MICROBIAL GROWTH
Bacteria divide and reproduce asexually. Binary Fission and everything is equally divided between the two daughter cells.
Time it takes for a single cell to go from one division to the next: generation time or doubling time. This is also the time it takes for a population to double. For many "typical" bacteria under "ideal" conditions this doubling time may be as fast as 20 minutes.
Bacterial population increases exponentially or logarithmically. That is:
(x)(2)n , where x=initial number of bacterial cells and n=the number of generations.
[Remember that if 2n=y, then n=log2y].
This is also a geometric progression: 2, 2x2, 2x2x2, 2x2x2x2, etc.
Bacterial Growth Curve:
When an organism is inoculated into a nutrient solution 4 distinct growth phases are noted:
1.) Lag Phase - where the organisms are "getting used to the medium and physical conditions" - that is they are inducing the necessary enzymes for growth.
2.) Logarithmic (Log) Growth Phase - This is the phase where the generation time is measured. The more ideal the conditions, the faster the growth - up to the maximum growth rate for the species.
3.) Stationary Phase - during this phase the number of new cells equals the number of dead cells so that there is no net increase in viable cells. Nutrient are becoming depleted, the pH is changing, toxic wastes are building up, oxygen levels are becoming depleted.
4.) Death Phase - Rate of cell death is faster than regeneration. Death may accelerate and become exponential.
Water
Carbon Source:
Inorganic carbon (autotrophs) - carbon dioxide
Nitrogen:
Nitrates and nitrites; elemental nitrogen if a nitrogen fixer. Amino acids and proteins (peptone, tryptone).
Sulfur
Phosphorus
Oxygen:
Many organism need it for respiration. It is a toxic gas and it forms toxic products during metabolism:
Hydrogen Peroxide - H2O2 and Superoxide - O2-
Cells living in the presence of oxygen need special enzymes to break these down:
2O2- + 2H+ ------superoxide dismutase----> H2O2 + O2
2 H2O2 -------catalase------> H2O + O2
Organisms can be classified based on their oxygen requirements and oxygen tolerance. :
Obligate aerobesMicroaerobic (formally called microaerophils). If these organism need increased carbon dioxide they are called capnophilic.
Facultative anaerobes
Obligate anaerobes
Minerals:
Mg, K, Fe, Ca, Zn, Mo, Co, Mn, Na, Cl. Note that there is a battle for iron between the human host and many infectious agents. We make lactoferrin and transferrin to capture and hold onto iron. Many bacteria make siderophores to capture and hold onto iron.
For many of these we commonly refer to optimum, maximum and minimum.
pH-- most bacteria of medical importance prefer neutral pH's: pH values between 6 - 8. Exception, Helicobacter pylori which inhabits the stomach with a pH approaching pH 1. This organism makes ammonia to neutralize the stomach acid around it.
many fungi prefer low pH; pH < or = 5. Exception, Candida albicans which likes neutral pH.
Temperature -- Psychrophiles have a low temperature optimum (Listeria monocytogenes grows best at low temperatures and cultures can be enriched by incubation at refrigerator temperature. Mesophiles have an optimum growth temperature around human body temperature. Thermophiles have a hot optimum growth temperature.
Cold temperatures are often used to slow microbial growth and thus preserve foods. Freezing tempertures do not kill microbes but preserve them in "suspended animation." Freeze-drying or lyophilization is often used to preserve microbial cultures.
Osmotic Pressure -- is most often adjusted by altering the salt or sugar concentration and is often used to preserve food. Some organisms, the halophiles, can tolerate and grow at high concentrations of salt or sugar. The Staphylococci are an important group of medical microbes which fall into this category.
What is meant by the terms: isotonic, hypertonic, and hypotonic?