Bacterial Homeostasis: Maintaining pH and Osmotic Pressure - Prof. Ranjan N. Chakraborty, Study notes of Health sciences

Download Bacterial Homeostasis: Maintaining pH and Osmotic Pressure - Prof. Ranjan N. Chakraborty and more Study notes Health sciences in PDF only on Docsity! Bacterial Physiology HSCI 4607/5607 Ch:15: Homeostasis Introduction: Homeostasis refers to the ability of living organisms to maintain constant internal environment despite the changes in the external environment. These involves maintenance of constant pH, and constant osmotic pressure inside the bacterial cells. Most of the time the mechanism underlying homeostasis are unknown, however it is a subject of active research in many laboratories. 2. Role of K+ in pH maintenance: The proton pumps are electrogenic and the number of protons can be pumped depends on the membrane potential. Thus in order to pump protons out to raise the internal pH, the excess membrane potential must be dissipited by either the influx of cation or the efflux of anions. In neutrophilic bacteria K+ influx dissipates the membrane potential, allowing more protons to be pumped out of the cell. There is much more uncertainity regarding the mechanism by which the neutrophiles decrease their internal pH by bringing the protons into the cells via H+/Na+ and H+/K+ antiporters. pH maintenance in Alkaliphile: Since alkaliphile live in basic external pH, they have to keep more acidic internal pH compared to the external pH by atleast two units. Thus they must bring protons into the cell. There are strong evidence that this is done with the help of Na+/H+ antiporters. If alkaliphiles are placed in a medium without Na+ at pH 10.5, the internal pH quickly rises to the external pH. This does not happen when Na+ is present in the medium. The sodium ion circuit is completed when sodium ion enters the cell via Na+/solute symporters that are driven by the membrane potential. pH Homeostasis in acidophiles: In acidophiles, the external pH is several units lower than the cytoplasmic pH. Thus the maintenance of the large pH requires an inverted membrane potential at low pHout, otherwise the proton efflux will be limited by the positive membrane potential as well as low pHout. The negative membrane potential as well as high pHin also will promote proton influx. Thus acidophilic bacteria like Thiobacillus ferroxidans have small membrane potentials of +10 at pHout of 2. The maintenance of inverted membrane potential in acidophiles is due to an inward flux of K+ greater than the efflux of protons. ATP dependent K+ pumps are known to exist for influx. Thus both neutrophiles and acidophiles rely on K+ flow for the maintenance of pH. Maintenance of Turgor Pressure: Bacteria are capable of maintaining the turgor pressure in response to the changes in external osmolarity. How bacteria detect and signal the external osmolarity is largely unknown. Adaptation to high osmolarity: When cells are placed in high osmolarity medium, cells increase the intracellular concentrations of certain solutes called ‘Osmolytes’ to ensure that the internal osmolaritry is higher than the external to maintain the turgor pressure. These osmolytes are also called ‘compatible solutes’. Some osmolytes are synthesized intracellularly while others are transported from outside. The common osmolytes include, K+, the amino acids glutamate, glutamine, and proline, the quarternary amine betaine (trimethylglycine or glycine betaine) and certain sugar like trehlose. Osmotic homeostasis in Halobacteria: Halobacteria live in water with the NaCl concentration of about 3 to 5 M. Under such conditions to maintain the level of water and the turgor pressure, the cytoplasm is maintained very salty. The salt which is used is KCl and not NaCl. Potassium ion uptake maintain the concentration in the order of 3 M. The sodium ion is exported from the cytoplasm. The intracellular proteins have to adapt the high ion concentration to avoid denaturation and unfolding due to high ion concentration. It is known that the proteins from halophiles require high salt concentration for their stability and activity. Even the cell envelopes are adapted and are disintegrated at low salt concentration. Effect of Osmolarity on Transcription and Enzyme activities: To adapt the high osmolarity medium bacteria synthesize new enzymes required for the synthesis or transport of osmolytes into the cells. Increased transcription of some of the genes activated by high osmolarity is due to the sigma factor s. The same factor is also responsible for transcription of many genes during starvation. Few examples of effect of osmolarity on transcription and enzyme activities are: 1. E.coli activates enzyme for the synthesis of periplasmic oligosaccharides. 2. S.aureus activates proline uptake system when shifted to high-osmolarity medium. 3. E.coli and S.typhimurium increase the transcription of proU, an operon that codes for a proline and betaine transport system. 4. E.coli increases the transcription of kdp operon involved in K+ transport.