Our Place in the Biosphere
No matter how you do the calculation, there are more bacteria than any other organism. They are the majority, easily out-numbering all other living beings. One estimate by microbiologists at the University of Georgia is that (at any moment) there are 5x1030 bacteria active and occupying every conceivable niche, including deep in the soil sediments and throughout every volume of water. Furthermore, bacteria cover the surfaces of all other organisms. We couldn’t possible out-number them because for each one of us, there are trillions of them, using each of us as habitats! According to scientists at the U.S. Geological Survey, there may be an additional 1018 bacteria circulating dust-like throughout the earth’s atmosphere. (You might want to linger on those big numbers. Have your students think about how to represent 5x1030. For example, if each bacterium is a sphere with a diameter of 1 micrometer, how big a cube could hold them all? Or if they were placed end to end, how long would the line be? This will give your students an opportunity to do some calculations with exponents and to convert from micrometers all the way to kilometers!)
Since the beginning of life on earth, 4 billion years ago, it has always been a bacterial world. Bacteria were the first organisms and for about 2 billion years were the only organisms. Multicellular organisms evolved later and humans evolved so recently that they scarcely appear on a time line that shows the history of life in billions. Try it with your students. Put a time line to scale, in billions across the chalk board beginning with the origin of the solar system, about 4.5 billion years ago, the origin of life, about 4 billion years ago, and the origin of multicellular organisms (fungi, animals, and plants) about 1 billion years ago. Add the “Cambrian Explosion” (when certain animal fossils became highly visible in the fossil record) at approximately 0.5 billion. And now find humans. We are about 1 million years old as a distinct lineage from the primates. There are 1000 million in 1 billion. Our lineage is briefer than the width of a chalk line on the billion year scale.
A few bacteria “view” us as yet another nutrient-rich surface, newly arrived compared to them but well worth inhabiting. However, the majority of bacteria are unaware of our existence. They dwell in the oceans, the deep sediments, the rain forests, and the polar ice caps with no direct interaction with us at all.
The Importance of Symbiotic Associations
Despite the bad publicity that bacteria often get, most are not pathogens. There are thousands of bacterial species, which associate with us, but only about 50 species that are truly pathogenic. Those pathogens typically are quite specific about when and where they act in destructive ways. Even for the pathogens, pathogenicity is not the usual occupation. For most bacteria living on and in us, apparently there are more advantages to a healthy host than a dead one.
Symbiotic associations are the usual or normal situations in which all organisms live. The world is full of interactions between organisms, some quite intimate such as the communities of microbial symbionts that we have with us at all times. For example, we humans are teeming inside and out with multitudes of microbes that far out-number even our own cells. We have about 1014 cells but only about 10% of those (1013) are human cells; the rest of “our” cells are our symbiotic bacteria.
(Here is another opportunity to remind your students about how to work with exponents and why 1014 is so much bigger than 1013.)
It is more difficult to find or to imagine an organism that is truly solitary or axenic (without associations of any kind with other species). Exceptions to the symbiosis “rule” are special laboratory colonies of axenic animals such as mice. These are taken from their mothers by sterile Cesarean section and maintained forever after on sterile food and water and in an atmosphere of sterile air. Some humans born with malfunctioning immune systems and therefore a much greater susceptibility to pathogens, can stay alive in sterile rooms with sterile air, food, and water—and unfortunately no direct contact with other humans.
How might we recognize and appreciate our bacterial symbionts? Their world is olfactory (a world of chemical smells). By paying attention to and analyzing the normal odors associated with humans, we catch a little glimpse (or whiff) of our symbionts. Furthermore, from a bacterial point-of-view, we are a vast, heterogeneous surface continuous with the soil and with our own food. Many of the same bacteria dwell in all three places. Some of our most distinctive body bacteria participate in making fermented foods and beverages, which are part of the cuisines of every human culture.This teacher sheet is a part of the The Ecology of Your Skin 1: Bacteria that Live on the Skin lesson.