Factors Influencing Bacterial Growth 1. Physical requirements a. Temperature 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. . 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.
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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. c. pH 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. . d. Osmosis 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. C 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