Online Textbook Bacteriology is continuously updated and includes information on Staphylococcus, MRSA, Streptococcus, E. coli, anthrax, cholera, tuberculosis, Lyme disease and other bacterial diseases of humans.
Kenneth Todar is the author of the Online Textbook of Bacteriology and an emeritus lecturer at the University of encourages people to wear a FDA approved face mask during the Covid-19 pandemic.
The Online Textbook of Bacteriology is a general and medical microbiology text and includes discussion of staph, MRSA, strep, Anthrax, E. coli, cholera, tuberculosis, Lyme Disease and other bacterial pathogens.
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Tag words: bacterial structure, flagellum, flagella, pilus, pili, fimbriae, capsule, S-layer, glycocalyx, slime layer, biofilm, outer membrane, LPS, cell wall, peptidoglycan, murein, teichoic acid, plasma membrane, cell membrane, phospholipid bilayer, transport system, proton motive force, pmf, ATPase, DNA, chromosome, nucleoid, ribosome, 30S subunit, 50S subunit, 16S rRNA, inclusion, PHB, glycogen, carboxysome, endospore, parasporal crystal.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Structure and Function of Bacterial Cells (page 10)

(This chapter has 10 pages)

© Kenneth Todar, PhD


A bacterial structure sometimes observed as an inclusion is actually a type of dormant cell called an endospore. Endospores are formed by a few groups of Bacteria as intracellular structures, but ultimately they are released as free endospores. Biologically, endospores are a fascinating type of cell. Endospores exhibit no signs of life, being described as cryptobiotic. They are highly resistant to environmental stresses such as high temperature (some endospores can be boiled for hours and retain their viability), irradiation, strong acids, disinfectants, etc. They are probably the most durable cell produced in nature. Although cryptobiotic, they retain viability indefinitely such that under appropriate environmental conditions, they germinate back into vegetative cells. Endospores are formed by vegetative cells in response to environmental signals that indicate a limiting factor for vegetative growth, such as exhaustion of an essential nutrient. They germinate and become vegetative cells when the environmental stress is relieved. Hence, endospore-formation is a mechanism of survival rather than a mechanism of reproduction.

Figure 28. Early and late stages of endospore formation. Drawing by Vaike Haas, University of Wisconsin Madison. During endospore formation, a vegetative cell is converted to a heat-resistant spore. There are eight stages, O, I-VII, in the sporulation cycle of a Bacillus species, and the process takes about eight hours. During the early stages (Stage II,) one bacterial chromosome and a few ribosomes are partitioned off by the bacterial membrane to form a protoplast within the mother cell. By the late stages (Stage VI) the protoplast (now called a forespore) has developed a second membrane and several wall-like layers of material are deposited between the two membranes.

Table 13. Differences between endospores and vegetative cells
Property Vegetative cells Endospores
Surface coats Typical Gram-positive murein cell wall polymer Thick spore coat, cortex, and peptidoglycan core wall
Microscopic appearance Nonrefractile Refractile
Calcium dipicolinic acid Absent Present in core
Cytoplasmic water activity High Very low
Enzymatic activity Present Absent
Macromolecular synthesis Present Absent
Heat resistance Low High
Resistance to chemicals and acids Low High
Radiation resistance Low High
Sensitivity to lysozyme Sensitive Resistant
Sensitivity to dyes and staining Sensitive Resistant

Figure 29. Bacterial endospores. Phase microscopy of sporulating bacteria demonstrates the refractility of endospores, as well as characteristic spore shapes and locations within the mother cell.

Figure 30. Electron micrograph of a bacterial endospore. The spore has a core wall of unique peptidoglycan surrounded by several layers, including the cortex, the spore coat and the exosporium. The dehydrated core contains the bacterial chromosome and a few ribosomes and enzymes to jump-start protein synthesis and metabolism during germination.


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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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