What are Bacteria ?
Kingdom bacteria is another group of prokaryotes (other is Archaea), and bacterial cell structure and function differ from Archaea. Bacteria are all single-celled. The bacterial cells are prokaryotic. It means they do not have a nucleus or any other fabrics which are surrounded by membranes. More giant bacterial cells may be visible using a light microscope; however, an electron microscope needs to see the details of the cell organelles.
Cell Surface Structures and Inclusions
Capsules and Slime layers
There are polysaccharide layers outside cell walls of many bacteria. Some polysaccharide layers are lightly bound to the cell wall and is called the capsules. In some other bacteria polysaccharide layer is only loosely bound to the cell wall. It is called the slime layer. Capsules typically adhere firmly to the cell wall, and some are even covalently linked to peptidoglycan. Some bacteria release a lot of polysaccharides to the outside environment. These are called Exo-polysaccharides.
- It prevents the cells from desiccation (drying).
- Attach to each other and to surfaces (as biofilm). Some bacteria can make layers of themselves attached to each other and to slid surfaces.
- To escape from phagocytosis (Phagocytes are parts of the immune system and they recognize bacteria by the cell walls of bacteria).
- These include acting as virulence factors in certain bacterial diseases.
- Some pathogenic bacteria use polysaccharides to attach onto host’s cell.
- Bacteria like Rhizobia exchange chemical signals with plat roots using exopolysaccharides.
- Some bacterial exopolysaccharides have significant industrial importance. For example, “Xanthan gum” from Xanthomonas and some other polysaccharides of bacteria used as gelling agents or thickening agents in food industry as well as pharmaceutical industry.
Pili and Fimbriae
These are hair like appendages around the cell. Fimbriae and pili are filamentous proteins that extend from the surface of a cell and can have many functions. A large number of fimbriae are there. They help the cells to attach onto surface, to make biofilms. Some pathogenic bacteria use fimbriae to attach on to host cells. In this case, fimbriae is a virulence factor of pathogenic bacteria. Ex. Salmonella typhimurium, Neisseria gonorrhoeae.
Pili are longer than fimbriae and shorter than flagella. Not like fimbriae, there are only few pili around the cell. These are helps to attach onto surfaces and they help bacterial conjugation. Conjugation is a rare type of exchanging DNA between cells. Two cells join together using pili (sex pilus) as a tube; nuclear material of one cell travels to the other cell through the pilus. Although there can be some DNA recombination it doesn’t make a diploid cell and therefore there is no meiosis.
Flagella are composed of a protein called flagellin. They are responsible for the movement of the cell. Some bacteria have flagella, while some do not. The presence of flagella can be demonstrated by the motility test using the hanging drop method. Some bacteria are motile and some of are non-motile. Motile bacteria have flagella and no-motile bacteria haven’t flagella like structures to move one place to another place. Flagella can attach to cells in different locations. In polar flagellation, the flagella are attached at one or both ends of a cell. Occasionally a group of flagella may arise at one. Flagella can rotate more than 100,000 R.P.M. propelling the cell at a rate of 20 body lengths per second.
Flagella are important in the ability of an organism to cause different diseases. For example,Helicobacter pylori, the bacterium that cause gastric ulcers, has powerful multiple flagella at one end of its cell. These flagella allow H.pylori to penetrate the viscous mucous gel that coats stomach epithelium. Another importance of flagella is, in some cases this helps for chemotaxis. Motile bacteria sense the presence of chemicals and respond by moving in a certain direction is called as chemotaxis.
Bacteria movement by rotation of flagella around the axis of flagella. The direction of movement is changed frequently due to a change in pattern of rotation. The frequency of changing the direction is decreased in favorable environments. This way they can remain longer in a favorable environment if they accidently end up in the favorable environment. When a bacterium falls into an unfavorable environment while moving randomly, the frequency of direction change is increased and it provides the possibility of escaping form he favorable environment.
Cell membrane is a delicate thin fluid structure that surrounds the cytoplasm and defines the boundary of the cell. Cell membrane is composed of two layers of phospholipids called lipid bilayer. The bilayer consists of two opposing leaflets composed of phospholipids. At one end of each phospholipid molecule containing glycerol, a phosphate group and other polar molecules which act as hydrophilic head. The other end of phospholipid molecule are two fatty acid chains , which act as hydrophobic tails. Lipid is an ester of glycerol and fatty acids. In a phospholipid molecule two hydroxyl groups are esterified with fatty acids while the 3rd hydroxyl group is esterified with a phosphate group.
There’re lot of proteins in the membrane (more than 200 protein have been found in E.coli). Archaea and Bacteria both have the same general structure of their membranes, but the lipid composition is distinctly different. The side chains of the membrane lipids of archaebacteria are connected to glycerol by a different type of chemical linkage. Although, the side chains are hydrocarbons rather than fatty acids. Large charged molecules do not travel across the lipid parts of the membrane. But, can travel through proteins of the membrane. Although, charged a small molecule like water can defuse across the membrane along the concentration gradient.
Cytoplasmic membrane allows to pass through freely across the membrane for only selected molecules. So. It is known as selective permeable membrane. These move through the membrane by a process called simple diffusion (Water, small hydrophobic molecules and gases such as oxygen and carbon dioxide). Some proteins in the membrane are transport proteins which help to transport specific substances into the cell or out to the environment. Some membrane proteins help to transport water molecules against concentration gradient.
Many bacteria have proteins in the membrane which helps movement of water against the concentration gradient. Bacterial cell membrane has electron transport chains in the cell membrane. Compounds like cytochrome. Iron-sulfur proteins are located in the membrane. ATP Synthase enzyme which is a protein is also located in the membrane. As a result of electron transport, in the electron transport chain which is within the membrane proteins move across the membrane to the outside (to periplasm). This is called proton translocation.
It develops on energy difference across the membrane (the pH difference and the charge difference). Protons cannot come back to the cytoplasm through the lipid membrane. Protons (H+) can come inside into the cytoplasm through ATP synthase. When protons go through the enzymes the 3D shape of protein changes temporally. A cavity in the enzyme structure is forced to open and close by the movement of H+. ADP and PO43- groups trapped in the cavity are forced to form bonds. Then the cavity is closed and it forms ATP. (Will discussed in details under Microbial Biochemistry section)
Not only ATP synthase, but many other proteins are located in the membrane. Some proteins help the digestion utilization of large molecules by the cell. And also, membrane provides anchorage to flagella. Flagella movement needs energy. This energy is provided by the activity of the membrane.
Storage materials in bacterial cells
When there is excess organic substances (substrate is the chemical used by organisms for ATP synthesis. Some bacteria use inorganic substrate for ATP synthesis), bacteria can store them in the form of either glycogen or poly-β-hydroxyalconates (PHA). Glycogen is a polymer of glucose. PHA is a lipid polymer. It is a group of compounds and the most common PHA is poly-β-hydroxybutyrate (PHB). The PHA appear in cells as granules. Usually the granules are covered by a protein membrane. The storage materials are utilized at low nutrient environment. PHA and glycogen are organic storage materials. In addition to that bacteria can store phosphate ions as polyphosphate granules.
Some bacteria accumulate sulfur granules. The sulfur granules are formed in different bacteria that use sulfide for other reduced forms of sulfur, as substrates for energy synthesis (ATP synthesis). They oxidize these compounds first in to S and then into SO42-. Sulfur can be seened as granules in the cell during this process (These bacteria are chemolithotrophic sulfer oxidizers Ex. Thiobacillus thioidans).
There are some phototrophic bacteria which use H2S as the electron donor to the photo system (plants use H2O as the electron donor to their photosystems). Sulfer is released as a result and it is accumulated in the cell as granules. Ex. Purple sulfur bacteria and Green sulfur bacteria.
Some bacteria accumulate Fe3O4 (Magnetite) as granules in the cytoplasm. In aquatic environments bacteria with magnetosmoes (Membranous structures present in magnetotactic bacteria. They contain iron-rich magnetite particles that are enclosed within a lipid bilayer membrane). move to the bottom. This is an advantage because they prefer low oxygen environments.
Some bacteria and cyanobacteria have gas vesicles with a protein membrane. And there can be hundreds of them within a single cell. Gases, but not H2O, flow freely into the vesicles, therefore decreasing the density of the sell. It helps them to float on the surface of layers of water. This helps phototropic bacteria to receive light.
Endospore is a resistant structure which is formed by some Gram-positive rod-shaped bacteria and one cocci called Sporosarcina. Bacteria produces these structures under unfavorable environmental conditions such as lack of water and nutrients. The bacterial cell recognizes lack of nutrients or water as a chemical signal which induces sporulating genes.
The vegetative cell develops a thick cell wall. Which is resistant to heat chemicals and UV significantly. In addition to the cell wall, sporulation generates a unique compound called dipicolinic acid, which combines with Ca2+ ions and occurs as Ca-dipicolinate in the cytoplasm. This compound binds water and this reduces the water availability within the cell. This reduces metabolic activities to a minimum amount. In addition, Ca-dipicolinate molecules bind to DNA and prevent DNA from heat damage.
Apart from this there are some small protein molecule (small acid soluble proteins-SASP) which binds to DNA and prevent damage from heat, UV and desiccation. The thick wall is resistant to chemicals. There is no exchange of matter through it. The spore can remain dormant under unfavorable environmental conditions. It germinates if the environment is favorable. As one cell produces only one spore which germinates in to one cell, this is not a method of reproduction. Number of cells should be increased due to reproduction. But in here, no increases of number of cells. Therefore, sporulation is not a reproduction method.
Bacteria like Bacillus, Clostridium are spore forming bacteria. Clostridium is an anaerobic organism and it is killed when exposed to oxygen. However, its endospore is not damaged by oxygen and therefore can survive dry heat (heat without moisture) up to about 150 °C. However, heat with water vapor (moist heat) can kill the endospore at 121 °C (autoclaving temperature).
Michael T. Madigan, John M. Martinko, Kelly S. Bender, Daniel H. Buckley, D. A. S. (2015). BROCK BIOLOGY OF MICROORGANISMS (14th ed.). Pearson Education,Inc
Eugene W. Nester; Denise G. Anderson; C.Evans Roberts; Nancy N. Pearsall; Martha T. Nester,(2004). Microbiology: A Human Perspective (4th ed.). McGraw-Hill Education.
Pasindu Chamikara – Microbiologist