Fundamental of bacterial cell morphology

Bacterial cell morphology

1. Cell Morphology

2. 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) Cell Morphology

3. 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 Cell Morphology

4. 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. Cell size and its significance

5. Capsule - Some species of bacteria have a third protective covering, a capsule made up of polysaccharides (complex carbohydrates). Capsules play a number of roles, but the most important are to keep the bacterium from drying out and to protect it from phagocytosis (engulfing) by larger microorganisms. The capsule is a major virulence factor in the major disease-causing bacteria, such as Escherichia coli and Streptococcus pneumoniae . Nonencapsulated mutants of these organisms are avirulent , i.e. they don't cause disease. Glycocalyx : 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

6. Pili : 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.

7. Fimbriae : 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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. 70S ribosome 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.

13. Most bacteria can locomotive 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.

14. 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. Endospore structure 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 .

15. Cytoskeleton The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons, but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells. In fact, homologues for all major cytoskeletal proteins in eukaryotes have been found in prokaryotes. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes.

16. Nutrient storage structures Most bacteria do not live in environments that contain large amounts of nutrients at all times. To accommodate these transient levels of nutrients bacteria contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteria store excess carbon in the form of polyhydroxyalkanoates or glycogen. Some microbes store soluble nutrients such as nitrate in vacuoles. Sulfur is most often stored as elemental (S ) granules which can be deposited either intra- or extracellularly . Sulfur granules are especially common in bacteria that use hydrogen sulfide as an electron source. Most of the above-mentioned examples can be viewed using a microscope and are surrounded by a thin nonunit membrane to separate them from the cytoplasm

17. Inclusions Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Volutin granules are cytoplasmic inclusions of complexed inorganic polyphosphate. These granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue.

18. Gas vacuoles Gas vacuoles are membrane-bound, spindle-shaped vesicles, found in some planktonic bacteria and Cyanobacteria , that provides buoyancy to these cells by decreasing their overall cell density. Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to perform photosynthesis. They are made up of a shell of protein that has a highly hydrophobic inner surface, making it impermeable to water (and stopping water vapour from condensing inside) but permeable to most gases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surrounding pressure increases. Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance to buckling, including an external strengthening protein, GvpC , rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse - the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles. Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column (fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers). The diameter of the gas vesicle will also help determine which species survive in different bodies of water. Deep lakes that experience winter mixing expose the cells to the hydrostatic pressure generated by the full water column. This will select for species with narrower, stronger gas vesicles.11

19. Microcompartments Bacterial microcompartments are widespread, membrane-bound organelles that are made of a protein shell that surrounds and encloses various enzymes. provide a further level of organization; they are compartments within bacteria that are surrounded by polyhedral protein shells, rather than by lipid membranes. These "polyhedral organelles" localize and compartmentalize bacterial metabolism, a function performed by the membrane-bound organelles in eukaryotes.

20. Carboxysomes Carboxysomes are bacterial microcompartments found in many autotrophic bacteria such as Cyanobacteria , Knallgasbacteria , Nitroso - and Nitrobacteria. They are proteinaceous structures resembling phage heads in their morphology and contain the enzymes of carbon dioxide fixation in these organisms (especially ribulose bisphosphate carboxylase / oxygenase , RuBisCO , and carbonic anhydrase ). It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Similar structures are known to harbor the coenzyme B12-containing glycerol dehydratase , the key enzyme of glycerol fermentation to 1,3-propanediol, in some Enterobacteriaceae (e. g. Salmonella).

21. Magnetosomes Magnetosomes are bacterial microcompartments found in magnetotactic bacteria that allow them to sense and align themselves along a magnetic field ( magnetotaxis ) . The ecological role of magnetotaxis is unknown but is thought to be involved in the determination of optimal oxygen concentrations. Magnetosomes are composed of the mineral magnetite or greigite and are surrounded by a lipid bilayer membrane. The morphology of magnetosomes is species-specific.