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**Microbial Nutrition and Growth Microbial Population Growth**

Microbiology of the Health Sciences

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**The Study of Microbial Growth**

Growth takes place on two levels Cell synthesizes new cell components and increases in size The number of cells in the population increases The Basis of Population Growth: Binary Fission

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Figure 7.13

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**The Rate of Population Growth**

Generation or doubling time: The time required for a complete fission cycle Each new fission cycle or generation increases the population by a factor of 2 As long as the environment is favorable, the doubling effect continues at a constant rate The length of the generation time- a measure of the growth rate of an organism Average generation time- 30 to 60 minutes under optimum conditions Can be as short as 10 to 12 minutes This growth pattern is termed exponential

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**Basic Nutrients for Growth**

ATP for cellular processes Carbon is necessary for the production of many macromolecules (proteins, lipids, and carbohydrates) Oxygen for metabolism Nitrogen for amino acid synthesis Sulfur for vitamins, amino acids, structural stability of proteins

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**Basic Nutrients for Growth**

Phosphorous makes ATP and membranes Trace elements are used for metabolic reaction in the cell and cell component stabilization cobalt Co potassium K molybdenum Mo magnesium Mg manganese Mn calcium Ca iron Fe zinc Zn

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**Basic Nutrients for Growth**

Organic growth factors such as vitamins, amino acids, and nucleic acids some growth factors cannot be synthesized by own cellular processes Water water activity

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**How Microbes Obtain Nutrients**

Heterotroph: uses organic carbon source Autotroph: uses inorganic carbon dioxide Phototroph: uses light as energy source Chemotroph: uses chemical compounds (ie. glucose) Saprobe Parasite

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**Microbial Nutrition Growth Requirements**

[INSERT FIGURE 6.1]

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**Culturing Microorganisms**

Inoculum introduced into medium (broth or solid) Environmental specimens Clinical specimens Stored specimens Culture – refers to act of cultivating microorganisms or the microorganisms that are cultivated

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**Culturing Microorganisms**

[INSERT TABLE 6.3]

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**Culturing Microorganisms**

Special Culture Techniques Techniques developed for culturing microorganisms Animal and cell culture Low-oxygen culture Enrichment culture

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Media Chemically defined Natural Living

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**Media Enriched Media added nutrient encourages**

the growth of microorganisms

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**Media Selective Media Selects form a microorganism**

while inhibiting most others Phenol Ethanol Agar Deoxycholate Agar

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**Media Differential Media Allow for the differentiation of**

microorganisms based on action that occurs on the media or a color change within the media that is based on a pH change Mannitol Salt Agar MacConkey Agar

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**Media Question What type of media is Blood Agar considered? Enriched**

Selective Differential

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**The Population Growth Curve**

A population of bacteria does not maintain its potential growth rate and double endlessly A population displays a predictable pattern called a growth curve The method to observe the population growth pattern: Place a tiny number of cells in a sterile liquid medium Incubate this culture over a period of several hours Sampling the broth at regular intervals during incubation Plating each sample onto solid media Counting the number of colonies present after incubation

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**The Rate of Population Growth**

Generation or doubling time: The time required for a complete fission cycle Each new fission cycle or generation increases the population by a factor of 2 As long as the environment is favorable, the doubling effect continues at a constant rate The length of the generation time- a measure of the growth rate of an organism Average generation time- 30 to 60 minutes under optimum conditions Can be as short as 10 to 12 minutes This growth pattern is termed exponential

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**Stages in the Normal Growth Curve**

Data from an entire growth period typically produce a curve with a series of phases Lag Phase Exponential Growth Phase Stationary Growth Phase Rapidly Declining Phase Death Phase

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**Lag Phase Relatively “flat” period**

Newly inoculated cells require a period of adjustment, enlargement, and synthesis The cells are not yet multiplying at their maximum rate The population of cells is so sparse that the sampling misses them Length of lag period varies from one population to another

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**Exponential Growth (Logarithmic or log) Phase**

When the growth curve increases geometrically Cells reach the maximum rate of cell division Will continue as long as cells have adequate nutrients and the environment is favorable The number of cells growing greatly out number the number of cells dying.

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**Stationary Growth Phase**

The population enters a survival mode in which cells stop growing or grow slowly The rate of cell inhibition or death balances out the rate of multiplication Depleted nutrients and oxygen Excretion of organic acids and other biochemical pollutants into the growth medium The number of cells growing will equal the amount of cells dying. Endospores begin to form in this phase.

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**Rapidly Declining Phase**

The curve dips downward Cells begin to die at an exponential rate The amount of cells dying out numbers the amount of cells growing. The dead cells become nutrients for the growing cells.

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**Death Phase The curve continues to dips downward**

Most cellular activity stops Endospores are formed and released from the parent cells.

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**Phases of Growth Basic phases of growth:**

1. Lag phase: new growth medium, period of delay while cells prepare to divide 2. Log phase (exponential growth phase): cellular reproduction most active during this period, generation time reaches a constant minimum 3. Stationary phase: state of equilibrium where number of cell deaths equals number of cell divisions

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Phases of Growth Basic phases of growth: 4. Rapidly Declining Phase: cells die logarithmically, endospores formed 5. Death phase: number of deaths exceeds number of new cells

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**Potential Importance of the Growth Curve**

Implications in microbial control, infection, food microbiology, and culture technology Growth patterns in microorganisms can account for the stages of infection Understanding the stages of cell growth is crucial for working with cultures In some applications, closed batch culturing is inefficient, and instead, must use a chemostat or continuous culture system

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**Graphing Bacterial Growth**

The data from growing bacterial populations are graphed by plotting the number of cells as a function of time If plotted logarithmically- a straight line If plotted arithmetically- a constantly curved slope To calculate thesize of a population over time: Nf = (Ni)2g Nf is the total number of cells in the population at some point in the growth phase Ni is the starting number g denotes the generation number

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Population Equation Nf = Ni (2)g

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