Germ Theory
Enter the Sci-lebrities
There are a few people we need to celebrate for the fact that we know about prokaryotes at all. Nowadays we know that many things in the world—both good and bad—are dependent upon bacteria. From ancient times until the mid-1800’s, however, the dominant theory held that diseases were caused by exposure to poisonous gases or "miasma". If you see a colonial-era lady in a movie saying she has caught "the vapors", that's what they're talking about.This miasma theory of disease was overtaken by the germ theory of disease, which is centered on the idea that bacteria, viruses, and some eukaryotic pathogens cause infectious diseases. We wouldn’t have gotten there without the work of a few curious scientists.
The first guy we want you to meet is Anton von Leeuwenhoek, aka "the Father of Microbiology". Van Leeuwenhoek was a Dutch businessman and microscope maker who was among the first people to observe cells of any kind. In the late 1600’s, he became the first person to describe single-celled organisms. He called them "animalcules".
Okay, so picture yourself back then. You had never heard of microbes and then this business guy starts talking about animalcules? As you’re probably thinking, his discoveries (though real!) were met with some skepticism at first. They were accepted within a few years, though, and microbiology was born.
Over the next 180 years or so, it became apparent that microbes were all around us, and that they were especially good at turning up in spoiled food. A popular theory, called the spontaneous generation theory held that the appearance of these microbes was a random, intrinsic property of the food. To say that another way, it was believed that the microbes appeared spontaneously from non-living manner. FAIL. We now know that nothing can spring to life out of nowhere.
The germ theory, on the other hand, held that life came from other life, that microbes grew and divided to make more microbial cells.
One experimental test of spontaneous generation depended on sterile broth. Boiling broth, or other liquids, kills microbes. If this process is done in a sealed container, the broth remains sterile. If the seal is broken, however, the broth turns cloudy and starts to spoil. Here is a diagram of this:
In the first part (1.), two flasks (one sealed, one open) are sterilized. The liquid inside is sterile and shows no sign of microbes. The heat is turned off and the flasks are left to sit (2.). After some time, (3.), the broth in the open flask begins to spoil, while the broth in the sealed flask remains sterile.
This result could be interpreted two ways, however. Germ-theory advocates believed that the sealed flask was sterile because germs couldn’t get in. Spontaneous generation advocates believed that air contained a life force necessary for growth of spontaneously generated microbes. The sealed flask, they argued, couldn’t access this life force.
Louis Pasteur
The Frenchman Louis Pasteur designed a pivotal experiment that showed that the microbes in spoiled food came from the growth and reproduction of other microbes. He designed special "swan-necked" flasks that allowed for sterile broth to access any "life force" in the air, but would not allow microbes to fall into the flask.
Here, as above, flasks are sterilized (1.) and left to sit (2.). The broth in both flasks is exposed to air, but only one flask allows microbes to access the broth, while the other flask traps them in the neck. In the panel (3.), the flask to the left shows evidence of spoiling while the swan-necked flask to the right remains sterile.
Check out Pasteur’s actual flasks here.
Meanwhile in England, John Snow was looking into an outbreak of the disease cholera. Cholera is caused by a bacterial infection, but no one knew that back then. He traced the source of the disease to a water pump that was being contaminated by a nearby leaking cesspit. Cesspits are one place that the water from, ummm, toilets ends up.
Snow looked at the well water under a microscope, but (1.) he didn’t know for sure that cholera was caused by bacteria and (2.) classifying bacteria can be tricky. All well water has SOME microbes in it, after all.
Snow was not able to determine if bacteria caused cholera, but his tracking of the epidemic to a specific contaminated well worked to disprove the miasma theory all the same. His work was a huge advance in the study of infectious diseases.
The miasma theory was put to rest by the work of a German named Robert Koch. Koch proved the germ theory of disease, showing that three diseases—anthrax, cholera, and tuberculosis—depended upon specific bacteria. We’re going to go over those experiments in the next section.