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A. Bacterial Growth Bacteria replicate by binary fission, a process by which one bacterium splits into two. http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmito.ht ml Bacteria increase their numbers by geometric progression. 1. Geometric progression refers to the population of bacteria doubling every generation time as a result of dividing by binary fission. 2. Generation time is the time it takes for a population of bacteria to double in number. For many common bacteria, the generation time is quite short, 20-60 minutes under optimum conditions. For most common pathogens in the body, the generation time is probably closer to 5-10 hours. Because bacteria grow by geometric progression and most have a short generation time, they can astronomically increase their number in a short period of time. The relationship between the numbers of bacteria in a population at a given time (Nt). The original number of bacterial cells in the population (No) The number of divisions those bacteria have undergone during that time (n) can be expressed by the following equation: Nt = No X 2n For example, Escheichia coli, under optimum conditions, has a generation time of 20 minutes. a. If one started with only 10 E. coli (No = 10) b. And allowed them to grow for 12 hours c. (n = 36; with a generation time of 20 minutes they would divide 3 times in one hour and 36 times in 12 hours) d. Then plugging the numbers in the formula, the number of bacteria after 12 hours (Nt) would be10 X 236 = Nt e. Or 687,194,767,360 E. coli Bacterial division begins with replication of the bacterium's DNA. 1. During DNA replication, each strand of the replicating bacterial DNA attaches to a different site on the cytoplasmic membrane. 2. As the bacterium grows, the newly replicated chromosomes become separated. 3. The cytoplasmic membrane then invaginates in the center of the bacterium and synthesizes a peptidoglycan septum to separate the two daughter cells Fig.1: Prokaryotic Cell (Bacillus megaterium) There are a number of factors the influence Bacterial Growth. We will begin our discussion of chapter 5 by review those described as Physical requirements. I. Temperature Bacteria have a minimum, optimum, and maximum temperature for growth. Most species can be divided into 3 groups based on their optimum growth temperature: 1. Psychrophiles are cold-loving bacteria. a. Their optimum growth temperature is between -5C and 15C. b. They are usually found in the Arctic and Antarctic regions and in streams fed by glaciers. 2. Mesophiles are bacteria that grow best at moderate temperatures. a. Their optimum growth temperature is between 25C and 45C. b. 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. a. Their optimum growth temperature is between 45C and 70C b. These organisms are commonly found in hot springs and in compost heaps. 4. d. Hyperthermophiles are bacteria that grow at very high temperatures. a. Their optimum growth temperature is between 70C and 110C. b. They are usually members of the Archae and are found growing near hydrothermal vents at great depths in the ocean. II. 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 are organisms that grow only in the presence of oxygen. They obtain their energy through aerobic respiration. 2. Microaerophiles 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. 3. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in fact, are often inhibited or killed by its presence. a. They obtain their energy through anaerobic respiration or fermentation. b. They have an inability to break down hydrogen peroxide 4. Aerotolerant anaerobes, like obligate anaerobes, cannot use oxygen to transform energy but can grow in its presence. a. They obtain energy only by fermentation and are known as obligate fermenters. 5. Facultative anaerobes are organisms that grow with or without oxygen, but generally better with oxygen. a. They obtain their energy through aerobic respiration if oxygen is present, b. but use fermentation or anaerobic respiration if it is absent. Most bacteria are facultative anaerobes. III. pH Microorganisms can be placed in one of the following groups based on their optimum pH requirements: 1. Neutrophiles grow best at a pH range of 5 to 8. 2. Acidophiles grow best at a pH below 5.5. 3. Allaliphiles grow best at a pH above 8.5. IV. Osmotic pressure Osmosis is the movement of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). 1. No energy is required. 2. To understand osmosis, one must understand what is meant by a solution. a. In terms of osmosis, solute refers to all the molecules or ions dissolved in the water (the solvent). b. When a solute such as sugar dissolves in water, it forms weak hydrogen bonds with water molecules. c. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not. d. 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, hypertonic, or hypotonic. (The prefixes iso-, hyper-, and hypo- refer to the solute concentration). Most bacteria require an isotonic environment or a hypotonic environment for optimum growth. 1. Organisms that can grow at relatively high salt concentration (up tp 10%) are said to be osmotolerant. 2. Those that require relatively high salt concentrations for growth, like some of the Archea that require sodium chloride concentrations of 20 % or higher halophiles. Nutrient are any substance (compound or molecule) needed by an organism for growth, maintenance, and reproduction. These substances used in biosynthesis and energy production, and thus required for microbial growth. All organisms need a source of energy and carbon There are 3 categories used to define microbial communities 1. Energy source 2. Carbon source 3. Electron source Energy Sources Microorganisms can be divided into two major categories based on energy sources: 1. Phototrophs - these organisms that use light as their energy source; capture radiant energy to phosphorylate ADP to ATP Green plants Algae cyanobacteria 2. Chemotrophs - most bacteria organisms that rely on oxidation of organic or inorganic chemical compounds to supply energy to phosphorylate ADP Electron Sources 1. Lithotrophs – microbes that used reduced inorganic material as their electron source 2. Organotrophs – microbes that use organic material as their electron source Subcategories of chemotrophs: (based on electron source) (1) chemo- organotrophs - extract electrons (e-) from organic compounds (2) chemo- lithotrophs (i.e. rock-eaters) - use reduced inorganic substances as energy sources Carbon Sources Two major divisions: 1. Autotrophs - those microorganisms that can use CO2 as their sole or principal source of carbon. Reduction of CO2 is energy expensive, so many microorganisms can not do this a. photoautotroph (photolithotrophic autotroph) - obtain energy from light and use CO2 as primary carbon source i. Sulfur-oxidizing bacteria (purple & green) ii. Eukaryotic algae iii. Cyanobacteria b. chemolithotrophic autotrophy - obtain energy from oxidation of inorganic compounds and carbon from CO2 2. Heterotrophs - microorganisms that use reduced, preformed (made by other organisms) organic molecule as carbon sources a. chemoheterotroph (chemoorganotrophic heterotroph) use organic compounds for energy, hydrogen, electrons, and carbon (nonphotosynthetic) b. photoheterotroph (photoorganotrophic heterotrophs)use organic compounds as carbon sources and radiant energy for ATP synthesis i. Purple non-sulfur bacteria ii. Green non-sulfur bacteria 3. methylotrophs – These fastidious (selective feeders) microorganisms grow on single carbon compounds, such as methane (family Methylomonadaceae) 4. mixotrophs- These microorganisms grow using a combination of chemolithotrophic and heterotrophic metabolic processes a. Purple non-sulfur bacteria are photoorganotrophic heterotrophs when O2 concentrations are low, but are chemotrophs when O2 levels are normal. b. Begiatoa use both inorganic and organic carbon sources