Spore forming inside a bacterium
You may have read elsewhere in this site that bacteria sometimes form protective spores to help them survive through tough times. Some other kinds of microbes do, too. Here's how that transformation takes place.
First off, you might think of a bacterial spore roughly as a mummified bacterium. The spore has a hard protective coating that encases the key parts of the bacterium—think of this coating as the sarcophagus that protects a mummy. The spore also has layers of protective membranes, sort of like the wrappings around a mummy. Within these membranes and the hard coating, the dormant bacterium is able to survive for weeks, even years, through drought, heat and even radiation. When conditions become more favorable again—when there’s more water or more food available—the bacterium "comes to life" again, transforming from a spore back to a cell. Some bacterial spores have possibly been revived after they lay underground for more than 250 million years!
Ok, so how do bacteria turn themselves into spores? First, the bacterium senses that its home or habitat is turning bad: food is becoming scarce or water is disappearing or the temperature is rising to uncomfortable levels. So it makes a copy of its chromosome, the string of DNA that carries all its genes.
Then, the rubbery cell membrane that surrounds the bacterial cell fluid begins pinching inward around this chromosome copy, until there’s a little cell within the larger bacterial cell. This little cell is called the "daughter cell" and the bigger, original one, what starts out as the "vegetative cell" in this illustration, is now called the "mother cell." Next, the membrane of the mother cell surrounds and swallows up the smaller cell, so that now two membrane layers surround the daughter cell. Between these two membranes a thick wall forms made out of stuff called peptidoglycan <pep-tid-oh-gly-can>, the same stuff found in bacteria’s rigid cell walls. Finally, a tough outer coating made up of a bunch of proteins forms around all this, closing off the entire daughter cell, which is now a spore. As the mother cell withers away or gets blasted by all kinds of environmental damage, the spore lies dormant, enduring it all, just waiting for things to get better.
Not all bacteria can form spores. But several types that live in the soil can. Bacteria in the Bacillus <buh-sill-us> and Clostridium <clah-strih-dee-um> groups are spore-formers. Their spores are called endospores.
Another group of bacteria called Methylosinus <meth-ill-oh-sigh-nus> produces spores called exospores. The difference between endospores and exospores is mainly in how they form. Endospores form inside the original bacterial cell, as described above. Exospores form outside by growing or budding out from one end of the cell. Exospores also don’t have all the same building blocks as endospores, but they’re similarly durable.
Members of the Azotobacter <ay-zoh-toe-back-ter>, Bdellovibrio <dell-oh-vih-bree-oh>, Myxococcus <mix-oh-cah-cuss> and Cyanobacteria <sigh-an-oh-back-teer-ee-uh> groups form protective structures called cysts <cists>. Cysts are thick-walled structures that, like spores, protect bacteria from harm, but they’re somewhat less durable than endospores and exospores.
Bacteria aren’t the only microbes that can form protective spores, however. Some protists can, too. For example, a group of parasitic protozoa called Microsporidia <mike-row-spore-ih-dee-uh> encase themselves in protective spores when they infect their hosts. Microsporidia are found mainly in the guts of insects and the skin and muscles of fish, although a few species can cause illness in people.
Microsporidium spore with tube thrust into host cells Courtesy of CDC
Microsporidia spores are usually round, oval or rod-shaped, although many species have elaborately shaped spores that may help hide them from their host immune systems. The spores help the protozoa survive while outside of a host’s body. Typically, hosts are infected when they swallow Microsporidia spores. Once the spores reach the gut, they poke a tube through their spore coats. This tube stabs through the host’s gut wall and other tissues. Then the Microsporidia cell fluid and nucleus—a cell's central command center—move through the hollow tube from the spore into the host cells. As Microsporidia reproduce in the host cells, new spores are formed that are typically passed out of the body with feces.
Note: The spores we’re talking about on this page are protective spores. A group of bacteria called Actinomycetes <ack-tin-oh-my-see-tees> and many kinds of fungi produce seed-like structures during reproduction that are also called spores. If you’ve ever stomped on a puffball mushroom, the brown cloud that jets out is a cloud of these reproductive spores. Like seeds, reproductive spores have tough outer coatings on them, but they aren’t as durable as protective spores or cysts.