Cellular world can be divided into two types, depending on the presence of nuclei inside cells. Eukaryotic cells have a well-defined nucleus surrounded by a nuclear envelope, whereas prokaryotic cells lack this compartment. All bacteria are prokaryotes. We will discover the structure and function of bacteria in this article.
What do bacteria look like?
When observing bacteria with microscope, it is not difficult to compare bacterial cell shapes and arrangements. There are many bacterial morphologies, each of them with a specific name.
- Rod-shaped (Baccilus) i.e. Escherichia coli, Bacillus cereus
- Spherical (Coccus): i.e. Staphylococcus epiderminis
- Curved (Vibrio, Spirochaete) i.e. Vibrio cholera, Rhodospirilium rubrum
- Square-shaped (Arcula)
- Star-shaped (Stella)
After cell division, the cells of different prokaryotic species can either stay separately or remain together in groups or clusters. The arrangements of these cells are often characteristic of certain genera.
- Pair i.e. Escherichia coli
- Chain i.e. Streptococcus
- Cluster i.e. Staphylococcus
Size of bacteria ranges between 0.2 µm and 700 µm in diameter, with the normal range of about 1-5µm in diameter. Bacteria are about 10 times smaller than eukaryotic cells, which leads to their unique features of growth. Small cells have more surface area relative to cell volume than large cells do, hence, they have higher surface-to-volume ratio (S/V ratio). This leads to obvious benefits such as higher nutrient uptake rate, faster growth and considerably shorter life cycle. In fact, bacteria cells can divide very rapidly, i.e. 20 minutes for E.coli , causing higher chance for mutations in bacterial genes to occur. Hence, bacteria can adapt quickly with changing environmental conditions and can explore new habitats much more quickly than eukaryotic cells.
Structures of cells
- Cell wall
- Cytoplasmic membrane
- Inclusion body
- Protect cells against osmotic shock (most important) and physical damage
- Regulation of substance transport into and out of cells.
- Contain genome.
- Contain supplemental genetic information such as resistance to antibiotics, production of toxins and tolerance to toxic environment.
- Take part in protein synthesis.
- Movement of cells.
- Mineral storage of cells.
- Attachment to host, bacterial mating.
- Tough, heat resistance structure that help bacteria survive in adverse conditions.
How do bacteria attach to surfaces?
Bacterial cells often attach to surfaces through specific structures. This is necessary for the overall survival of bacteria, especially pathogenic ones. This process is supported by glycocalyx, pili and fimbriae.
- Structure: Polysaccharide layers; can be thick and stable like capsule or loosely attached to cell wall like slime layer.
- Function: Assist cells in adhesion to solid surface, and also protect pathogenic bacteria from the attack of the host's immune system.
- Structure: Short, thin, straight, hairlike projections form surface of some bacteria. Composed of protein pilin, carbohydrate and phosphate. Pili are usually few.
- Function: Take part in adhesion of pathogen to specific host tissues. Sex pili are involved in genetic material exchange between mating bacterial cells.
- Structure: Similar to pili, but shorter and more abundant on the cell surface.
- Function: Adhesion of cells to surface and formation of pellicles (biofilms) containing thin sheets of cells on a liquid surface.
The cell wall of bacteria protects the cell from osmostic shock and physical damage. In addition, it also confers rigiditiy and shape of bacterial cells.
Although bacterial cell walls all consist of peptidoglycan, also known as murein or mucopeptide, they differ in certain properties in two groups of bacteria, namely gram-negative and gram-positive. The properties of peptidoglycan are discussed below.
- Polysaccharide backbone - consists of 2 alternately repeating sugars such as NAG (N-acetylglucosamine) and NAM (N-acetylmuramic) .
- Tetrapeptide - links the two polysaccharide backbones, forming a peptidoglycan subunit. Here, some unusual amino acids such as L-Alanine, D-Glutamaic acid, D-Lysin and D-Alanin are found. Note that D-type amino acids are very rare in all organisms.
- Peptide cross bridge - links peptidoglycan subunits together . Variations in cross bridges show the diversity of peptidoglycan subuints. In Gram-positive bacteria i.e. Staphylococcus aureus, the cross-linkage is a glycin pentapeptide. In Gram-negative bacteria i.e. E.coli, cross-linkage is formed by the direct link between diaminopimelic acid (DAP) of one chain to terminal D-alanine of another chain.
The table below shows the differences of cell wall structure in gram-positive and gram-negative bacteria.
The cytoplasmic membrane encloses the cytoplasm. It regulates the specific transport of substance between the cell and the environment. The cytoplasmic membrane contains 2 main components: lipid and protein.
The lipid component of the bacterial cell is phospholipid bilayer.
- Thickness: 6-8nm.
- Unit: amphipathic phospholipid, consisting of 1 phosphate group (hydrophilic ) and unbranched fatty acid chains (hydrophobic).
- Distribution of 2 portions: hydrophilic heads are exposed to the external environment or the cytoplasm. The fatty acid chains point inward, facing each other due to hydrophobic effects (staying away from water).
Membrane proteins are located in various positions within the membrane, through specific interactions with phospholipid molecules. These proteins consist of 3 main groups: integral proteins, outer-surface proteins and inner-surface proteins. They play distinctive roles in cellular activities.
- Integral proteins: firmly embedded in the membrane, transport substance across the cytoplasmic membrane in 3 main mechanisms known as uniport, symport and antiport.
- Outer-surface proteins: usually in Gram-negative bacteria, interact with periplasmic proteins in the transport of large molecules into the cells.
- Inner-surface proteins: cooperate with other proteins in enery yeilding reactions and also other important cellular functions.
How do bacteria store genetic information?
Genetic information in bacteria is stored in the sequence of DNA in two forms, that is bacterial chromosome and plasmid.
The following are the properties of a bacterial chromosome.
- Location: Within nucleoid region , not surrounded by nuclear envelope.
- Number: 1 chromosome each cell.
- Size: E.coli 4640 kbp.
- Component: Single, double stranded, circular DNA. Also contains RNA and proteins that take part in DNA replication, transcription and regulation of gene expression. DNA does not interact with protein histone.
- Information: Contain genes essential for cellular functions.
In addition to chromosome, bacterial cells may also contain another genetic element, plasmid. Features of plasmid are analysed below.
- Location: In cytosol of bacterial cells.
- Number: From 1 to several.
- Size: Much smaller than chromosomes.
- Components: Single, double stranded, circular DNA.
- Information: Contains drug resistant genes as well as heavy metal resistant genes. Not essential for growth and metabolism of bacteria.
What helps bacteria synthesise protein ?
Protein synthesis is a very important process for both eukaryotes and prokaryotes. In this process, nucleotide sequence in a segment of DNA is translated into the specific sequence of amino acids in a protein. Translation occurs at ribosomes; ribosomes consist of RNA and proteins. While eukaryotic cells have 80S ribosomes, bacterial cells contain 70S ribosomes, which have the folllowing components.
- 30S subunit: 21 proteins and 16S rRNA.
- 50S subunit: 34 proteins, a 23S rRNA and a 5S rRNA
- Combination of 2 subunits to form functional ribosome requires magnesium ions and chemical energy.
- Activity of 70S ribosomes is blocked by antibiotics like erythromycin and streptomycin.
Notes: S is Svedburg unit, which represents how rapidly particles or molecules sediment in an ultracentrifuge. The larger a substance, the greater its S value.
What helps bacteria move?
Most bacteria can locomote to different parts of their environment, which helps them to find new resources to survive. This process is due to flagellum (plural, flagella) pushing or pulling the cell through a liquid medium.
Structure of flagella
- Long filamentous appendages containing a filament, hook and basal body.
- Filament: consists of protein flagellin.
- Hook: single type of protein, connects filament to the basal body.
- Basal body: contains a rod and several rings in gram-negative bacteria. ( Gram-positive bacteria only have the inner pair of rings). This contributes to rotation of flagella, using energy from the activity of proton pumps.
Types of Flagella distribution
- Monotrichous flagella: one flagellum, if it originates from one end of the cell, it is called polar flagellum. Rapid swimming caused by the rotation of flagella.
- Peritrichous flagella: flagella surround the cell. Bundled peritrichous flagella give rise to slower forward motion than polar flagella.
- Many other types exist but not discussd here.
Function of flagella
- Chemotaxis: movement of bacteria toward or away from chemical stimuli
- Magnetotaxis: movement along the Earth's magnetic field. Happen in magnetotatic bacteria, which contain magnetosomes including iron.
- Phototaxis: response to differences in light density. Bacteria swim to areas of particular light intensities.
Bacterial endospore - The sleeping bacteria
Bacillus and Clostridium are among the few bacterial genera known to be able to produce endospores. An endospore, a heat-resistant and non-growing structure, can retain its viability over long periods of time under adverse environmental conditions. When the environment becomes more favourable, the endospore then germinates to a vegetative cell.
- Exosporium: Outer-most layer consisting of protein.
- Spore coat: Several layers of spore-specific proteins.
- Cortex: Loosely cross-linked peptidoglycan.
- Core: Core wall, cytoplasmic membrane, cytoplasm, nucleoid, ribosomes and other cellular compartments. Additionally. dipicolinic acid-calcium complex maintains dehydrated conditions inside the spore and helps to stablise DNA against heat denaturation.
Madigan., M.T., Martinko., J.M., Stahl., D.A. and Clark., D.P. (2012). Biology of Microorganisms. 13 edition. Boston and London. Pearson.
Atlas., R.M. (1995). Principles of Microbiology. 1st edition. London and New York. Mosby-Yearbook, Inc.
Talaro., K and Talaro., A. (1993). Foundations in Microbiology. Wm. C. Brown Communication, Inc.
Hogg., S. (2006). Essential Microbiology.Wiley.