Chapter 9- Microbial growth Flashcards
(114 cards)
1
Q
In prokaryotes, reproduction is always
A
Asexual. Genetic reproduction can occur in the form of horizontal gene transfer.
2
Q
What form is bacterial DNA usually in?
A
Most bacteria have a single circular chromosome
3
Q
Binary fission (3)
A
The most common mechanism of cell replication in bacteria.
1. The cell grows and increases its number of cellular components
2. DNA replication begins at the origin of replication
3. The center of the enlarged cell constricts until two daughter cells are formed
4
Q
Origin of replication
A
The region of the bacterium’s circular chromosome that is attached to the inner cell membrane. This is where DNA replication begins during binary fission. Replication continues in opposite directions along the chromosome until it reaches the terminus
5
Q
How do the offspring resemble the original cell after binary fission?
A
The offspring are clones of the original cell. They receive a complete copy of the parental genome and a division of the cytoplasm through cytokinesis
6
Q
FtsZ
A
A protein that directs the process of cytokinesis and cell division in bacteria. It assembles into a Z ring on the cytoplasmic membrane. The Z ring is anchored by FtsZ binding proteins and provides a plane for cell division
7
Q
FtsZ
A
A protein that directs the process of cytokinesis and cell division in bacteria. It assembles into a Z ring on the cytoplasmic membrane
8
Q
Divisome
A
During binary fission, additional proteins are added to the Z ring to form this structure. The divisome will eventually produce a peptidoglycan cell wall wall and will build a septum that divides the two daughter cells
9
Q
Division septum
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Separate the two daughter cells during binary fission. This is where the cell’s cell wall and outer membranes are remodeled to finish the division process
10
Q
Generation time
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In prokaryotes, this is also called the doubling time- the time it takes for the population to double through one round of binary fission. Different species of bacteria have different generation times
11
Q
Growth curve
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The growth pattern of microorganisms plotted as a logarithm of bacterial cells (y axis) and time (x) axis
12
Q
Culture density
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The number of cells per unit volume. In a closed environment, this is also a measure of the number of cells in the population
13
Q
Phases of the growth curve (4)
A
- Lag phase- no increase in the number of living bacterial cells
- Log phase- exponential increase in the number of living bacterial cells- positive slope
- Stationary phase- plateau in number of living bacterial cells, the rate of cell division is equal to the rate of cell death
- Decline phase- exponential decrease in the number of living bacterial cells
14
Q
Inoculum
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The small number of cells that are initially added to a fresh culture medium
15
Q
Culture medium
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A nutritional broth that supports growth
16
Q
Lag phase
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The part of the growth curve where cells are gearing up for the next phase of growth. The cells grow larger and are metabolically active so that they can synthesize necessary proteins for growth in that medium
17
Q
Logarithmic (log) growth phase
A
The part of the growth curve where cells are actively dividing by binary fission- their number increases exponentially. Generation time under specific growth conditionals is genetically determined. Cells in this phase have constant growth and uniform metabolic activity. This is the stage where bacteria are most susceptible to disinfectants and antibiotics that affect protein, DNA, and cell wall synthesis
18
Q
Intrinsic growth rate
A
The genetically predetermined generation time of bacteria in specific conditions
19
Q
Stationary phase
A
A plateau in the number of living bacterial cells in the growth curve. At this point, the rate of cell division is equal to the rate of cell death, so the total population of living cells is mostly stagnant. This occurs because as the number of cells increase, waste products accumulate, oxygen is depleted, and nutrients are used up- this creates unfavorable living conditions and cells start to die. During this phase, cells switch to a survival mode of metabolism. Many cells undergo sporulation if they are capable of producing endospores. Cells synthesize secondary metabolites, including endospores, in this phase. Virulence factors are also synthesized
20
Q
What determines a culture’s carrying capacity?
A
The carrying capacity/maximum culture density depends on the types of microorganisms in the culture and the specific conditions of the culture. However, carrying capacity is constant for a given organism grown under the same conditions
21
Q
How do cells’ metabolism change during the stationary phase?
A
Metabolism switches to survival mode. As the growth of the cells slows down, so does the synthesis of peptidoglycans, proteins, and nucleic acids. This means that stationary cultures are less susceptible to antibiotics that disrupt these processes
22
Q
Virulence factors
A
Products that contribute to a microbe’s ability to survive, reproduce, and cause disease in a host organism. S. aureus bacteria produce enzymes that can break down human tissue and cellular debris, which allows bacteria to spread to new tissue where nutrients are more plentiful. These products are typically synthesized during the stationary phase
23
Q
Death phase
A
The phase of the growth curve where cells die in greater numbers, and the number of dying cells exceeds the number of dividing cells. This occurs because the culture medium begins to accumulate toxic waste and nutrients are exhausted. Some cells lyse and release nutrients, which allows some of the other cells to survive and form endospores.
24
Q
Persisters
A
Cells that are characterized by a slow metabolic rate. These cells are associated with chronic infections that do not respond to antibiotic treatment, such as tuberculosis
25
Chemostat
A culture vessel that is used to maintain a continuous culture where nutrients are supplied at a constant rate. Bacterial suspension is removed at the same rate at which nutrients flow in to maintain an optimal growth environment
26
Why is it important to estimate the number of bacterial cells in a sample?
The number of bacteria in a clinical sample can indicate the extent of an infection. Estimates of bacterial counts in drinking water, food, medication, and cosmetics are used to detect contamination and prevent the spread of disease
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Direct cell count
Refers to counting the cells in a liquid culture or colonies on a plate. It is a direct method of estimating how many organisms are present in a sample
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Direct microscopic cell count
Involves transferring a known volume of a culture to a calibrated slide (Petroff-Hausser chamber) and counting the cells under a light microscope. It involves a counting chamber which is etched into squares of different sizes. A sample of the culture is added to a chamber. The concentration of cells in the original sample can be estimated by counting individual cells in a certain number of squares, and determining the volume of the sample observed. Cells in several small squares are counted and averaged in order to get an accurate measurement.
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Petroff-Hausser chamber
The calibrated slide used in a direct microscopic cell count. It is similar to a hemocytometer used to count red blood cells.
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Pros and cons of the direct microscopic cell count method
Pros- it is a cheap, easy to use, and fast method
Cons- the counting chamber does not work well with dilute cultures. Also, it can be difficult to differentiate between living and dead cells with this method
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Fluorescence staining
Used to distinguish between living and dead bacteria. These stains bind to nucleic acids, but the primary and secondary stains differ in their ability to cross the cytoplasmic membrane. The primary stain is fluorescent green and can penetrate intact cytoplasmic membranes, and stains living and dead cells. The secondary stain is red and can stains dead cells, because it only stains a cell if the cytoplasmic membrane is damaged.
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Coulter counter
An electronic cell counting device that detects and counts the changes in electrical resistance in saline solution. Cells are drawn through a class tube, and an electrode is located inside and outside the tube. The electrodes measure the brief change in resistance between them as the cells pass through. Each resistance change represents a cell. This method is fast and accurate within a range of concentrations. However, if the culture is too concentrated, multiple cells could pass through at one time and skew the results. It also can't differentiate between living and dead cells
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Viable
Living- refers to live cells in this context
34
In which situations is it necessary to estimate the number of living cells in a sample?
Counts of live cells are needed to determine the extent of an infection, the effectiveness of antimicrobials, or the contamination of food and water. In these cases, direct cell counts aren't helpful
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Viable plate count
Produces an estimated count of viable cells, based on the principle that viable cells replicate and give rise to visible colonies when incubated. The results are expressed as colony-forming units per millimeter. Microbiologists use plates with 30-300 colonies to get the most reliable numbers.
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What are the limitations of a viable plate count?
More than one cells can occupy one spot, which is why the results have to be expressed as colony-forming units per millimeter. Some samples of bacteria grow in clusters or chains that are difficult to disperse, and a colony may represent multiple cells. Also, some cells can't form colonies on solid media. These limitations mean that viable plate counts are low estimates of the actual number of cells
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2 approaches to a viable plate count
1. Pour plate
2. Spread plate
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Serial dilution
The first step of a viable plate count, before a pour plate or a spread plate. It is used in order to obtain plates that have between 30-300 colonies. Serial dilutions are usually done in multiples of 10 to simplify the process, but the number of dilutions is determined based on culture density. The culture is diluted in sterile broth, and the dilution continued until the culture is a specific concentration. The sample is plated on solid medium using either the pour plate method or the spread plate method, and the plates are incubated until colonies appear. 2-3 plates are used and the number of colonies on each plate are averaged.
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CFU/mL
Stands for colony forming units per millimeter
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Pour plate method (4 steps)
1. The bacterial sample is mixed with warm agar (45-50 degrees)
2. The sample is poured onto a sterile plate
3. Sample is swirled to mix and allowed to solidify
4. The plate is incubated until bacterial colonies grow
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Spread plate method (3 steps)
1. A .1 mL sample is poured onto solid medium
2. Spread sample evenly over the surface
3. The plate is incubated until bacterial colonies grow on the surface of the medium
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Membrane filtration technique
Known volumes of a sample are vacuum-filtered aseptically though a membrane with a pore size small enough to trap microorganisms. The membrane is transferred to a Petri plate containing a growth medium, and the colonies are counted after incubation. Cell density is calculated by dividing the cell count by the volume of filtered liquid. This method is used for a very dilute sample that doesn't contain enough organisms to use either of the plate count methods
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Most probable number (MPN) method
A statistical procedure for estimating the number of viable microorganisms in a very dilute sample. It evaluates detectable growth by observing changes in turbidity or color due to metabolic activity. It can be used for water and food samples
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Turbidity
The cloudiness of a sample of bacteria in a liquid suspension. A spectrophotometer is the instrument used to measure turbidity. This is an indirect cell count method
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How does a spectrophotometer work?
A light beam is transmitted through a bacterial suspension, and a detector measures the amount of light that is passing through the sample. The amount of light that reaches the detector is converted into a percent transmission or a logarithmic value called absorbance/optical density. As the numbers of bacteria in a suspension increase, the turbidity also increases and causes less light to reach the detector. The decrease in light passing through the sample and reaching the detector is associated with a decrease in percent transmission and increase in absorbance measured by the spectrophotometer
46
How is the actual number of cells estimated from turbidity readings?
By performing a viable plate count of samples that have been taken from cultures with a range of absorbance values. Turbidity can be plotted as a function of cell density, making a calibration curve. The calibration curve can be used to estimate cell counts for all samples obtained or cultured under similar conditions
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Measuring dry weight of a culture
An indirect method of evaluating culture density without directly measuring the cell count. The cell suspension is concentrated (by filtration or centrifugation), washed, and dried, and measurements are then taken. This is useful for filamentous microbes that are difficult to count directly
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Fragmentation
Multiple nucleoids accumulate in an enlarged cell or along a filament, allowing multiple cells to develop at once. The new cells then split from the parent filament and float away (this is the fragmentation process). It occurs in cyanobacteria, and is commonly observed in Actinomycetes (gram positive, aerobic, found in soil). The new cells contain one nucleoid and can develop into spores that give rise to new colonies
49
Which example of cell division in prokaryotes is similar to that of birth in animals?
Several daughter cells fully develop inside of the parent cell. The parent cell disintegrates and releases the new cells. Occurs in Epulopiscium bacteria
50
Budding
A process where species of prokaryotes form a long, narrow extension. The tip of the extension enlarges and forms a smaller cell, as a bud that detaches from the parent cell as a fully formed cell. This is common in yeast but is also found in prosthecate bacteria and some cyanobacteria.
51
Biofilms
Complex and dynamic ecosystems that form on a variety of environmental surfaces. They can form on solid surfaces or liquid surfaces, like with microbial mats that float on water. They are beneficial to the survival of microorganisms
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How are biofilms structured?
The structure depends on environmental conditions. Some biofilms (streamers) are filamentous, and are anchored to the substrate by a head- the tail floats downstream in a current of water. In still or slow moving water, a biofilm might have a mushroom-like shape. Under a microscope, biofilms seem to contain clusters of microorganisms embedded in a matrix interspersed with open water channels. There is an extracellular matrix composed of EPS- it makes up 50-90% of the biofilm's weight
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Extracellular polymeric substances (EPS)
The substance making up the extracellular matrix in biofilms, secreted by the organisms making up the biofilm. It is a hydrated gel made of polysaccharides and other macromolecules like proteins, nucleic acids, and lipids. It provides the integrity and function of the biofilm. EPS contain channels that allow the movement of nutrients, wastes, and gases through the biofilm, which keeps the cells hydrates. EPS also protects organisms in the biofilm from predation by other microbes or cells (like WBCs)
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Planktonic cells
Free-floating microbial cells that live in an aquatic environment. To form a biofilm, planktonic cells attach to a substrate, where they become sessile (attached to a surface)
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Stages of biofilm formation (5)
1. Planktonic cells attach to a surface that is coated with a conditioning film of organic material. This stage is reversible
2. The first colonizers become irreversibly attached within seconds or minutes. The cells express new phenotypes that allow a sessile lifestyle
3. Growth and cell division occurs within hours or days.
4. Production of EPS and formation of water channels. The biofilm develops necessary and characteristic structures, and appendages interact with the EPS
5. Attachment of secondary colonizers and dispersion of microbes to new sites
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Dispersal
The last stage of the biofilm life cycle, where cells on the periphery of the biofilm revert to a planktonic lifestyle. They fall off of the mature biofilm to colonize new sites. This occurs days to months into the cycle
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Quorum sensing
The mechanism by which cells in a biofilm coordinate their activities in response to environmental stimuli. It can occur between cells of different species within a biofilm. It enables microorganisms to detect their cell density through the release and binding of autoinducers
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Autoinducers
Small, diffusible molecules. When the cell population in a biofilm reaches a critical threshold (a quorum), the autoinducers initiate a cascade of reactions that activate genes associated with cellular functions that are beneficial once the population reaches a critical density. With some pathogens, virulence factors are only produced when there are enough cells to overwhelm the host's immune defenses
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How do different species of microorganisms establish metabolic collaborations?
The waste product of one organism becomes the nutrient for another. Aerobic microorganisms consume oxygen, creating anaerobic regions that are beneficial for anaerobic microorganisms
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2 classes of signaling molecules in quorum sensing
1. N-acylated homoserine lactones, used by Gram negative bacteria
2. Small peptides, used by Gram positive bacteria
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Steps of quorum sensing (3)
1. When a threshold concentration of signaling molecules is reached, the autoinducer binds to its specific receptor
2. A cascade of signaling events leads to changes in gene expression
3. Biological responses linked to quorum sensing are activated. This includes the increase in production of signaling molecules
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What is the significance of biofilms to human health?
Some biofilms are pathogenic. Plaque on teeth is a biofilm that causes dental and periodontal disease, and biofilms can form in wounds and cause infections. Pathogens embedded in biofilms have a higher resistance to antibiotics
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Why are pathogens in biofilms more resistant to antibiotics?
Cells located deep in a biofilm are metabolically inactive and can be less susceptible to antibiotics that disrupt bacterial metabolism. EPS could also slow down the diffusion of antibiotics and prevent them from getting to cells deep in the biofilm. Phenotypic changes and exchange of genes in biofilms can contribute to antibiotic resistance and to traits that allow the bacteria to remove the antibiotic from their cells
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Reactive oxygen species
Unstable ions and molecules that come from partial reduction of oxygen. They can damage almost any macromolecule or structure they come in contact with. Examples include superoxide, peroxide (H2O2), singlet oxygen, and others
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Thioglycolate tube cultures
Uses a thioglycolate medium that contains a low percentage of agar to allow motile bacteria to move in the medium. Thioglycolate is a reducer, and an autoclave is used to remove oxygen. Bacterial cultures are added to the medium to determine their molecule oxygen requirements. Oxygen slowly diffuses through the culture, and bacterial density increases in the area where oxygen concentration is best for that specific organism
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Obligate aerobes
Strict aerobes that can't grow without an abundant supply of oxygen. They grow at the top of a tube in a thioglycolate tube culture. They can be found in the environment where anaerobic conditions exist
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Obligate anaerobes
Microorganisms that are killed by oxygen. They grow at the bottom of the tube in a thioglycolate tube culture.
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Facultative anaerobes
Organisms that thrive in the presence of oxygen, but they can grow without oxygen using fermentation or anaerobic respiration. They grow mostly at the top but also throughout the tube in a thioglycolate tube culture
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Aerotolerant anaerobes
Don't use oxygen because they use fermentation, but they are not harmed by the presence of it. They grow evenly through the tube in a thioglycolate tube culture
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Microaerophiles
Bacteria that require a minimum level of oxygen for growth, well below the percentage of oxygen found in the atmosphere. They grow at the top of the tube in a thioglycolate tube culture
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Where are anaerobic environments found?
Anaerobic environments exist naturally in the intestinal tract of animals, but are also found in still waters, in the bottom of the ocean where there is no photosynthetic life, and in deep sediments of soil. Obligate anaerobes, mainly Bacteroidetes, represent a large fraction of the microbes in the human gut. Tissues become anaerobic and die when blood flow is cut off, and are an ideal environment for obligate anaerobes
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C. difficile
An obligate anaerobe. Prolonged use of
antibiotics for other infections increases the probability of a patient developing a secondary C. difficile infection. Antibiotic treatment disrupts the balance of microorganisms in the intestine and allows the colonization of the gut by C. difficile, causing a significant inflammation of the colon
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Why are deep puncture wounds associated with tetanus?
The infection starts in necrotic tissue that does not receive oxygen, which is good environment for C. tetani, an obligate anaerobe
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Anaerobic jar
Include chemical packs that remove oxygen and release carbon dioxide. Used to grow obligate anaerobic bacteria
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Anaerobic chamber
An enclosed box where all oxygen is removed. Gloves are sealed to openings in the box to allow for handling of the cultures without exposing them to air. Used to grow obligate anaerobic bacteria
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Examples of facultative anaerobes
Staphylococci and Enterobacteriaceae. Enterobacteriaceae are found primarily in the gut and upper respiratory tract but can sometimes spread to the urinary tract, where they are capable of causing infections. Sometimes, there are mixed bacterial infections in which the facultative anaerobes use up the oxygen, creating an environment for the obligate anaerobes to flourish
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Examples of aerotolerant anaerobes
Lactobacilli and streptococci, both found in the oral microbiota
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Example of a microaerophile
Campylobacter jejuni, which causes gastrointestinal infections
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Optimum oxygen concentration
The ideal concentration of oxygen for a particular microorganism.
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Minimum permissive oxygen concentration
The lowest concentration of oxygen that allows the microbe to grow. The organism will not grow outside the range of oxygen levels found between the minimum
and maximum permissive oxygen concentrations.
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3 enzymes that break down the toxic byproducts of aerobic respiration
Superoxide dismutase, peroxidase, and catalase. Obligate anaerobes do not have any of these enzymes
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Which reactions are catalyzed by peroxidase?
A reduced compound (electron donor like NADH) oxidizes hydrogen peroxide (H2O2), producing water. The enzyme limits the damage caused by peroxidation of membrane lipids
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Which reactions are catalyzed by superoxide dismutase?
Superoxide (O2) anions are broken down to produce hydrogen peroxide and oxygen
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Which reactions are catalyzed by catalase?
Hydrogen peroxide is converted to water and oxygen
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Capnophiles
Bacteria that grow best in a higher concentration of carbon dioxide and a lower concentration of oxygen. A candle jar is used to grow them, because a burning candle consumes most of the oxygen present and releases carbon dioxide
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How do pH changes affect macromolecules?
The hydrogen bonds that hold together strands of DNA break apart at a high pH. A basic pH hydrolyzes lipids. The proton motive force that is used to make ATP depends on the concentration gradient of hydrogen ions across the plasma membrane. If those ions are neutralized by hydroxide (OH) ions, then concentration gradient collapses and impairs energy production. Also, moderate changes in pH change the ionization of amino acid functional groups and impair hydrogen bonding, causing proteins to denature
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Optimum growth pH
The most favorable pH for the growth of an organism
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Minimum and maximum growth pH
The lowest and highest pH values that an organism can tolerate. These values can cover a wide range
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Neutrophiles
Organisms that grow optimally at a pH within one or two pH units of the neutral pH (7). This describes most bacteria, and some bacteria like E. coli and Salmonella can't survive in the acidic environment of the stomach. Pathogenic strains of these bacteria are resistant to stomach acid
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Acidophiles
Organisms that grow optimally at a pH less than 5.55. Sulfur oxidizing sulfolobus archaea found in the hot springs in Yellowstone National Park are extreme acidophiles. Acidophiles have mechanisms to survive in these environments. Proteins show increased negative surface charge that stabilizes them at a low pH, and pumps eject hydrogen ions out of cells.
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Lactobacillus bacteria
Acidophiles that are part of the normal microbiota of the vagina. They can tolerate environments with a pH between 3.5 and 6.8, and they contribute to the acidity of the vagina (pH of 4, except at the onset of the menstruation) through their metabolic production of lactic acid. The acidic environment of the vagina inhibits the grow of other microbes that can't tolerate acidity
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Alkaliphiles
Microorganisms that grow best at a pH between 8 and 10.5. Extreme alkaliphiles have adapted to their environment through modifications of lipid and protein structure and mechanisms to maintain the proton motive force in an alkaline environment. For example, some bacteria derive energy from a sodium ion gradient rather than a proton one. Vibrio cholerae, the pathogenic agent of cholera, grows best at the slightly basic pH of 8.0; it can survive pH values of 11.0 but is inactivated by the acid of the stomach.
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Optimum growth temperature
The temperature at which growth rates of the microbe are highest
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Minimum and maximum growth temperature
The lowest and highest temperatures at which the microbe can survive and replicate
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Mesophiles
"Middle loving", microbes that are adapted to moderate temperatures. Their optimal growth temperatures range from room temperature (20 degrees) to 45 degrees. The core temperature of the body is 37 degrees, so normal human microbiota and pathogens are mesophiles
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Psychotrophs
Microorganisms that live in cooler temperatures, from 25 to 4 degrees (refrigeration temperature). They are found in many natural environments and are responsible for the spoilage of refrigerated food
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Psychrophiles
Cold loving organisms. They grow at temperatures of 0 degrees and below, have an optimum growth temperature of 15, and usually can't survive above 20. Proteins in these cells are rich in hydrophobic residues, are more flexible, and have a lower number of secondary stabilizing bonds. They also have antifreeze proteins and solutes that decrease the freezing temperature of the cytoplasm. Growth rates are much slower than those encountered at moderate
temperatures.
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Thermophiles
Organisms that grow between 50 and 80 degrees. They don't multiply at room temperature. They are found in hot springs, geothermal soils, and manmade environments like garden compost piles
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Hyperthermophiles
Grow between 80 and 110 degrees. The hydrothermal vents at the bottom of the ocean are an example of an optimal environment
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How do low temperatures affect a cell?
Membranes become less fluid and are damaged by ice crystal formation. Chemical reactions and diffusion slow down. Proteins become too rigid to catalyze reactions and can denature
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How do high temperatures affect a cell?
Membranes become more fluid, which impairs metabolic processes that can occur in the membrane. Proteins and nucleic acids are denatured. Heat is a component of multiple sterilization techniques.
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How are macromolecules in thermophiles and hyperthermophiles different from those in mesophiles?
There are more saturated lipids than polyunsaturated lipids, which limits the fluidity of the cell membrane. Their DNA sequences have more guanine-cytosine bases, which require more hydrogen bonds to stay together than adenine-thymine. Additional secondary structures, like ionic and covalent bonds, help the proteins to resist denaturation
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Halophiles
"Salt loving" organisms, which require high salt concentrations for growth. They are found in marine environments with a salt concentration of around 3.5%. Dunaliella spp. counters the tremendous osmotic pressure of the environment with a high cytoplasmic
concentration of glycerol and by actively pumping out salt ions. Halobacterium spp. accumulates large concentrations of K+ and other ions in its cytoplasm. Its proteins are designed for high salt concentrations and
lose activity at lower salt concentrations
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Halotolerant organisms
Don't need high concentrations of salt for growth, but will survive and divide in the presence of high salt. Halotolerant pathogens are a cause of food borne illnesses because they can survive and multiply in salty food
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Barophiles
Organisms that require high atmospheric pressure for growth. Bacteria living at the bottom of the ocean live in high pressures.
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In most natural environments, how does the solute concentration compare to that of microorganisms' cytoplasm?
Most natural environments tend to have lower solute concentrations than the cytoplasm of most microorganisms. Rigid cell walls protect the cells from bursting in a dilute environment. In areas of high osmotic pressure, there aren't many protective mechanisms. Water leaves the cell along its concentration gradient, and the cell can shrink
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Photoautotrophs and photoheterotrophs
Depend on sufficient light intensity at the wavelengths absorbed by their pigments. Photoautotrophs include cyanobacteria and green sulfur bacteria, and photoheterotrophs include purple nonsulfur bacteria. Energy from light is captured by pigments and converted into chemical energy that drives
carbon fixation and other metabolic processes.
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Photosynthetically active radiation (PAR)
The portion of the electromagnetic spectrum that is absorbed by photoautotrophs and photoheterotrophs. It lies within the visible light spectrum ranging from 400 to 700 nanometers (nm) and extends in the near infrared for some photosynthetic bacteria. . A number of accessory pigments, such as fucoxanthin in brown algae and phycobilins in cyanobacteria, widen the useful range of wavelengths for photosynthesis and compensate for the low light levels available at greater depths of water.
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Enriched media
Contains growth factors, vitamins, and
other essential nutrients to promote the growth of fastidious organisms- organisms that cannot make certain nutrients and require them to be added to the medium
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Chemically defined medium
When the complete chemical composition of a medium is known. For example, in EZ medium, all individual chemical components are identified and the exact amounts of each is known
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Complex media
The precise chemical composition of the medium is not known. It contains variable extracts and digests of yeasts, meat, or plants. Nutrient broth, tryptic soy broth,
and brain heart infusion, are all examples of complex media.
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Selective media
Media that inhibit the growth of unwanted microorganisms and support the growth of the organism of interest by supplying nutrients and reducing competition. One example is MacConkey agar. It contains bile salts and crystal violet, which interfere with the growth of many gram-positive bacteria and favor the growth of gram-negative bacteria, particularly the Enterobacteriaceae
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Enrichment culture
Fosters the preferential growth of a desired microorganism that represents a fraction of the organisms present in an inoculum
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Differential media
Makes it easy to distinguish colonies of different bacteria by a change in the color of the colonies or the color of the medium. Color changes are the result of end products created by interaction of bacterial enzymes with differential substrates in the medium. In the case of hemolytic reactions, it is caused by the lysis of red blood cells in the medium
