that sounds like a bunch of hokum it was not far from the truth, at least in terms of the requirements for a number of living organisms. It was around this same time Laurent Lavoisier demonstrated the oxygen requirement of living organisms, and Spallanzani was back to square one.

In 1858 Rudolf Virchow introduced the concept of biogenesis: living cells can arise only from preexisting cells ("Life from life").

Louis Pasteur demonstrated that microorganisms are in the air everywhere and offered proof of biogenesis with a set of elegant experiments in 1861.

To allow air to enter the flasks and at the same time prevent air-borne bacteria from gaining entry, Pasteur bent the necks of his flasks after he added broth.

He then boiled the broth, killing any microorganisms that were present.  If the theory of biogenesis was valid there should be no growth in the sterilized broth. 

And sure enough, that's exactly what happened.  As a matter of fact, some of the original flasks are still on display at the Pasteur Institute today.  (The personnel in charge of the flasks did eventually seal them to prevent jokesters from trying to blow bubbles, plug the ends with their gum, etc.)

Pasteur’s discoveries led to the development of aseptic techniques used in laboratory and medical procedures to prevent contamination by microorganisms that are in the air.