Chapter 6 - Microbial Growth
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I. This chapter is replete with vocabulary. This vocabulary is a necessary part of the professional lexicon you must develop.
A. Look for commonalities in the terms
B. Terms tend to be descriptive
II. The Requirements of Growth
A. Physical Requirements
1. Temperature
a. Microorganisms, as a whole, favor a much wider range of temperature than humans. Each species has a narrower range in which they can grow, enabling us to separate them on this characteristic.
b. If you plot the growth rate of a microorganism versus temperature you get a hump with three distinct points.
i. Minimum Growth Temperature - the lowest temperature at which growth still occurs
ii. Maximum Growth Temperature - the highest temperature at which growth still occurs
iii. Optimum Growth Temperature - the temperature at which the growth rate is the highest
c. Categories based on temperature
i. Psycrophile - organisms that reproduce in cold temperatures (i.e. ~10 °C)
ii. Psycrotroph - also called facultative psycrophile, organisms that grow best at food storage temperatures
iii. Mesophile - organisms that grow best at moderate temperatures (~30 °C)
iv. Thermophile - organisms that grow best at high temperatures (i.e. >60 °C)
v. Hyperthermophile - organisms whose optimum growth temperature is above 80 °C.
d. The optimum growth temperature for an organism reflects its primary niche. Therefore pathogens typically have an optimum near the body temperature of their host.
2. pH
a. Most bacteria prefer a near neutral pH (i.e. 6.5 - 7.5)
b. Acidophile - organism that can grow at acidic pH.
c. Fungi typically have a lower pH optimum than bacteria.
3. Osmotic Pressure
a. Governed by osmosis (see chap. 4)
b. The most common solute for purposes of classification is salt.
i. Obligate Halophile - organism that requires high salt concentrations in its media for growth.
ii. Facultative Halophile - organism that does not require elevated salt concentrations, but can grow in slightly elevated conditions.
B. Chemical Requirements
1. Given the nature of the chemicals used in biology (i.e. proteins, carbohydrates, etc.) certain chemicals will be required by the cell.
2. Major Requirements
a. Carbon
i. Necessary for all organic chemicals
ii. Heterotrophs derive their carbon from organic compounds
iii. Autotrophs derive their carbon from CO2
b. Nitrogen
i. Amino acids
ii. Nucleic acids
c. Sulphur
i. Some amino acids
ii. Some vitamins
d. Phosphorus
i. Nucleic acids
ii. Phospholipids
3. Trace Elements - other elements needed in very small amounts for organisms to grow.
4. Growth media has to contain a source of all thesemajor and minor chemicals.
5. Oxygen (Fig. 6.1)
a. Molecular oxygen (O2) is a compound that separates microorganisms into two camps.
i. Aerobe - organism that can use O2
ii. Anaerobe - organism that does not use O2
b. Each class can be broken into two subclasses: Obligate (i.e. Obligate aerobe) and Facultative
i. An obligate can only grow in that condition, an obligate aerobe can only grow in the presence of oxygen.
ii. A facultative can grow under the opposite condition, just not as well.
c. Special Conditions
i. Aerotolerant - organism that can not grow in O2, but does tolerate it.
ii. Microaerophile - aerobic organism that requires a concentration lower than what is present in the air.
6. Organic Growth Factors - chemicals that an organism can not produce and they have to acquire, similar to vitamins in humans
*END OF LECTURE ON 9/22*
III. Culture Media
A. Culture media - the nutrient material used in laboratory to grow microorganisms
1. Microorganisms introduced into media are referred to as an inoculum
2. When microorganisms multiply in a medium it is referred to as a culture
B. Media must be sterile (contain no living microorganisms) to start
C. Media can be solidified with a compound called agar
D. Chemically Defined Media - a medium whose exact chemical composition is known
E. Complex Media - A medium whose components include extracts of things such as yeast, meat, or plants
F. Anaerobic media has to take special precautions to avoid contaminating O2, these are referred to as reducing media
1. Work with anaerobes requires special equipment to maintain an oxygen-free atmosphere
G. Some bacteria require even more stringent requirements
1. Mycobacterium leprae (the cause of leprosy) can not be cultured in the lab, but is cultured in the footpads of nine-banded armadillos.
2. Some organisms require higher than normal CO2 concentrations (capnophiles)
IV. Selective and Differential Media
A. Many media are designed to show a preference for specific microbes
B. Selective media - media designed to suppress the growth of unwanted microbes and promote desired microbes
C. Differential media - media that makes it easier to distinguish colonies with desired traits
D. Enrichment culture - media that encourage the amplification of a small minority population in a mixed culture
E. Colony - a visible growth of microbes that theoretically arise from a single spore or cell
V. The Growth of Bacterial Cultures
A. Phases of Growth
1. Lag Phase - the delay that microbes experience when introduced into medium where little growth occurs
2. Log Phase - the stage of fastest generation rate
3. Stationary Phase - the stage of equilibrium where cells are slowed in metabolism and waste products build-up
4. Death Phase - decline of a population
B. In the environment, very few microbes are in log phase, mostly they live in stationary phase
C. Generation Time – the amount of time required for a microbial cell to reproduce, some bacteria can accomplish this in as little as 10-15 minutes.
VI. Enumerating Bacteria
A. Direct Methodologies
1. Pour Plate Counts – Making a series of known dilutions of a bacteria culture and then counting the number of colonies that appear on the plate.
2. MPN (Most Probable Number) – Making a series of dilutions and using a statistical test to determine the most likely concentration for the number and dilutions of tubes that show growth (not used much anymore)
3. Direct Cell Counting – A representative amount of the culture is viewed under a microscope and individual cells are counted with a special etched chamber (the most tedious)
B. Indirect Methodologies
1. Optical Density – Determining the concentration of microbes by their scattering of light.
2. Biochemical Activity – Measuring the activity of some enzyme activity in the cells.
3. Indirect methods have to be calibrated to the type of microbe tested.