PROKARYOTIC CELL ANATOMY:
BACTERIAL ENDOSPORES

Bacterial Endospores

Fundamental Statements for this Lesson: 

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.

 

Common Course Objectives

  1. Identify the parts of a bacterium and their physiological purpose.
  2. Articulate why endospore formation enhances bacterial survival and recall the medically important endospore formers and the diseases they cause.
  3. Provide a diagnosis for a "patient" when given a list of symptoms and a description of the bacterium.

 

Detailed Learning Objectives for this Lesson

 

1. Name 2 common genera of bacteria capable of producing endospores and state which is an obligate anaerobe.

2*. Briefly discuss the function of a bacterial endospore.

3. Describe the structure of a bacterial endospore.

4*. Define sporulation and germination.

5. Name three infections that may be transmitted to humans by endospores.

(*) = Common theme throughout the course

(**) = More depth and common theme

 

Highlighted Bacterium

1. Read the description of Clostridium tetani and match the bacterium with the description of the organism and the infection it causes.

 

TPS Question

 

 


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.

 


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 that live in soil while Clostridium species (see Fig. 2) are obligate anaerobes 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

Endospore stain of Bacillus megaterium as viewed through a light microscope. Endospores appear as green ovals within red vegetative cells. Note green endospores within pink bacilli.

Copyright © Gary E. Kaiser 

Endospore stain of Clostridium tetani as viewer through a light microscope. Endospores appear as clear ovals within gray vegetative cells. The endospore within the rod gives the bacterium a tennis racquet shape shown at arrows. Note the endospore within the rod gives the bacterium a "tennis racquet" shape (arrows).

Copyright © Gary E. Kaiser 

 

Scanning electron micrograph of Clostridium botulinum with endospore.

Courtesy of Dennis Kunkel's Microscopy.

Scanning electron micrograph of Bacillus anthracis endospores.

Image provided by Janice Haney Carr.

Courtesy of the Centers for Disease Control and Prevention.

Scanning electron micrograph of Clostridium botulinum with endospores. The spore-containg end appears swollen or club-shaped. Flagella are also visible.

Scanning electron micrograph of Clostridium botulinum with endospores. The spore-containg end appears swollen or club-shaped. Flagella are also visible.

Scanning electron micrograph of Bacillus anthracis endospores; courtesy of CDC.

 

 

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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.

First the DNA replicates (slideshow Fig. 3, step 1) and a cytoplasmic membrane septum forms at one end of the cell (slideshow Fig. 3, step 3). A second layer of cytoplasmic membrane then forms around one of the DNA molecules (slideshow Fig. 3, step 4) - the one that will become part of the endospore - to form a forespore (slideshow 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 (slideshow Fig. 3, step 6) that lies next 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 (slideshow Fig. 3, step 7). Sometimes an outer membrane composed of lipid and protein and called an exosporium is also seen (slideshow Fig. 3, step 8).

Finally, the remainder of the bacterium is degraded and the endospore is released (slideshow Fig. 3, step 9). Sporulation generally takes around 15 hours. The process is summarized in (slideshow Fig. 3).

 

Fig. 3, steps 1-9: Formation of endospores

  Slideshow Activity 

 

 

GIF animation showing endospore formation

GIF animation showing endospore germination

 

 

  

 Ordering Activity  

 

B. Endospore Structure

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 (slideshow Fig. 3, step 10).

 

Transmission electron micrograph of an endospore of Bacillus stearothermophilus.

Image by Stu Pankratz.

Courtesy of the Microbe Zoo web page of Michigan State University.

Transmission electron micrograph of an endospore of Bacillus stearothermophilus showing white cortex, multilayered spore coat, and outer endosporium.

 

  Slideshow Activity 

 

(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, allowing outgrowth of a single vegetative bacterium as shown in (slideshow Fig. 3, steps 11 and 12). 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) bacteria have also reportedly been isolated from fluid inclusions in salt crystals dating back over 250 million years!

 

Fig. 4: Scanning Electron Micrograph of the Germination of an Endospores of Clostridium sporogenes.

Scanning Electron Micrograph of the Germination of an Endospores of Clostridium sporogenes. Image provided by Kathryn Cross, Institute of Food Research.

Image provided by Kathryn Cross, Institute of Food Research.

 

 

Fig. 3, steps 11-12:Germination of endospores

  Slideshow Activity 

 

 

Bacterial endospores 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:

 

TPS Question

 

 

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:

1. Anthrax, caused by Bacillus anthracis; endospores can be inhaled, ingested, or enter wounds where they germinate and the vegetative bacteria subsequently replicate.

Endospore stain of Bacillus anthracis

Endospore stain of Bacillus anthracis as viewed through a light microscope. Clear endospores can be seen within blue vegetative cells.

Copyright © Gary E. Kaiser 

2. Tetanus, caused by Clostridium tetani; endospores enter anaerobic wounds where they germinate and the vegetative bacteria subsequently replicate.

Endospore stain of Clostridium tetani

Endospore stain of Clostridium tetani as viewed through a light microscope. Clear endospores can be seen within gray vegetative cells.

Copyright © Gary E. Kaiser 

3. Botulism, caused by Clostridium botulinum; endospores enter the anaerobic environment of improperly canned food where they germinate and subsequently replicate.

Endospore stain of Clostridium botulinum

Courtesy of the Public Health Image Library (Images), Centers for Disease Control and Prevention. Licensed for use, ASM MicrobeLibrary.

Endospore stain of Clostridium botulinum showing green endospores within red vegetative cells. Courtesy of the Public Health Image Library (Images), Centers for Disease Control and Prevention. Licensed for use, ASM MicrobeLibrary.

4. Gas gangrene, caused by Clostridium perfringens;•endospores enter anaerobic wounds where they germinate and the vegetative bacteria subsequently replicate.

Endospore stain of Clostridium perfringens

Image provided by Dr. Holderman.

Courtesy of the Centers for Disease Control and Prevention.

Endospore stain of Clostridium perfringens as viewed through a light microscope. Clear endospores are seen within blue vegetative bacilli.

5. Pseudomembranous colitis caused by 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.

Endospore stain of Clostridium perfringens

Image provided by Lois S. Wiggs.

Courtesy of the Centers for Disease Control and Prevention.

Scanning electron micrograph of Clostridium difficile from a stool sample showing bacillus-shaped bacteria.

 

 

Highlighted Bacterium: Clostridium tetani

Click on this link, read the description of Clostridium tetani, and be able to match the bacterium with its description on an exam.

 

Concept map for Bacterial Endospores

 

 

E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.

  • Bacillus anthracis
  • Clostridium tetani
  • Clostridium perfringens
  • Clostridium botulinum

 

 

 

Self Quiz for Bacterial Endospores

 

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