Airborne microbes are everywhere. But since we cannot see them, people do not realize that they exist. In fact, because of airborne microbes, until the last century people believed that life was spontaneously made from non-living material, a process called "spontaneous generation." This was because invisible microbes floating in the air would settle down on food and then multiply and turn previously dead matter to life. Louis Pasteur finally devised an experiment to exclude airborne microbes from food (broth) to prove that sterile food would not spontaneously spoil.
So far, no known microbe lives its entire life cycle in the air. At some point they must come down to surface where there is water and nutrients to complete the life cycle. Some scientists speculate, though there is no proof, that certain microbes may grow inside of clouds, where there is enough water and some nutrients to support growth. Maybe you will be the one to discover that microbes do indeed grow inside of clouds.
Many airborne microbes are in a resting state called spores. Spores are produced by bacteria and by fungi and plants (we will not discuss plant spores). One of the functions of spores is to spread organism through the air to distribute the organism geographically. Spores survive well in the air because they have several important adaptations. Most spores have thick skins to prevent them from drying out (desiccation). They have a slow metabolism and therefore require little food (which is missing in the atmosphere). Spores can resist the harmful effects of ultraviolet radiation (U.V. light). U.V. light is a potent carcinogen for us humans and is deadly to unprotected microbes. Spores that are resistant to U.V. have dark colored pigments that filter out harmful rays - in much the same way that dark glasses protect our eyes from sunburn.
Many bacteria form spores as a way to resist harsh conditions such as too little food or water, or excessive heat. Bacterial spores are most prevalent in spring and fall.
Most airborne spores are from fungi. Many fungi have adaptations that allow them to shoot their spores into the air so that they may be dispersed a long way. Fungal spores appear to be most abundant in June, July and August.
Watch airborne microbes grow! Moisten a slice of bread. Allow the bread to become inoculated with spores by letting the bread sit on the counter for several hours. Put the bread in a loose baggy. Every day, look at the bread to see what happens.