II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL ANATOMY
3. Cellular Components Located Within the Cytoplasm
Fundamental Statements for this Learning Object:
1. Endospores are dormant alternate life forms produced by a few genera of bacteria.
2. The genus Bacillus (an obligate aerobe often living in the soil) and the genus Clostridium (an obligate anaerobe living in the gastrointestinal tract of animals) produce endospores.
3. Under conditions of starvation, a single endospore forms within a bacterium through a process called sporulation, after which the remainder of the bacterium is degraded.
4. The completed endospore consists of multiple layers of resistant coats (including a cortex, a spore coat, and sometimes an exosporium) surrounding a nucleoid, some ribosomes, RNA molecules, and enzymes.
5. Endospores are quite resistant to high temperatures (including boiling), most disinfectants, low energy radiation, and drying.
6. The endospore survives until a variety of environmental stimuli trigger germination, allowing outgrowth of a single vegetative bacterium.
7. Infectious diseases such as anthrax, tetanus, gas gangrene, botulism, and pseudomembranous colitis are transmitted to humans by endospores.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Anatomy we are looking at the various anatomical parts that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
We will now look at bacterial endospores.
Endospores are dormant alternate life forms produced by the genus Bacillus, the genus Clostridium, and a number other genera of bacteria, including Desulfotomaculum, Sporosarcina, Sporolactobacillus, Oscillospira, and Thermoactinomyces.
Bacillus species (see Fig. 1) are obligate aerobes (def) that live in soil while Clostridium species (see Fig. 2) are obligate anaerobes (def) often found as normal flora of the gastrointestinal tract in animals.
|Fig. 1: Endospore stain of Bacillus megaterium||Fig. 2: Endospore stain of Clostridium tetani|
Note green endospores within pink bacilli.
Note the endospore within the rod gives the bacterium a "tennis racquet" shape (arrows).
A. Formation of Endospores
Under conditions of starvation, especially the lack of carbon and nitrogen sources, a single endospores form within some of the bacteria. The process is called sporulation (def).
First the DNA replicates (Fig. 3, step 1)and a cytoplasmic membrane septum forms at one end of the cell (Fig. 3. step 3). A second layer of cytoplasmic membrane then forms around one of the DNA molecules (Fig. 3, step 4) - the one that will become part of the endospore - to form a forespore (Fig. 3, step 5). Both of these membrane layers then synthesize peptidoglycan in the space between them to form the first protective coat, the cortex (Fig. 3, step 6) that lies adjacent to the germ cell wall that will eventually form the cell wall of the bacterium upon germination.
Calcium dipocolinate is also incorporated into the forming endospore. A spore coat composed of a keratin-like protein then forms around the cortex (Fig. 3, step 7). Sometimes an outer membrane composed of lipid and protein and called an exosporium is also seen (Fig. 3, step 8).
Finally, the remainder of the bacterium is degraded and the endospore is released (Fig. 3, step 9). Sporulation generally takes around 15 hours. The process is summarized in Fig. 3.
B. Endospore Structure (see Fig. 3, step 10)
The completed endospore consists of multiple layers of resistant coats (including a cortex, a spore coat, and sometimes an exosporium) surrounding a nucleoid, some ribosomes, RNA molecules, and enzymes.
- To view an electron micrograph of an endospore of Bacillus stearothermophilus, see the Microbe Zoo web page of Michigan State University.
(Some bacteria produce spore-like structures distinct from endospores. Exospores are heat resistant spores produced by a budding process in members of the genus Metylosinus and Rhodomicrobium. Cysts are resistant to drying and are formed singly within vegetative cells by Azotobacter, Myxococcus, and Sporocytophaga. Conidia are heat-susceptible asexual reproductive spores produced by various genera of branching bacteria belonging to the group Actinomycetes.)
C. Function of Endospores
An endospore is not a reproductive structure but rather a resistant, dormant survival form of the organism. Endospores are quite resistant to high temperatures (including boiling), most disinfectants, low energy radiation, drying, etc. The endospore can then survive until a variety of environmental stimuli trigger germination (def), allowing outgrowth of a single vegetative bacterium (def) as shown in Fig 3, step 11 and step 12 and in Fig. 4. Viable endospores have reportedly been isolated from the GI tract of a bee embedded in amber between 25 and 40 million years ago. Viable endospores of a halophilic (salt-loving) bacterium have also reportedly been isolated from fluid inclusions in salt crystals dating back over 250 million years!
Bacterial endospores (def) are resistant to antibiotics, most disinfectants, and physical agents such as radiation, boiling, and drying. The impermeability of the spore coat is thought to be responsible for the endospore's resistance to chemicals. The heat resistance of endospores is due to a variety of factors:
- Calcium-dipicolinate, abundant within the endospore, may stabilize and protect the endospore's DNA.
- Small acid-soluble proteins (SASPs) saturate the endospore's DNA and protect it from heat, drying, chemicals, and radiation. They also function as a carbon and energy source for the development of a vegetative bacterium during germination.
- The cortex may osmotically remove water from the interior of the endospore and the dehydration that results is thought to be very important in the endospore's resistance to heat and radiation.
- Finally, DNA repair enzymes contained within the endospore are able to repair damaged DNA during germination.
D. Endospores and Infectious Disease
Although harmless themselves until they germinate, they are involved in the transmission of some diseases to humans. Infections transmitted to humans by endospores include:
- Anthrax, caused by Bacillus anthracis; endospores can be inhaled, ingested, or enter wounds where they germinate and the vegetative bacteria subsequently replicate.
- Tetanus, caused by Clostridium tetani; endospores enter anaerobic wounds where they germinate and the vegetative bacteria subsequently replicate.
- Botulism, caused by Clostridium botulinum; endospores enter the anaerobic environment of improperly canned food where they germinate and subsequently replicate.
- Gas gangrene, caused by Clostridium perfringens); endospores enter anaerobic wounds where they germinate and the vegetative bacteria subsequently replicate.
- Pseudomembranous colitis (Clostridium difficile); antibiotics destroy the normal microbiota of the intestines that keep the growth of C. difficile in check while the endospores of C. difficile survive and subsequently germinate and replicate before the microbiota is restored.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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Updated: March, 2014
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