Prokaryotic Cell Structure: Endospores

II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL STRUCTURE

3. Structures Located Within the Cytoplasm

e. Endospores

The overall purpose of this Learning Object is:
1) to learn the structure and the functions associated with bacterial endospores;
2) learn several genera of bacteria are able to produce endospores; and
3) learn the role of endospores play for a few specific bacteria in transmitting infectious disease.

LEARNING OBJECTIVES FOR THIS SECTION

In this section on Prokaryotic Cell Structure we are looking at the various organelles or structures that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane;
a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and
often various external structures such as a glycocalyx, flagella, and pili.

Structures located within the cytoplasm of bacteria include the nucleoid, ribosomes, and in some bacteria, plasmids, endospores, inclusion bodies, and organelles used for photosynthesis. We will now look at bacterial endospores.

Endospores (def)

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 are obligate aerobes (def) that live in soil while Clostridium species are obligate anaerobes (def) often found as normal flora of the gastrointestinal tract in animals.

Scanning electron micrograph of Clostridium botulinum with endospore; courtesy of Dennis Kunkel's Microscopy.

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 and a cytoplasmic membrane septum forms at one end of the cell. A second layer of cytoplasmic membrane then forms around one of the DNA molecules (the one that will become part of the endospore) to form a forespore. Both of these membrane layers then synthesize peptidoglycan in the space between them to form the first protective coat, the cortex. Calcium dipocolinate is also incorporated into the forming endospore. A spore coat composed of a keratin-like protein then forms around the cortex. Sometimes an outer membrane composed of lipid and protein and called an exosporium is also seen.

Finally, the remainder of the bacterium is degraded and the endospore is released. Sporulation generally takes around 15 hours. The process is summarized in Fig. 1.

GIF animation showing endospore formation

GIF animation showing endospore germination

YouTube animation of endospore formation by http://biology-forums.com

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

B. Endospore Structure (see Fig. 1J)

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 1K. 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!

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.

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:

anthrax, caused by Bacillus anthracis;

tetanus, caused by Clostridium tetani;

botulism, caused by Clostridium botulinum; and

gas gangrene, caused by Clostridium perfringens).

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