Most prokaryotes range in size from 1-10 micrometers (µm).
Reading assignment: Cowan Chapter 4, p. 86-113
Listen to Audio Overview of Chapter 4
Prokaryotic cells are the smallest, simplest and most abundant cells on Earth. Prokaryotes include bacteria and archaea (ancient bacteria). Prokaryotes lack a nucleus and complex organelles. Most prokaryotes range in size from 1-10 micrometers (µm). In contrast, most Eukaryotic cells (plants, animals, fungi, protozoans) range from micrometers (µm) to millimeters (mm) in size. The picture on the left below shows the tip of a surgical needle (shown in purple) contaminated with bacteria (shown in yellow).
Bacterial cell structure can be organized into 3 categories or layers:
1. External structures (appendages & coverings): flagella, fimbriae, sex pilus and glycocalyx
2. Cell envelope: cell membrane, peptidoglycan cell wall or an outer lipid membrane (only found in Gram-negative cells)
3. Internal Structures: Cytoplasm, nucleoid, bacterial chromosome, plasmid, ribosomes, and storage granules
Can you identify the different Prokarotic cell structures and
match each structure with the function it performs?
The external structures of a bacterial cell include appendages for movement (flagella) and adhesion (pili, fimbriae, and glycocalyx) to surfaces.
Fimbria are small, fingerlike projections (see color photo below) that aid in bacterial cell attachment to surfaces. Escherichia coli uses fimbriae to attach to the intestinal wall of its host (see black and white photo below)
A sex pilus is a long, rigid tube made of protein called, pilin. Most bacteria reproduce asexually through a process called binary fission. However, some bacteria use the sex pilus or pili (plural) to "mate" with other bacterial cells. During conjugation (sexual reproduction), bacteria use the sex pilus to transfer genetic material between the two cells (see photo below).
Flagella (flagellum-singular) are long, hair-like appendages that function like propellers. They allow bacteria to swim and move. Unlike the flagella found on human sperm, bacterial flagella rotate 360 degrees. If the flagellum rotates counterclockwise, the cell swims in a smooth line called a "run." During "tumbles" the flagellum "reverses" direction and the cell stops and changes its course.
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The number and arrangement of flagella on a bacterial cell can vary.
Flagella can have one of the following arrangements:
axial filament, where the flagellum is located INSIDE the bacterial cell giving the bacteria cell a "corkscrew" appearance (found in spirochetes such as Treponema pallidum)
monotrichous, where there is 1 flagellum located at one 1 end of the cell (found in Bdellovibrio, a predatory bacteria)
lophotrichous, where there are small tufts of flagella located at 1 end of the cell (Vibrio fischeri, a marine bacteria)
amphitrichous, where there are flagella on both ends of the cell (Aquaspirillum)
peritrichous, where there a several flagella dispersed around the surface of the cell
Bacterial flagella can have a monotrichous, lophotrichous, amphitrichous, or peritrichous arrangement. Can you correctly identify these different arrangements?
The surface of bacteria is covered with a sticky coating called a glycocalyx. The glycocalyx is composed of polysaccharides (sugars) and proteins. The bacterial glycocalyx has 2 forms, a rigid capsule and a loose slime layer. Capsules are found on many pathogenic (disease-causing) bacteria including, Streptococcus pneumoniae, which causes a respiratory infection of the lungs. The glycocalyx has several functions including: protection, attachment to surfaces, and formation of biofilms.
A biofilm is a living layer of bacteria that is attached to a surface. For example, biofilms are commonly found in showers, toilets, catheters, medical equipment, and even in your mouth! Dental plaque is an example of a biofilm commonly found in humans.
The glycocalyx helps protect the bacteria cell by preventing immune cells from attaching to it and destroying it through phagocytosis. Biofilms can have serious medical implications, because once they form on surfaces, they are difficult to eliminate. They can form on damaged tissues, teeth, and medical devices (catheters, artificial hip joints, IUDs).
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Bacteria are protected by an outer cover called the cell envelope. The cell envelope consists of 2-3 layers of protection: 1) cell wall; 2) cell membrane; and 3) an outer membrane (only found in Gram-negative bacteria).
The bacterial cell wall surrounds and protects the fragile cell or plasma membrane, much like a bicycle tire protects the inner tube. The cell wall provides protection and structural support for the cell. It also determines the shape of the bacterial cell.
The bacterial cell wall contains peptidoglycan, which is made from long glycan (sugar) chains cross-linked (held together) by peptides, much like a chain-link fence. There are 2 types of glycan chains that make up peptidoglycan: N-acetyl glucosamine (NAG) andN-acetyl muramic acid (NAM).
Bacteria can be classified into 3 groups based on differences in the thickness or composition of the cell wall structure: Gram-positive, Gram-negative, and Acid-fast. TheGram stain is a technique used to distinguish between Gram-positive cells that have a thick layer of peptidoglycan and stain purple and Gram-negative cells which have a thin layer of peptidoglycyan and stain pink.
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Gram-Positive Cell Wall
The Gram-positive cell wall consists of a thick (20-80nm) layer of peptidoglycan. This thick layer is porous making the cell wall absorbent like a sponge. The Gram-positive cell wall also contains teichoic acid, which functions in cell wall maintenance and gives the cell surface an acidic (-) charge.
Gram-Negative Cell Wall
TheGram-negative cell wall consists of thin (1-3nm) layer of peptidoglycan. Because the Gram-negative cell wall is so thin, these bacterial cells require an extra layer of protection, called the outer membrane. The outer membrane consists of a phospholipid membrane, similar to the cell or plasma membrane. This membrane has tiny holes or openings calledporins. Porins block the entrance of harmful chemicals and antibiotics, making Gram-negative bacteria much more difficult to treat than Gram-positive cells. In many antibiotic resistant bacteria, these porins are connected to drug pumps, which pump out any drugs or harmful chemicals that enter through the porins.
Attached to the outer membrane is is a highly-branched fatty sugar called lipopolysaccharide (LPS). LPS acts as an endotoxin because it induces fever and shock in the human host.
Acid-fast bacteria contain a waxy substance called mycolic acid and a small amount of peptidoglyan. Due to their waxy cell wall, these bacteria are highly resistant to staining and treatment. Mycobacterium tuberculosis , the causative agent of TB, is one example of a bacterial cell with an acid-fast cell wall. These bacteria must be heated and treated with an acid-alcohol in order to stain them in the lab (See image below).
Another type of bacteria with an unusual cell wall is Mycoplasma pneumoniae. This bacteria attaches to the epithelial cells in the lungs, causing pneumonia. The cell wall of this bacteria contains large amounts of sterols (rigid lipids), which make it difficult to treat. Due to the waxy content of their cell wall, these bacteria are pleiomorphic and vary in shape from long and filamentous to round.
Can you identify each of the key structural features associated with
Gram-positive, Gram-negative, and Acid-Fast bacteria?
The cell or plasma membrane is a thin, fragile membrane located just beneath the bacterial cell wall. The plasma membrane consists of a phospholipid bilayer that contains negatively-charged phosphate heads that are hydrophilic and hydrophobic lipid tails that are insoluble in water. The plasma membrane also contains proteins that aid in the transport of materials into and out of the cell. The plasma membrane is selectively permeable, which allows it to regulate the flow of materials into and out of the cytoplasm. Materials that dissolve in lipids (fat soluble) pass between phospholipids. Materials that cannot dissolve in lipids must pass through the transport proteins. The plasma membrane is often targeted and disrupted by detergents and antibiotics.
Inside the flexible plasma membrane is the cytoplasm or "guts" of the cell. The cytoplasm is a gelatin-like substance made of water, protein , carbohydrates and salt. The other internal structures, such as the nucleoid, plasmid, ribosomes, storage granules, and endospores are suspended in the cytoplasm.
Like all prokaryotes, bacteria do not have a nucleus. Instead, the bacterial chromosome is found floating within a dense region of the cytoplasm called the nucleoid region. Unlike humans, bacteria only have 1 or at most a few chromosomes, which are tightly-coiled, circular pieces of DNA that contain all of the information required for survival.
In addition to the bacterial chromosome, some bacteria also contain plasmids. Plasmids are small, circular pieces of DNA that are not required for survival. Plasmids are transferred between bacterial cells during conjugation via the sex pilus. Plasmids carry genes (information) for antibiotic resistance. They also allow bacteria to produce a sex pilus for sexual reproduction (conjugation).
Although bacteria lack many of the complex organelles found in Eukaryotic cells, they do contain up to 10,000 ribosomes per cell. Ribosomes consist of 2 subunits (50S/30S) or parts that resemble a hamburger bun. The large subunit (top of the bun) is made of ribosomal RNA (rRNA) and has the molecular size of 50 Svedberg units (50S). The smaller subunit (bottom of the bun) has a size of 30S. These subunits join together to form a protein factory where messenger RNA (mRNA) is translated into long chains of amino acids to form proteins.
Bacteria use granules to store minerals and nutrients (lipids, carbohydrates, phosphates, sulfur or metals) for the cell to use when needed.
Some bacteria have the ability to produce endospores. Endospores are an internal storage compartment that consists of 3 layers of protection: peptidoglycan, calcium (dipicolinic acid) and keratin. Endospores provide long-term storage and protection of the genetic material when moisture and nutrients are not available. Sporulation, or the formation of an endospore, occurs when nutrients and moisture are low. This process takes about 6-8 hours and begins with duplication of the bacterial chromosome. During the process of sporulation, the duplicated chromosome is sealed inside 3 thick layers of protection. Once moisture and nutrients return, the endospore quickly uncoats during a process called germination.
Endospores are highly resistant to conditions that would kill most bacteria. They can withstand extremely high and low temperatures, pH, treatment with chemicals, and exposure to ultraviolet (UV) radiation. Endospores can survive these extreme conditions for up to 250 million years! This makes endospore-forming bacteria the very difficult to treat. Not all bacteria are able to form endospores. Bacillus anthracis (anthrax), Clostridium botulinum (botulism) and Clostridium tetani (tetanus) are examples of endospore-forming.
Please watch this animation about endospore formation
Watch Bacterial Shapes Video
Prokaryotes (bacteria) are smaller (1-10 micrometers, um) than Eukaryotic cells and can be distinguished by the lack of a membrane-bound nucleus. Prokaryotic cells have 3 major shapes: cocci, bacilli or spirilla.
Bacterial cells can be arranged in different groups or patterns. These cells can be arranged singly, in pairs (diplo), groups of 4 (tetrads), chains (strepto), clusters (staphylo), packets of 8 or 16 (sarcinae), or hinged together (palisades).
The following types of bacteria have a rod or bacillus shape: Bacillus anthracis (anthrax), Lactobacillus acidophilus (yogurt), and Bacillus megaterium, which consists of long chains of rods called (streptobacillus). Corynebacterium diphtheriae (diphtheria) and Clostridium tetani (tetanus) are club-shaped bacteria hinged together in an arrangement called palisades.
Many Gram-positive bacteria have a spherical shape called coccus. These cocci can be arranged in groups of 2 (diplococci) like Streptococcus pneumoniae (pneumonia) and Neisseria gonorrhoeae (gonorrhea), or in long chains like Streptococcus pyogenes (strep throat), or in clusters like Staphylococcus aureus (food poisoning and skin infections).
Other bacteria are spiral-shaped. For example, Vibrio cholera (cholera) forms a comma-shaped spiral. Other bacteria, such as Treponema pallidum (syphilis), are considered spirochetes, because they form a more tightly-coiled spiral.
Bacteria that have a waxy cell wall, such as Mycoplasma pneumoniae have an irregular shape called pleiomorphic. These bacteria vary in shape from long and filamentous to round.
Can you correctly identify each of the following bacterial shapes?
Can you correctly identify each of the following bacterial cell arrangements?
All bacteria belong to the domain Prokaryotae because they lack a true nucleus. Bergey's Manual of Determinative Bacteriology breaks this domain down into 4 major divisions (which are similar to phylums) based upon the structure of their cell wall: Gracilicutes (Gram-negative), Firmicutes (Gram-positive), Tenericutes (no cell wall), and Mendosicutes (archaea).
For example, the taxonomic classification of the intestinal bacteria, Escherichia coli would be as follows:
Kingdom: (none assigned)
Division (similar to Phylum): Gracilicutes (Gram -)
Class: Scotobacteria (non-photosynthetic)
The scientific name always includes both the Genus and species ( Escherichia coli or E. coli), but can also include the strain or subspecies ( E. coli 0157:H7).
Escherichia coli 0157:H7 is the pathogenic strain which causes bloody diarrhea and hemorrhaging.
These groups can also be organized based upon their genetic structure, oxygen requirements (aerobes versus anaerobes) or their shape (coccus, bacillus, spirilla).
Bacteria are classified based upon: cell wall structure, aerobes vs. anaerobes, shape, genetic structure and can be identified using the following methods described in Chapter 17 (p. 514-522) and referenced in your lab manual.
Macroscopic morphology (appearance of bacterial colonies on petri dish
Microscopic morphology (bacterial shape & arrangement under the microscope)
Physiological / biochemical characteristics (metabolism: aerobes vs anaerobes)
Chemical analysis (cell wall composition)
Serological analysis (antibodies)
Genetic & molecular analysis (DNA & rRNA sequence)
Can you identify which category each of the following bacteria belongs to?
Listen to Audio Overview of Chapter 4
4.1 General Features of Prokaryotes
(4.2-4.4) 3 Types of Bacterial Structures:
External structures :
Cell Envelope :
Gram-Positive Bacteria :
Gram-Negative Bacteria :
Acid-Fast Bacteria :
4.5 Bacterial Size, Shape & Arrangment
4.6 Identification and Classification:
Bacteria are classified based upon cell wall structure, aerobes vs. anaerobes, shape, genetic structure using the following laboratory tests (Chapter 17, p. 514-522)
Can you identify the different Prokarotic cell structures and match each structure with the function it performs?
Can you identify each of the key structural features associated with Prokaryotes and Eukaryotes?