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Bacteria require several nutrients for their growth. Structural components such as cell wall and cell membrane restrict the entry of several molecules inside the cell. Therefore, the specific mechanism for nutrient uptake is highly important for the bacterial cell.
Image Source: Dr. Kenneth Todar
There are different transport mechanisms involved in the process which are the following:
Some of the molecules such as glycerol can pass the plasma membrane by Passive Diffusion. This is a process by which molecules present at a higher concentration move towards the lower concentration level. The rate of passive diffusion is dependent on the difference of size of the gradient present inside or outside of the cell. Molecule transport by passive diffusion requires a fairly large concentration gradient outside the cell while the concentration of the gradient inside the cell needs to be low. Small molecules such as water, oxygen, carbon dioxide, etc. can move across the plasma membrane by passive diffusion.
This process requires carrier proteins such as permease to transport the solute across the membrane. Due to the involvement of the carrier proteins, the rate of diffusion is higher than Passive Diffusion. The rate of diffusion increases with the concentration gradient much more rapidly and at a lower concentration of diffusing molecule than that of passive diffusion each of the carrier proteins involved in transporting specific molecules. Though this process requires carrier proteins for movement of molecules, but the movement depends on the concentration of the gradient, no extra energy is required for the process. In this process, the carrier protein complex spans the membrane. After the attachment of the solute at the outside of the carrier, the protein changes its conformation and releases the solute inside of the cell. At the end of this step, the carrier again changes to its previous conformation to carry more molecules. As the process is dependent on the gradient concentration, the solute can come out of the cell if the concentration inside the cell is higher than outside. Glycerol is often transported inside a bacterial cell by facilitated diffusion. The process is also found in different Eukaryotes.
Active transport is a process to transport the solute at a higher concentration, i.e., against the gradient concentration. Microorganisms often live in environments that lack nutrients, therefore, this process plays an important role to overcome those situations. Moreover, this process requires energy to carry forward the nutrient uptake. ATP binding cassette transporters (ABC) are the important examples of active transport system which is present in bacteria, Achaea and eukaryotes. These transporters consist of two hydrophobic membrane domains along with two ATP binding domains. ABC transporters facilitate the involvement of special substrate-binding proteins which binds with the solute and interacts with the membrane transport protein to transport the solute inside the cell. ABC transporters also involve in pumping out antibiotics in several antibiotic-resistant bacteria. Molecules entering the gram-negative bacteria need to pass through the outer membrane before ABC transporters and active transport systems can take action. An example of this movement is the transport of phosphate molecules in E. coli. The inorganic phosphate molecules cross the outer membrane by the involvement of the porin protein channel.
Electron transport during the energy-conserving process generates a proton gradient in prokaryotes; the protons are at higher concentration outside the cell. The transport process can be described by using the example of lactose uptake by E. coli. Lactose permease is a single protein that transports lactose molecule inward as a proton simultaneously enters the cell. This linked transport of two substances is called symport. Here the energy in form of a proton gradient drives transport. Transport proteins are present as outward and inward-facing conformations when proton and lactose bind to the specific binding proteins, those proteins alters the conformations to uptake the sugar and proton.
Apart from this, the proton gradient can indirectly involve in active transport through the formation of sodium ion gradient. In E. coli, the sodium transport system pumps sodium outside of the cell when the protons move inside. This type of transport is known as antiport. The proton antiport system facilitates the uptake of sugars or amino acids. In this case, sodium ions attache to the carrier protein and the protein alters its shape. Then the carrier binds with the sugars or amino acids and orients the binding sites towards the interior of the cell. Due to low intracellular sodium concentration, the sodium ion dissociates from the carrier.
In this case, the solute is chemically modified when it is transported inside the cell. It is also a type of active transport as metabolic energy is used during the nutrient uptake. The process can be described by the sugar-phosphate transferase system (PTS). This system helps to transport many sugars by phosphorylating them using phosphoenolpyruvate (PEP). PEP is used for ATP synthesis, but, in PTS the energy present in PEP is used to energize the uptake molecule. The transfer of phosphate from PEP requires different proteins. In E. coli and Salmonella, two enzymes (Enzyme I and Enzyme II) and one low molecular weight heat-stable protein (HPr) is connected with PTS. Enzyme II is made up of three domains: EII A (cytoplasmic and soluble), EII B (hydrophilic), EII C (hydrophobic). Phosphate is transferred from PEP to EII by the help of EI and HPr. Then a sugar molecule is phosphorylated as it carries across the membrane by EII. EII transport is specific for sugars and varies in each PTS, but EI and HPr are the same in different PTs systems.
Many bacteria require iron for cytochromes and several enzymes. Ferric ion is highly insoluble and it makes it challenging to transport iron molecules inside the cell. Siderophores are used by many bacteria and fungi to overcome the challenge. These are low molecular weight organic molecules that can bind with the ferric ions and make it available for bacteria. These iron-transport molecules are normally either hydroxamates or phenolates-catecholates. Ferrichrome is a hydroxamate produced by many fungi; enterobactin is the catecholate formed by E. coli.
Microorganism secretes siderophores when iron concentration is low at the medium. Siderophore makes a complex with the iron molecules and binds to the siderophore receptor protein which is present on the surface of the cell. Then the iron is either released inside the cell or the whole siderophore iron complex transported inside by ABC transporters.