What is spontaneous generation?

Louis Pasteur

Louis Pasteur, Image via Wikipedia

The concept of Spontaneous Generation has been a scientific controversy in many ways equivalent to anthropomorphic climate change (global warming,) but perhaps lacking some of the political hyperbole that the latter field experiences. Spontaneous generation can be succinctly described as the premise that cells can arise from non-living matter, and apparently conclusive experiments either supporting or rejecting this hypothesis were conducted over the course of centuries. The first chapter of Bauman describes how this concept fell into and out of favor.

The Greeks, beginning with Aristotle, observed that non-living environments such as dry lake beds rapidly demonstrated an abundance of living organisms, following a summer rain shower. It was thought that these organisms arose via a process called abiogenesis, due to the infusion of water stimulating a vital force, resulting in the growth of animals. This way of thinking shaped the scientific understanding of biology for centuries, until the Italian physician Francesco Redi conducted a series of experiments in the late 1600’s that contradicted the premise of spontaneous generation. Redi placed meat into several containers, which were either open to the air or had covers on them. The open containers rapidly became spoiled and maggots appeared on the meat. The closed containers also became spoiled, but flies were kept from the meat, and no maggots appeared in the containers.  Redi’s experiments supported the contention that animals could only arise from other animals, but did not however disprove that the microorganisms could not arise by spontaneous generation.

John Needham was a British naturalist of the mid-18th century, and performed a series of experiments to demonstrate that broths contained a “life force” that promoted microorganisms to appear from non-living material. Needham prepared a series of beef infusions, and boiled them extensively. Following the boiling treatment, no living cells were observable, and the containers were tightly sealed by inserting corks. However after several days, the vials were cloudy with the growth of microorganisms, leading him to conclude that bacteria could arise spontaneously.

In 1799, an Italian scientist  named Lazzaro Spallanzani readdressed Needham’s experiments. Spallanzani boiled his beef infusions for an hour, then sealed the containers by melting the thin glass necks of his culture flasks. These cultures remained clear until the flask necks were broken, at which point they quickly demonstrated microbial growth. This result appeared to conclusively disprove the hypothesis of spontaneous generation, however critics of Spallanzani claimed that the critical “life force” (possibly oxygen?) was prevented from entering his flasks, and therefore preventing life from arising.

Illustration of the Swan-necked bottle used in...

Pasteur's Swan-necked flask: Image via Wikipedia

The final experiment which laid this debate to rest was carried out by the father of modern microbiology, Louis Pasteur in the 1860’s. Pasteur conducted an experiment essentially like Spallanzani’s, where he extensively boiled beef extracts in glass flasks. Pasteur however utilized a novel flask, called a “swan-neck” flask, to boil his beef extracts for an hour. These flasks remained open to the air (and available to the mysterious “life force”), but remained clear of all microbial growth once they cooled to room temperature.  The bend in the neck trapped microorganisms that fell from the air, and prevented them from reaching the sterilized broth. However, if the flask were tipped, so that broth entered the bend and then returned to the flask, the culture became cloudy within days, demonstrating that the broth still maintained the potential to allow microorganisms to grow. The Pasteur Institute in Paris has an example of his original swan-neck flask that still exhibits no microbial growth 150 years after being sterilized.

Bonus Opportunity: based on what we have learned to date, what do you think is going on with the work of Needham? Why did he observe the results that he did, given that the experiment of Spallanzani was not greatly different?

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About ycpmicro

My name is David Singleton, and I am an Associate Professor of Microbiology at York College of Pennsylvania. My main course is BIO230, a course taken by allied-health students at YCP. Views on this site are my own.

Posted on February 4, 2011, in A bit 'o history, Bonus!, Important, Lecture. Bookmark the permalink. 8 Comments.

  1. I think from Needham’s experiment the corks allowed some amount of oxygen into the containers which gave the microorganisms the nutrients they needed to grow. Also, in Spallanzani’s experiment the microorganisms didn’t grow until they became intact with the air. Which allowed the nutrients to come into contact and provide growth.

    • Good guess, but remember that we are learning in this chapter that there are plenty of organisms that have no requirement for oxygen, and will grow fine in the absence of it. Oxygen is actually a bit of a red herring in all of this, although it was certainly used by critics to explain the negative results of Spallanzani. Pasteur’s response was an elegant way to address this criticism, but I could imagine a scenario where Spallanzani’s tightly sealed cultures (which excluded oxygen) could be teeming with growth, for exactly the same reason as Needham’s.

  2. Is it possible that Needham’s experiment allowed for contamination of the broth. Although he boiled the broth extensively, between boiling and putting the cork on, there could have been time for contamination to happen and other microorganisms or outside baceteria to “fall” into the broth. Is this possible?

    • Contamination is always an issue with microbiology, and is a potentially likely source in this instance. If we assume that he did use good aseptic technique though, are there any other possibilities?

  3. The difference with the two experiments outcomes possibly had to do with the different containers that they were held in. Possibly the difference with the glass in Spallanzani’s experiment and whatever was holding the different extracts in Needham’s experiment maybe had something in them or the corks provided an environment that wasn’t seen in Spallanzani’s experiment.

    • Again, contamination due to the containers is always an issue. Let’s presume that glass is glass, and that isn’t the source of the microbial growth. I’m thinking along the lines of the organisms: are there bacteria, or bacterial structures that are resistant to boiling?

      • Yes, after looking back in the book I found that there are structures in bacteria called spores that help a bacteria in boiling and/or dry heat. The only way to get around them is by using pressure or something called an “autoclave” that incorporates pressure and can get the temperature of the water higher around 121 degrees F which is ideal to kill these bacteria with spores.

        • We actually came across “spores” twice already in the course. In Chapter 3, endospores are those bacterial structures that are formed when a cell comes under environmental conditions that are not conducive for normal growth. In Chapter 4 (and in lab 3) we learned about the differential endospore stain to help us to identify bacterial species that are able to form endospores. I believe that Needham’s cultures were undoubtedly contaminated with endosprores, likely from the same genus (Bacillus) that we looked at in lab.

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