Bacterial Cell Structure
Structure of Bacterial Cell
Capsule
The outermost part of a bacterial cell is the capsule, often described as the glycocalyx. Most capsules are composed of polysaccharides, although in some species the capsule is made of polypeptides. Capsules can be visualised
under light microscopy by using staining techniques.
The main function of bacterial capsules is to provide protection from adverse environmental conditions, prolonging the period of survival in such conditions. The capsule also facilitates adherence to surfaces and interferes with host cells involved in phagocytosis.
Cell Wall
The cell wall lies between the cell membrane (inner) and the capsule (outer) and protects the bacteria from mechanical damage and osmotic lysis. Cell walls are non-selectively permeable and are only able to exclude large molecules. Species dependant differences in the structural and chemical composition of the cell wall creates variation in the pathogenicity of the cell and also influences the staining properties of the cell which is important for species identification. Peptidoglycan (a polymer unique to prokaryotic cells) provides the cell wall with rigidity.
Bacteria can be divided into two major groups on the basis of the colour of the cell wall when stained using the Gram Method. The groups are called “Gram Positive” and “Gram Negative”. Gram positive bacteria stain blue and have a thick cell wall composed mainly of peptidoglycan and teichoic acids. Gram negative bacteria stain red and their walls have a much more complex structure containing
an outer membrane, a periplasmic space and an inner membrane. For further information on the structure of both types of cell wall please see Bacterial structure
Antibiotic treatments such as penicillin interefere with the ability of the bacterial cell to produce peptidoglycan and therefore cannot produce their cell wall making them more vulnerable to the environment.
Cytoplasmic Membrane
Bacterial cytoplasmic membranes are flexible structures composed of phospholipids and proteins and are similar to the lipid bi-layer membranes found in eukaryotic cells. Only a limited number of small molecules such as water, carbon dioxide and lipid-soluable compounds can enter bacterial cells by passive diffusion. Nutrients and waste metabolites are transferred via active transport using ATP (adenosine triphosphate).
The cytoplasmic membrane is also the site of electron transport for bacterial respiration and also contains enzymes and carrier
molecules that function in the biosynthesis of DNA, cell wall polymers and membrane lipids.
Cytoplasm
The cytoplasm is enclosed by the cytoplasmic membrane and is an aqueous fluid containing nuclear material, ribosomes, nutrients and the enzymes involved in most cellular functions. Storage granules can often be seen in the cytoplasm under certain environmental conditions. Storage granules mainly contain starch and glycogen.
Nuclear Material
The bacterial genome is composed of a single haploid circular chromosome containing double-stranded DNA (dsDNA). Bacterial genomes
vary in size depending on species but often has a folded structure to form a dense body which is visible using a scanning electron microscope. During replication the DNA helix unwinds and both daughter cells (produced by binary fission) receive
a copy of the original genome.
The cytoplasm also contains Plasmids. Plasmids are small circular pieces of DNA that are separate from the genome and are capable of autonomous replication. Several different plasmids can be within the cytoplasm of a single bacteria. Plasmids
can be transferred between bacteria during binary fission or through a process called conjugation. Plasmid DNA codes for characteristics including antibiotic resistance and endotoxin production.
Flagella & Pili/Fimbrae
Motile bacteria have flagella allowing them to move into suitable micro-environments in response to physical or chemical stimuli. It is mainly gram negative bacteria that possess flagella and they are rarely present in cocci species. Flagella are normally several times longer than the bacterial cell and are composed of the protein flagellin. Flagella are usually anchored
to the cell wall.
Pili are straight, hair-like appendages composed of pilin and also anchored to the cell wall. Pili are most common on gram negative
bacteria. In pathogenic bacteria pili function as adhesions for receptors on mammalian cells.
Endospores
Endospores are dormant bodies that are highly resistant to the environment. The only genera of pathogenic bacteria that are able to
produce endospores are Bacillus and Clostridium. Endospores have a “spore coat” and are effectively in a dehydrated state with negligible metabolic activity. Due to the thermostability of endospores they can only be destroyed with certainty by moist heat at 121C for 15 mins.
An endospore will reactivate in response to environmental factors such as exposure to heat, abrasion of the spore coat or environmental acidity. Reactivation occurs in three stages; activation, initiation and outgrowth. In the correct conditions, germination will occur in which the spore coat is degraded and water is absorbed.
To Incorporate
Bacterial genome:
- Contains double-stranded DNA
- Prokaryotic DNA differs to eukaryotic DNA:
- Few repeated sequences
- Most of the DNA is transcribed
- No intervening sequences within structural genes
Cytoplasm:
- Does not contain mitochondria, lysosomes or Golgi bodies (found in eukaryotic cells)
- Contains mesosomes- thought to be primitive endoplasmic reticulum
Surface components:
- Fimbriae- also known as pili, these are hair-like structures that allow bacteria to adhere to each other
- F-type pili- also known as sex pili, these act as conjugation tubes during sexual reproduction
- Capsules/slime- serve to adhere bacteria to cells and provide protection from phagocytosis and dehydration, e.g. hyaluronic acid
- Flagella- help the bacteria move around
One way bacteria can be classified is by the structure of the cell wall:
- Gram-positive bacteria: cell wall consists of peptidoglycan layer, with teichoic polymers attached, e.g. Staphylococcus
- Gram-negative bacteria:peptidoglycan layer is thinner, but surrounded by outer membrane of lipopolysaccharides and lipoproteins, e.g. salmonella
| Component | Gram positive | Gram negative |
|---|---|---|
| Peptidoglycan | Yes | Yes |
| Teichoic acid | Yes | No |
| Lipoprotein | No | Yes |
| Lipopolysaccharide | No | Yes |
| Phospholipid | No | Yes |
Another method of classification is the shape and arrangement of the bacteria themselves:
- Cocci- round shape, e.g. Streptococci, Staphylococci, Neisseria
- Rods or bacilli- long shape, e.g. Coliforms, Bacillus, Spirochaetes
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