Microbial Growth Definitions 1. 2. 3. 4.
Psychrophiles ……………………………………and many more… Thermphiles Halophiles Mesophiles
Optimum Temperature- The temperature at which a cell grows best. Lag phase: The period of adaptation and rapid aerobic growth Exponential (Logarithmic Phase): A period during the growth cycle of a population in which growth increases at an exponential rate. Stationary Phase: The period during the growth cycle of a population in which growth ceases. Death Phase: The final growth phase of cell culture, during which nutrients have been depleted and cell number decreases.
• The growth of bacteria refers to the increase in the number of cells in the population, not the increase in cell size. • Bacteria multiply in a process called binary fission. During binary fission, the bacterial cell copies its chromosome and other materials and increases in size. Then, cell membrane and wall form between the two chromosomes, splitting the cell in two. Generation Time • The time it takes for a bacterial cell to divide, or for a population to double, is called the generation time. Generation time for bacteria can range from as little as 10 minutes, to as long as 24 hours. Many of the familiar disease-causing bacteria divide in 20-30 minutes. • Because generation times are so short, it doesn’t take long for disease-causing organisms to multiply to dangerous levels. • The formula to find the total number of cells after the generation time is: Initial number of cells x 2number of generations = Number of cells Bacterial Growth Curve •
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When bacteria are introduced into a new environment that has fixed resources, they will exhibit a pattern of growth that can be plotted as a bacteria growth curve. This curve shows the changes in the number of bacteria over time. There are four parts of a bacterial growth curve. They are: the lag phase, the Logarithmic (or exponential) phase, the stationary phase, and the death phase.
Lag phase •
When bacteria are placed into a new environment, there will be no increase in numbers in the population. During this phase, the bacteria are adjusting to the new
environment and synthesizing the necessary enzymes to break down new food sources.
Exponential (Logarithmic) Phase • After the bacteria have made the necessary enzymes, they begin growing at a rapid rate, and the population doubles at regular intervals. This phase, by the way, is the phase in which the bacteria are most metabolically active. Stationary Phase • As nutrients run out, and wastes begin to accumulate, growth slows, and the number of new cells begins to equal the number of dying cells. Death Phase • As growth slows even farther due to these conditions, the number of dying cells becomes greater than the number of new cells, and the population enters the death phase.
Conditions That Affect Growth 1. 2. 3. 4.
Temperature Osmotic Pressure Oxygen Nutritional requirements
Temperature • • • •
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All cells have a range of temperatures at which they survive. This range is defined by the lowest survivable temperature (minimum temperature) and the highest survivable temperature (maximum temperature) Optimum Temperature- The temperature at which a cell grows best. Most bacteria and eukaryotes survive best at moderate temperatures, but some prokaryotes have the ability to tolerate very low or very high temperatures. Based on their temperature preferences, cells are divided up into three main categories: 1. Psychrophiles (cold-loving organisms) 2. Mesophiles (moderate-loving organisms) 3. Thermophiles (heat-loving organisms) The psychrophiles and the thermophiles can be farther divided into organisms that are more or less extreme. psychrotrophs are less extreme than psychrophiles.
Hyperthermophiles are more extreme than thermophiles. The optimum temperatures of these types of organisms are given below. 1. psychrophile= -5-15oC 2. psychrotroph= 20-30oC 3. mesophile= 25-45oC 4. thermophile= 45-75oC 5. hyperthermophile= 70-110oC pH
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From: The Facts On File Dictionary of Environmental Science, Third Edition.
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A unit used to express the strength of an acidic or basic solution; calculated as the negative logarithm of the hydrogen ion concentration. Values commonly range from 0 to 14: less than 7.0 is acidic and greater than 7.0 is basic. A pH of 7.0 is considered neutral. Because the units are derived from common logarithms, a difference of one pH unit indicates a 10-fold (101) difference in acidity; a difference of two units indicates a 100-fold (102) difference in acidity.
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Most bacteria grow best at a ph of 7, neutral ph. Fungi are more tolerant and can grow well in a ph of 5-6 The prokaryotes that can tolerate of very low ph are called acidophiles. It is believed that these prokaryotes survive at these conditions because they secrete and enzyme called urease, which breaks down urea and releases ammonia. The ammonia would pick up the H+, raising the local ph to a survivable level. Other prokaryotes, called alkalophile, can tolerate conditions of very high ph (up to about 12.5) One strategy used by these prokaryotes is to transport H+ into the cell, which helps to maintain the neutrality of the cytoplasm.
Here is a list of all the Extremophiles and their strategies: Extremophile Thermophiles and hyperthermophiles
Growth Condition High Temperatures
Psychrophiles and Psychrotrophs
Low Temperatures
Acidophiles
Acidic conditions
Potential Problem High temperatures cause plasma membranes to become too fluid and may denature (unfold) enzymes. Low temperatures cause plasma membranes to become too rigid. Lower kinetic energy results in less molecular motion. Low ph may denature enzymes.
Strategy More rigid plasma membrane (saturated fatty acids) enzymes that maintain structure in high temperatures. More flexible plasma membrane (unsaturated fatty acids), enzymes that maintain flexibility in cold. Ability to produce compounds that
Alkalophile
Basic conditions
Halophiles
High salt environments
High ph may denature enzymes, cause RNA to break down. A high salt environment is hypertonic to the cell. Water will leave the cell, causing the cell to shrink (plasmolyse).
buffer local ph. Protect intracellular ph by pumping H+ out of the cell. Import H+ into the cell to maintain neutrality of the cytoplasm. Balance intracellular osmotic conditions with those outside the cell by pumping on K+ to prevent water loss. Proteins ma be modified to withstand high salt conditions.
Osmotic Pressure Most cells require conditions of about 90 percent water to function. If cells are placed on a hypertonic environment, water will leave the cell by osmosis. In bacterial cells, this causes the cell to collapse, drawing the plasma membrane away from the wall. This condition is called plasmolysis. When cells are plasmolysed, they may not be dead, but they cannot grow. Prokaryotes that grow in high salt environments are called halophiles. Facultative halophiles grow in typical environments but can also adjust and survive in higher salt environments. Extreme halophiles have protein and membrane modifications that allow them to live in high salt environments. Oxygen Oxygen has a strong pull for electrons (it is electronegative). This characteristic makes it both very useful and very dangerous to cells. OF a cell can use oxygen to perform aerobic respiration, it is very useful to the cell because it allows the cell to make lots of ATP from its food molecules. However, because molecular oxygen (O2) accepts electrons, it can easily be converted to more dangerous free radicals, such as superoxide (O2-) and hydrogen peroxide (H2O2) Superoxide and hydrogen peroxode are both very reactive and will damage organic molecules by stealing electrons from them. This cellular damage can kill cells. In order for a cell to survive in the presence of oxygen, it must have protective enzymes that allow it to detoxify oxygen free radicals. One such enzyme is superoxide dismutase, which converts twp superoxide radials into hydrogen peroxide and water: O2- + O2- + 2H+ H2O2 + O2 Hydrogen peroxide may be neutralized of the organism has catalase of peroxidase, which catalyze the following reactions: Catalase: 2 H2O2 2 H2O2 + O2 Peroxidase: H2O2 +2H+ 2 H2O
The effects of oxygen on a cell depends on what metabolic pathways the cell uses (aerobic respiration, anaerobic respiration, or fermentation) and whether the cell can make the enzymes to detoxify free radicals. Based on their response to oxygen, organisms can be placed in the following categories:
Category Obligate anaerobe Aerotolerant anaerobe Facultative anaerobe Microaerophile Obligate aerobe
Response to Oxygen (all oxygen words should be replaced with O2) Cannot survive in the presence of oxygen Can survive in the presence of oxygen, but does not use oxygen in its metabolism; grows the same with or without oxygen. Can use oxygen in its metabolism and so grows better if oxygen is present, but can still survive if oxygen is absent. Grows best at low concentrations of oxygen. Can only grow if oxygen is present.
Nutritional Requirements All cells need essential elements in order to grow and make necessary molecules The elements that make up the bulk of the necessary lipids, carbohydrates, proteins, and nucleic acids are required in large amounts by cells. These elements are called macronutrients. • The macronutrients can be remembered by this phrase “See Hopkins Café. Mighty Good” This phrase represents the element symbols CHOPKNS Ca Fe Mg, which stands for carbon, hydrogen, oxygen, phosphorous, potassium, nitrogen, sulfur, calcium, iron, and magnesium (actually, it may be a lot easier just memorizing these). • Microbes obtain these elements from food or from inorganic salts that are present in the soil and water. • There are also elements that are required in very small amounts. They are called micronutrients or trace elements and include elements such as: zinc, copper, cobalt, manganese, and molybdenum. Most of the elements are required for enzyme function. Sufficient quantities of trace elements are usually found in water.