Typed lecture notes covering chapter 5 through 6 of Microbiology: An Evolving Science, the textbook used in the "General Microbiology" course (BioM122) at UCI. Aligns with lectures 5 and 6.
Control of Microbial Growth (Ch.5-6, Lec. 5-6) Active learning (10/12/20):
Monday, October 12, 2020 11:54 AM • Interference-- base of phase contrast microscopy. Light waves can either collide or disrupt
each other.
• 5.1: Environmental limits on growth • What medium is good for photoautotrophs? A defined/synthetic media.
• Normal growth conditions: sea level, b/w temp of 20-40C, neutral pH, and 0.9% salt and ample nutrients.
○ Any ecological niche outside of this window is "extreme." -> Extremophiles: microbes that can grow in
extreme temperatures, pH levels, etc.
• Bioinformatic analysis uses DNA sequence of a gene to predict function of its protein product. -> We can
study the biology of organisms that we cannot culture. -> We can analyze microbial growth and defenses
against environmental stresses.
• Microbes are classified by their environment: temp, pH, osmolarity, oxygen, and pressure.
• Growth rate and temperature:
○ Microbes that grow at higher temperatures achieve higher growth rates.
○ Growth rates roughly double for every 10C rise.
○ Cell growth is limited to a narrow temp range; typically optimal temp will span 30-40C.
• Microorganisms are classified by their growth temperature:
○ Psychrophiles: 0-20C
○ Mesophiles: 15-45C
○ Thermophiles: 40-80C
○ Hyperthermophiles: 65-121C
• Heat-shock response: caused by rapid temp changes during growth -> activating stress-response genes.
Include enzymes that change membrane lipid composition and…
○ Chaperone: protein-product of heat-shock response; maintain protein's shape.
○ Response documented in all living organisms.
• Barophiles/piezophiles: organisms adapted to grow at high pres-- up to 1,000atm.
• Place of high pres is bottom of ocean, at 2C. -> Many barophiles are also psychrophiles.
○ INCed hydrostatic pre + cold temp = reduced membrane fluidity. -> Specially-designed membranes and
protein structures are formed to combat this.
• 5.2: Osmolarity
• Water activity (aw): measure of available water for use. Ratio of solution's VP relative to pure water.
○ Most bacteria require aw levels of >0.91.
• Osmolarity: # of solute molecules in a soln; inversely related to aw.
○ Important to a cell's semipermeable plasma membrane. Ex. In a hypertonic medium (super salty),
water will leave the cell to equalize osmolarity across the membrane.
• Aquaporins: membrane-channel proteins that allow water to transverse the membrane much faster than by
diffusion. Help protect the cell from osmotic stress.
• Microbes have 2 more mechanisms to minimize osmotic stress:
1. In HYPERtonic media, bacteria will protect their internal water by making/importing compatible solutes.
-> Equalize osmotic levels across the membrane.
2. In a HYPOtonic media, mechanosensitive (pressure-sensitive) channels leak solutes out of the cell.
• Halophiles: require high salt conc. 2-4M NaCl.
○ To achieve low internal Na+ conc, halophiles will use ion pumps to replace sodium w/ K+.
• 5.3: Hydronium and Hydroxide Ion Conc (pH)
• Extreme conc of hydroxide ions or hydronium in a soln will limit growth.
• Bacteria regulate internal pH to keep their enzymes functioning optimally.
○ A lot of fermentative bugs are able to tolerate low pH b/c they produce acidic molecules already.
○ Membranes are impermeable to protons. -> Uncharged, organic acids cross the membrane, THEN
dissociate inside the cell, releasing a proton and disrupting internal pH.
• Bacteria are differentiated by the pH range they grow optimally in:
○ Neutralophiles: pH of 5-8. include most pathogens.
○ Acidophiles: pH of 0-5. often are chemo-autotrophs.
○ Alkaliphiles: pH of 9-11. typically found in soda lakes.
• Soda lakes have high salt conc and pH values.
• Their cell surface barrier sequesters fragile cytoplasmic enzymes away from the harsh
extracellular pH. Cell wall has acid polymers and excess hexosamines; cell membrane has high
levels of di-ether lipids.
• Rely on Na+/H+ antiporters to bring protons into the cell. -> Can raise their internal pH.
• Under acidic conditions, when internal pH becomes too low microbes can prevent unwanted flux of protons
by bringing in extracellular K+ and bring out intracellular H+.
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