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One of the reasons microbes have survived and flourished for billions of years is the different ways they reproduce and how fast they do it.
Bacteria usually reproduce by simply dividing in two. Each new bacterium is a clone of the original—they each contain a copy of the same DNA. This is called binary fission (bye-nair-ee fish-un). If conditions are just right, one bacterium could become a BILLION (1,000,000,000) bacteria in just 10 hours through binary fission!
Sometimes bacteria do have a form of sex called conjugation (con-ju-gay-shun). One bacterium reaches out to another using a thread-like structure called a pilus. The first bacterium transfers part of its DNA to the other bacterium through the pilus.
By this exchange, bacteria can quickly create or pass along new traits that help them adapt to different environments.
Some yeasts, which are a type of fungus, also reproduce through binary fission. Other yeasts reproduce by budding. A parent yeast cell forms a growth, or bud, on its surface. As this bud gets bigger, the parent cell's nucleus divides into two by a process called mitosis (my-toe-sis). One of these nuclei transfers into the growing bud, which eventually pinches off. One yeast cell can produce up to 24 daughter cells.
Other fungi reproduce through spore formation. Fungi can produce spores either without sex or when two fungal cells fuse and share DNA. Spores are kind of like seeds in that they can be carried on the wind or rain, spreading the fungus to new places. Some species can do both sexual and asexual reproduction, depending on environmental conditions.
Microbes in the protist category reproduce in a number of ways. Algae, for example, can reproduce without sex. The nucleus of the algal cell divides by mitosis and moves to opposite ends of the cell. Then the cell divides into two new cells—this is called cytokinesis (sigh-toe-kin-ee-sis). Some species can reproduce sexually. Some do both. Protozoa reproduce by binary fission, budding or a process called schizogony (skiz-og-uh-knee). Schizogony is multiple fission: the cell's nucleus divides several times before the cell itself divides into multiple new cells, each with one of those new nuclei.
Viruses cannot reproduce on their own, which is one reason people debate whether they should be counted as living creatures. But when they're inside a suitable host cell, all they do is reproduce and they do it well. They take over the host cell's machinery and use it to make copies of their genes and proteins. These genes and proteins come together to make new viruses, which eventually escape the host cell and drift off to infect new cells.
So why aren't we up to our elbows in bacteria, fungi and protists if microbes can reproduce so quickly? It's because conditions are almost never perfect in the real world. Once there are too many microbes in one place, food runs out, their wastes pile up, they crowd each other and eventually they start dying off.
But there are still billions of them under your feet and all around you!