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Topic_6 - Topic 6 Fundamentals of...

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Unformatted text preview: Topic 6 Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: Following this lecture, you should understand the concept of exponential growth, and how growth is measured in the laboratory. You should also realize that microorganisms have preferred conditions, and that these differ according to the specific type of microorganism. Different microorganisms have different optima for temperature, pH, water availability, and oxygen. Population growth Growth rate = change in cell # / time Generation time (g) = time for formation of two cells from one = doubling time If the generation time of a culture remains constant, the growth rate will change in an exponential fashion. This is called exponential growth (see Figure 6.8). The generation time (g) can be calculated if the number of cells in a culture at the beginning and end of a given time period are known (Figure 6.9). Growth cycle of populations A typical growth curve for a batch culture is shown in Figure 6.10. Lag phase: the time required for the organisms to adapt to new culture conditions or recover from injury. Exponential phase: exponential growth Stationary phase: no net change in cell number Death phase: some populations are unable to maintain stationary phase indefinitely, and there will then be a net reduction in cell number Measurement of growth The number of cells in a culture can be measured by either direct microscopic count (Figure 6.14) or viable count (Figures 6.15 and 6.16). One of the limitations of the direct method is that it does not distinguish living from dead cells. Another very useful method of estimating cell numbers is to measure the culture turbidity (Figure 6.17). In culture, there is a proportional relationship between turbidity and cell number. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld So far, we have been considering growth only under ideal conditions. In the real world, most microorganisms are not growing in log phase (we know why that could not be possible!) and there are several factors, besides nutrient availability and growth inhibition by toxic substances, which can limit growth. In this lecture, we will look at some of these factors. Temperature Each type of microorganism has upper and lower temperature limits (Figure 6.18), beyond which they are not able to grow. Between these upper and lower limits, there is an optimum temperature at which growth is maximum. Maximum growth temperature: inactivation of at least one important protein. Minimum growth temperature: freezing of the cytoplasmic membrane. There is a wide variation in the temperature limits and optima for different microorganisms (Figure 6.19). • • • • Psychrophiles: low temperature optima Mesophiles: midrange temperature optima Thermophiles: high temperature optima Hyperthermophiles: very high temperature optima Low Temperature Growth • Psychrophiles: optimal growth temperature 15°C or lower, maximum growth temperature below 20°C, minimal growth temperature 0°C or lower Psychrotolerant: grow at 0°C, optimal growth temperature 20 ­40°C • Many psychrophiles are actually killed by temperatures above 20°C. They are often very difficult to study, since they must always be kept below room temperature. Their intolerance to heat is due to the denaturation of some of their enzymes at even moderate temperatures. As expected, the cytoplasmic membranes of psychrophiles are more fluid at low temperatures, and this is due to a greater proportion of unsaturated fatty acids in the membrane phospholipids. High Temperature Growth • Thermophiles: optimal growth temperature above 45°C • Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Hyperthermophiles: optimal growth temperature above 80°C Examples of hot environments include: soil surface in midsummer sun: 50°C compost piles: 65°C hot springs: boiling point of water (100°C) ocean floor hydrothermal vents: 350°C hot water heaters: ~50°C Archaea are the most thermophilic of all organisms, followed by bacteria, and then by eukaryotes (see Table 6.1). For thermophiles and hyperthermophiles to survive and thrive at high temperatures, their proteins must be resistant to thermal denaturation (i.e. thermostable). Their membrane phospholipids must also have a high proportion of saturated fatty acids (for the bacterial thermophiles) or lipid monolayer (for the archaeal hyperthermophiles Figure 4.8d). Enzymes from thermophiles and hyperthermophiles are valued for their use in industrial processes, which are often preferentially carried out at high temperatures. Growth at Low or High pH • • • Acidophiles: pH optimum between 2 and 6 Neutrophiles: pH optimum between 6 and 8 Alkalophiles: pH optimum between 8 and 11 Osmotic Effects on Microbial Growth Water availability is expressed as water activity (aw) (see Table 6.2). Solute concentration is an important contributor to this value. The higher the aw value, the easier it is for the microorganism to obtain water. If the solute concentration outside of the cell is lower than inside (i.e. high water concentration outside) then the water tends to diffuse into the cell. If the solute concentration is higher outside the cell, then the water tends to diffuse out of the cell. In nature, water activity is commonly influenced by the NaCl concentration. • • Halophiles: growth optimum above 1% NaCl, often requiring sodium for growth Halotolerant: can tolerate the presence of solute, but grow better in the absence of solute • • Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Osmophiles: can grow in presence of high sugar Xerophiles: can grow in very dry conditions To cope with low water activity, many microorganisms are able to increase the internal solute concentration (Figure 6.26, Table 6.3). They do this by increasing the intracellular concentration of compatible solutes, which do not inhibit biochemical processes. Oxygen Effects on Microbial Growth Please see Table 6.4 for an outline of how different microorganisms are affected by O2. Note that some organisms are very sensitive to O2, others require O2 for growth, and others can grow either in the presence or absence of O2. Note that reduction of O2 produces toxic byproducts, which are very good oxidizers (i.e. causes other compounds to oxidize). Organisms that are able to grow in the presence of O2 are usually able to detoxify these byproducts (Figure 6.30) using specific enzymes such as catalase, peroxidase and superoxide dismutase. Since O2 is not very soluble in water, to culture aerobic microorganisms, it is necessary to aerate the culture by either shaking the tube or flask or by bubbling air into the culture medium. In contrast, growth of obligate anaerobes requires that O2 be excluded from the culture medium. Thioglycolate broth, along with the redox indicator dye resazurin, can be used as a test to demonstrate the aerobic or anaerobic growth characteristics of a given microorganism (Figure 6.27). ...
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