The Mechanisms of Bacterial Nutrition and Transport of Nutrients into Bacterial Cells
Bacterial cells are generally characterized by holophytic type of nutrition. This mode of nutrition has some common essential traits:
- a) there are no specialized cellular organelles for nutrition in bacteria;
- b) the nutrients are absorbed by the whole surface of bacterial cell; this requires special mechanisms for their transport across the layers of bacterial envelope;
- c) only relatively small molecules (usually about 600 Da or even less) can be easily delivered into bacterial cell. In the latter case many saprophytic bacteria and fungi produce the number of exo-enzymes that make extracellular digestion of various polymeric substrates (proteins, carbohydrates, lipids and others). These substrates undergo transformation into low molecular weight substances (amino acids, mono- and oligosaccharides, etc.) that become available for microbial cells. This is known as saprotrophic nutrition.
A hydrophobic phospholipid nature of bacterial cytoplasmic membrane poses the impermeable barrier for hydrophilic nutrients delivered from outside. This resulted in creation of versatile transport systems harnessed for the delivery of nutrients into the cells and backward transportation of wastes out of the cells.
Usually such transport systems act against a concentration gradient resulting in accumulation of nutrients inside the cell. This process ultimately requires the energy in some available form.
There are four basic mechanisms providing the transport of substances across the bacterial membranes: facilitated diffusion, chemiosmotic-driven transport, binding protein-dependent (active) transport, group translocation.
Facilitated diffusion does not need energy for transportation. It is driven by established concentration gradient of substances, where the external concentration of the substance is higher than internal one. This stimulates the passive diffusion of a nutrient through the cell membrane. It is evident, that substrate internal concentration never overcomes the levels of its external concentration. As an example, glycerol is one of the few substrates that enter into bacterial cell by the mechanism of facilitated diffusion.
Chemiosmotic-driven transport provides the translocation of molecules across the cytoplasmic membrane using the energy of primarily established membrane gradient of protons, known as proton-motive force. It also involves other ions, such as Na+.
Three basic kinds of chemiosmotic-driven transport are determined: uniport, antiport and symport.
Uniporters carry the substrate across the membrane regardless of any other accompanied substance. Antiporters stimulate the parallel delivery of two similarly charged substances in opposite directions using the same carrier (e.g., H+ and Na+). And symporters provide the simultaneous movement of two substances towards the same direction by a common carrier. For example, an established H+ gradient activates the symport of certain oppositely charged compounds (like amino acid glycine) or the neutral nutrients (such as galactose). Chemiosmotic-driven mechanism plays a substantial role in trans-membrane transport in bacteria. For instance, more than 40% of nutrients, acqusited by E. coli, exploits chemiosmotic-driven transport.
Binding protein-dependent (or active) transport is energy-dependent nutrient delivery across the cytoplasmic membrane against the existing concentration gradient. It is governed by specific substrate-binding proteins. They transfer the substrate to specialized membrane-located protein transport complex. In gram-negative bacteria these complexes are present within the periplasmic space. The process of transportation requires ATP energy or in some situations other high-energy substances (e.g., acetylphosphate).
Another 40% of nutrients, delivered for E. coli, uses this universal mechanism.
Group translocation as the mechanism of trans-membrane transport is characterized by temporary change of structure of translocated substances. It is used, for example, for successful uptake of nutrient sugars by bacterial cells (e.g., glucose or mannose). This process is performed by bacterial phosphoproteins. They posphorylate the sugars outside the membrane and move them into the cell in posphorylated form.