II. BACTERIAL GROWTH AND MICROBIAL METABOLISM
C. Factors that Influence Bacterial Growth
LEARNING OBJECTIVES FOR THIS SECTION
Factors Influencing Bacterial Growth
1. Physical requirements
Bacteria have a minimum, optimum, and maximum temperature for growth and can be divided into 3 groups based on their optimum growth temperature:
1. Psychrophiles (def) are cold-loving bacteria. Their optimum growth temperature is between -5C and 15C. They are usually found in the Arctic and Antarctic regions and in streams fed by glaciers.
2. Mesophiles (def) are bacteria that grow best at moderate temperatures. Their optimum growth temperature is between 25C and 45C. Most bacteria are mesophilic and include common soil bacteria and bacteria that live in and on the body.
3. Thermophiles (def) are heat-loving bacteria. Their optimum growth temperature is between 45C and 70C and are comonly found in hot springs and in compost heaps.
4. Hyperthermophiles (def) are bacteria that grow at very high temperatures. Their optimum growth temperature is between 70C and 110C. They are usually members of the Archae and are found growing near hydrothermal vents at great depths in the ocean.
b. Oxygen requirements
Microorganisms show a great deal of variation in their requirements for gaseous oxygen. Most can be placed in one of the following groups:
1. Obligate aerobes (def) are organisms that grow only in the presence of oxygen. They obtain their energy through aerobic respiration (def).
2. Microaerophiles (def) are organisms that require a low concentration of oxygen (2% to 10%) for growth, but higher concentrations are inhibitory. They obtain their energy through aerobic respiration (def).
3. Obligate anaerobes (def) are organisms that grow only in the absense of oxygen and, in fact, are often inhibited or killed by its presense. They obtain their energy through anaerobic respiration (def) or fermentation (def).
4. Aerotolerant anaerobes (def), like obligate anaerobes, cannot use oxygen to transform energy but can grow in its presence. They obtain energy only by fermentation (def) and are known as obligate fermenters.
5. Facultative anaerobes (def) are organisms that grow with or without oxygen, but generally better with oxygen. They obtain their energy through aerobic respiration (def) if oxygen is present, but use fermentation (def) or anaerobic respiration (def) if it is absent. Most bacteria are facultative anaerobes.
Microorganisms can be placed in one of the following groups based on their optimum pH (def) requirements:
1. Neutrophiles (def) grow best at a pH range of 5 to 8.
2. Acidophiles (def) grow best at a pH below 5.5.
3. Allaliphiles (def) grow best at a pH above 8.5.
Osmosis (def) is the diffusion of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). Osmosis is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy. While water molecules are small enough to pass between the phospholipids in the cytoplasmic membrane, their transport can be enhanced by water transporting transport proteins known as aquaporins (def). The aquaporins form channels that span the cytoplasmic membrane and transport water in and out of the cytoplasm (see channel proteins below).
To understand osmosis, one must understand what is meant by a solution (def). A solution consists of a solute (def) dissolved in a solvent (def). In terms of osmosis, solute refers to all the molecules or ions dissolved in the water (the solvent). When a solute such as sugar dissolves in water, it forms weak hydrogen bonds with water molecules. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not (see Fig. 4C and Fig. 4D).Therefore, the higher the solute concentration, the lower the concentration of free water molecules capable of passing through the membrane.
A cell can find itself in one of three environments: isotonic (def), hypertonic (def), or hypotonic (def). (The prefixes iso-, hyper-, and hypo- refer to the solute concentration).
- In an isotonic environment (see Fig. 5A), both the water and solute concentration are the same inside and outside the cell and water goes into and out of the cell at an equal rate.
- If the environment is hypertonic (see Fig. 5B), the water concentration is greater inside the cell while the solute concentration is higher outside (the interior of the cell is hypotonic to the surrounding hypertonic environment). Water goes out of the cell.
- In an environment that is hypotonic (see Fig. 5C), the water concentration is greater outside the cell and the solute concentration is higher inside (the interior of the cell is hypertonic to the hypotonic surroundings). Water goes into the cell.
Most bacteria require an isotonic environment (def) or a hypotonic environment (def) for optimum growth. Organisms that can grow at relatively high salt concentration (up tp 10%) are said to be osmotolerant (def). Those that require relatively high salt concentrations for growth, like some of the Archea that require sodium chloride concentrations of 20 % or higher halophiles (def).
2. Nutritional requirements
In addition to a proper physical environment, microorgaisms also depend on an available source of chemical nutrients. Microorganisms are often grouped according to their energy source and their source of carbon.
a. Energy source
1. Phototrophs (def) use radiant energy (light) as their primary energy source.
2. Chemotrophs (def) use the oxidation (def) and reduction (def) of chemical compounds as their primary energy source.
b. Carbon source
Carbon is the structural backbone of the organic compounds that make up a living cell. Based on their source of carbon bacteria can be classified as autotrophs or heterotrophs.
1. Autotrophs (def): require only carbon dioxide as a carbon source. An autotroph can synthesize organic molecules from inorganic nutrients.
2. Heterotrophs (def): require organic forms of carbon. A Heterotroph cannot synthesize organic molecules from inorganic nutrients.
Combining their nutritional patterns, all organisms in nature can be placed into one of four separate groups: photoautotrophs, photoheterotrophs, chemoautotrophs, and chemoheterotrophs.
1. Photoautotrophs (def) use light as an energy source and carbon dioxide as their main carbon source. They include photosynthetic bacteria (green sulfur bacteria, purple sulfur bacteria, and cyanobacteria), algae, and green plants. Photoautotrophs transform carbon dioxide and water into carbohydrates and oxygen gas through photosynthesis (def).
Cyanobacteria, as well as algae and green plants, use hydrogen atoms from water to reduce carbon dioxide to form carbohydrates, and during this process oxygen gas is given off (an oxygenic process). Other photosynthetic bacteria (the green sulfur bacteria and purple sulfur bacteria) carry out an anoxygenic process, using sulfur, sulfur compounds or hydrogen gas to reduce carbon dioxide and form organic compounds.
2. Photoheterotrophs (def) use light as an energy source but cannot convert carbon dioxide into energy. Instead they use organic compounds (def) as a carbon source. They include the green nonsulfur bacteria and the purple nonsulfur bacteria.
3. Chemolithoautotrophs (def) use inorganic compounds such as hydrogen sulfide, sulfur, ammonia, nitrites, hydrogen gas, or iron as an energy source and carbon dioxide as their main carbon source.
4. Chemooganoheterotrophs (def) use organic compounds (def) as both an energy source and a carbon source. Saprophytes (def) live on dead organic matter while parasites (def) get their nutrients from a living host. Most bacteria, and all protozoans, fungi, and animals are chemoorganoheterotrophs.
c. Nitrogen source
Nitrogen is needed for the synthesis of such molecules as amino acids, DNA, RNA and ATP (def). Depending on the organism, nitrogen, nitrates, ammonia, or organic nitrogen compounds may be used as a nitrogen source.
Sulfur is needed to synthesisize sulfur-containing amino acids and certain vitamins. Depending on the organism, sulfates, hydrogen sulfide, or sulfur-containing amino acids may be used as a sulfur sorce.
Phosphorus is needed to synthesize phospholipids (def), DNA, RNA, and ATP (def). Phosphate ions are the primary source of phosphorus.
3. potassium, magnesium, and calcium
These are required for certain enzymes to function as well as additional functions.
Iron is a part of certain enzymes.
5. trace elements
Trace elements are elements required in very minute amounts, and like potassium, magnesium, calcium, and iron, they usually function as cofactors (def) in enzyme reactions. They include sodium, zinc, copper,molybdenum, manganese, and cobalt ions. Cofactors usually function as electron donors or electron acceptors during enzyme reactions.
f. Growth factors
Growth factors are organic compounds such as amino acids (def), purines (def), pyrimidines (def), and vitamins (def) that a cell must have for growth but cannot synthesize itself. Organisms having complex nutritional requirements and needing many growth factors are said to be fastidious (def).
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Updated: July 12, 2006
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