Lecture 11 - microbial nutrition Flashcards
What are essential nutrients?
Nutrients that must be supplied from the environment
Bacterial macronutrients
Major elements in macromolecules: CHONPS
Metal ions for nucleic acid/protein structure and function: Mg, Ca, Fe, K
What is Mg used for in the cell
Nucleic acid structure and stability
Why can iron be limiting to bacterial growth?
Bacteria take iron from the environment for electron transfer
Micronutrients for bacteria
Trace elements necessary for enzyme function: Co, Cu, Mn, Zn
Autotroph
Can assimilate inorganic carbon into organic carbohyrates
Heterotroph
Assimilates preformed organic carbon-containing molecules
Why do some bacteria require additional growth factors?
Evolved away from production of all necessary molecules.
- pathogenic species have evolved to depend on the host for certain materials
Minimal medium
Contains only compounds needed for an organism to grow
Prototrophs
Can synthesize complex compounds from simpler organic molecules (ex: wild type E. coli can make all of its amino acids from glucose)
Auxotrophs
Must obtain biologically important molecules in a pre-formed state from the environment or from hosts
- bacteria are usually referred to as auxotrophic for a particular substance that they need
Do bacterial nutrients generally cross the membrane?
No, normally hydrophilic or charged
- ammonia is an exception
How do nutrients get into the cell?
Permeases: substrate-specific carrier proteins
How does LacY (lactose permease) transport lactose into the cell?
Secondary active transport using proton gradient
Group translocation
Process that uses energy to chemically alter a substrate during transport
Example of group translocation in bacteria
Phosphotransferase system (PTS)
How does the phosphotransferase system work?
Uses energy from phosphoenolpyruvate (PEP) to phosphorylate specific sugars during transport.
- can be hooked up to different carbohydrate importers
Why are sugars phosphorylated on import?
Phosphorylated sugars are different than non-phosphorylated sugars. Keeps the concentration gradient tending towards import of sugars.
Catabolite repression between glucose and lactose
E. coli will preferentially use glucose until consumed. Then it will switch to lactose metabolism.
Mechanism of catabolite repression between glucose and lactose
Glucose shuts down expression of the lacZ gene
- if lots of glucose is moving through the transporter, it is phosphorylated at such a high rate that parts of the LacY system can’t be phosphorylated
- LacY stops import of lactose
- decreased import of lactose –> repression of lac operon expression
What do ABC transporters typically move?
- Import of carbohydrates and AAs
- export of small molecules like hemolysin or cytotoxic materials
Where are ABC transporters located?
Inner membrane
How do Gram negative bacteria import materials?
- Outer membrane transporters bring material into periplasmic space
- periplasmic binding proteins shuttle material to the ABC transporter
- ABC transporter brings material into cell
Siderophores
Secreted molecules that bind ferric ions (Fe 3+) for transport into the cell
General structure of siderophores
Flat molecules that fold up into a cage to encapsulate charged iron
Mechanism of siderophore uptake into the cell
Active transport by TonB-ExbB-ExbD system
- EbB and D perform a mechanical up/down motion thru ATP hydrolysis that is relayed to TonB
- TonB displaces a plug in an outer membrane transporter that allows siderophore into periplasmic space
Iron piracy
Some cells will make transporters for particular siderophores without actually making the siderophore –> can steal packaged iron from other bacteria
Hemin
Molecule that sequesters iron
- analogous structure to siderophores
- can be broken down by hemolysin from bacteria
Hemin uptake in Gram + bacteria
Iron-bound hemin is passed between Isd molecules through the cell wall to the membrane where it can be transported inside
What the issue with energy sourcing for outer membrane transport?
The periplasm does not contain ATP because it would be subject to loss to the environment. There must be another way to get energy
