Back To CourseUExcel Microbiology: Study Guide & Test Prep
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
In the early 1980s, archaeologists discovered a 7,500-year-old bacteria in lake sediment. This probably sounds incredible. You may be thinking that no viable organism could possibly be older than that. Would you believe that the record for the oldest living organism not only beats the age of the lake sediment bacteria but completely blows it away? And, by blows it away, I mean by 25 million years!
The champion is a member of the bacterial genus Bacillus. And, by the end of this lesson, you will understand exactly why this is the bacteria you should suspect. Bacillus was found in 1995 in the stomach of a fossilized bee that had become trapped in amber around 25 million years ago. The scientists who discovered the bee were able to examine its stomach contents and found some very interesting things: viable bacterial endospores.
But, before we hand out the award to the bee stomach Bacillus, there is considerable controversy surrounding how to prove how old DNA is and what constitutes a living organism. So, don't be surprised if a new champion emerges as scientists explore new environments that could harbor extremely old bacteria. Now, back to those endospores.
I said at the beginning that the 25 million-year-old bacteria were alive, then said that the scientists found endospores, not cells (as you might expect from living bacteria). So, what is an endospore?
An endospore is an extremely resistant dormant cell structure produced by some bacterial species. If you break down the term endospore, 'endo-' means 'inside' and '-spore' refers to the 'dormant structure,' so the endospore is a structure formed inside the cell. There are many examples of endospore-forming bacteria. The two most common are Clostridium and Bacillus. In favorable conditions, these bacteria are actively growing and dividing cells. If a nutrient, such as carbon or nitrogen, becomes scarce or if the population becomes too dense, the bacteria can become stressed. They will enter a stasis phase, which is their equivalent of survival mode. The bacteria can survive in stasis until better growth conditions return. This is kind of similar to the science-fiction concept of cryogenic preservation, where a human body could be frozen in stasis until some distant time in the future, when they can be thawed and resume living.
So, now that we understand why a cell would form an endospore, how exactly do bacteria make endospores? We just discussed endospore formation as a strategy employed by some bacteria to survive in unfavorable conditions. Well, when these conditions are encountered, the process of sporulation begins, which is simply the process of endospore formation.
During the first phase of sporulation, the bacterium will replicate its DNA and undergo an incomplete cell division. The cell division is incomplete because one copy of the DNA will segregate to a small region of the cell and be engulfed into the original mother cell membrane. This smaller cell that results from the initial sporulation process is called the prespore. It will eventually become the mature endospore, while the larger mother cell will end up degrading.
Inside the prespore is a small amount of cytoplasm, the bacterial DNA, and dipicolinic acid. Dipicolinic acid stabilizes the DNA and proteins, preventing degradation during stasis. The mother cell then lays down thick layers of peptidoglycan and protein to act as a protective and resistant coat on the outside of the endospore. With the coat in place, the mother cell degrades, leaving behind a mature endospore.
One key thing you must remember is that sporulation is not reproductive. When Clostridium and Bacillus need to reproduce, they undergo the classic process of binary fission like other bacteria. In contrast, during sporulation, the cell does replicate the DNA chromosome, but only the mature endospore remains at the end of the process. Instead of two new growing cells, there is a single endospore in a new dormant life stage.
When favorable conditions return, endospores can quickly come out of stasis and begin growing again. The process usually requires three steps: activation, germination, and outgrowth. The activation process is not entirely understood, but activation can usually be accomplished by an increase in the temperature of the endospore for a minute or two, though not so high as to kill the endospore. Activation triggers the next step: germination. During germination, the protective coat softens and the dipicolinic acid is released. In the final step, outgrowth, water is taken up, the cell swells, and new DNA, RNA, and proteins are synthesized. The newly emerged cell is now fully out of dormancy and can grow and divide again.
Now that we know the why and how of endospores, let's take a look at some of the major features. Endospores are generally considered the most resistant living structures known. Endospores are able to resist desiccation (which just means drying), extreme heat and cold, radiation, chemical reactions, acids, and the effects of long periods of time, as illustrated by the endospores found in the bee belly.
Endospores have survived boiling for two hours, submersion in 70% ethanol for 20 years, and upwards of one million rems of radiation. As a comparison for that last one, humans usually die if exposed to only 500 rems.
What I find the most fascinating, however, is that even after all that abuse, returning the endospore to favorable conditions for even minutes will cause the protective coat to break down and the cell to begin growing!
Endospores are not indestructible despite nature's attempt at making them so. In laboratory and hospital settings, the best way to eliminate endospores is the autoclave. An autoclave is a machine that uses steam coupled with very high temperatures and pressures to sterilize objects and liquids.
What if you need to sterilize something that will melt, like some plastics or large pieces of machinery? There are also a few chemicals capable of inactivating endospores. Gases such as ethylene oxide and chlorine dioxide are capable of killing endospores, but these gases can be dangerous to work with and harmful to humans. The liquid glutaraldehyde is also sporicidal. As with the gases, this chemical is unstable and potentially dangerous to the people using it. Common bactericidal chemicals like bleach, phenols, and alcohols usually have absolutely no effect on endospores, making the more dangerous chemicals a necessary risk.
You might assume that if the methods for eliminating endospores are so toxic to humans, why take the risk of exposure? Why can't we just let the endospores be? Typically with bacteria, the answer is disease. Several very serious illnesses are caused by spore-forming bacteria. Perhaps the most serious is anthrax. Bacillus anthracis is the causative agent of anthrax and also a spore-former. Anthrax spores that get inhaled or contaminate a wound will quickly germinate and begin secreting toxins.
The genus Clostridium also forms spores and causes disease. Tetanus, botulism, and gas gangrene can result from Clostridium spores germinating in a favorable environment. Clostridium requires an environment without any oxygen present, so any wounds, dead tissues, or vacuum-sealed foods could be a potential growth medium.
Another infamous endospore-forming bacteria is Clostridium difficile, which you may have heard called C. diff. These bacteria invade the intestines of sick patients, where they can form endospores that survive antibiotic treatment. After the antibiotics wear off, the endospores can germinate and cause severe gastrointestinal disease and sometimes death.
Let's take a quick look back at what we've just learned. Bacteria in the genera Bacillus and Clostridium are able to enter a state of dormancy during unfavorable growth conditions. These bacteria form endospores that are extremely resistant to environmental extremes, chemical degradation, and the effects of time. When conditions improve, the endospores can germinate and rapidly return to normal growth.
While in endospore form, most common disinfectants and sterilization methods are unable to penetrate the endospore coat. But, endospores are susceptible to the effects of autoclaves, glutaraldehyde, and sterilizing gases, although these gases and chemicals pose risks to the people using them.
Several endospore-forming bacteria pose a significant threat as disease-causing organisms. Bacillus and Clostridium endospores can be inhaled or infect wounds as well as survive antibiotic treatments, leading to rebound infections.
At the end of this lesson, you'll be able to:
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Back To CourseUExcel Microbiology: Study Guide & Test Prep
28 chapters | 241 lessons