Microbial Growth and Control Lecture 4 PDF

BIO 311 - Microbiology part 4 Chapter 5 Microbial Growth and Its Control Prepared by Charbel Al Khoury, Ph.D. [email protected] Lebanese American U...

BIO 311 - Microbiology part 4 Chapter 5 Microbial Growth and Its Control Prepared by Charbel Al Khoury, Ph.D. [email protected] Lebanese American University  Microbial Growth and Its Control IV. Environmental Effects on Growth: pH, Osmolarity, and Oxygen 12. Effects of pH on Microbial Growth 13. Osmolarity and Microbial Growth 14. Oxygen and Microbial Growth  12. Effects of pH on Microbial Growth the optimum PH pH expresses the acidity or alkalinity of a solution. pH 7 = neutral (Figure 1) acidic pH < 7, alkaline pH > 7 Each microbe has a pH range ~2–3 pH units within which growth is possible. Most natural environments are pH 3–9. (Table 1) Figure 1. The pH Scale  12. Effects of pH on Microbial Growth For example, the bacterium Escherichia coli is a neutrophile Neutrophiles: organisms that grown optimally at pH 5.5–7.9 Acidophiles: organisms that grow best at low pH (< 5.5) A critical factor governing acidophily is the stability of the cytoplasmic membrane. When the pH is raised to neutrality, the cytoplasmic membranes of strongly acidophilic bacteria are destroyed and the cells lyse. This stability of cytoplasmic membrane critical indicates that these organisms are not just acid-tolerant but that high concentrations of protons are actually required for cytoplasmic membrane stability. Some are obligate acidophiles—membranes destroyed at neutral pH.  12. Effects of pH on Microbial Growth Alkaliphiles: organisms that grow best at high pH (≥ 8) found in soda lakes and high-carbonate soils used commercially (e.g., secreted proteases and lipases that are added to laundry detergents)because they excrete hydrolytic enzymes such as proteases and lipases that maintain their activities at alkaline pH. These enzymes are added to laundry detergents to remove protein and fat stains, respectively, from clothing Some have sodium (Na+) motive force rather than proton motive force.  12. Effects of pH on Microbial Growth The optimal pH for growth of an organism refers to the extracellular environment only; The intracellular pH must stay relatively close to neutral (pH 5–9) even if the external pH is highly acidic or basic. Microbial culture media typically contain buffers to maintain constant pH. the intracellular pH must be maintained at a value consistent with the stability of macromolecules, a range of about 4 pH units from pH 5 to 9. Thus, despite conditions in their habitats, extreme acidophiles and alkaliphiles maintain cytoplasmic pH values nearer to neutrality. Table 1. Relationships of Microorganisms to pH  13. Osmolarity and Microbial Growth Water availability is an important factor affecting the growth of microorganisms. Depends on how moist or dry an environment is It is a function of the concentration of solutes (salts, sugars, and other substances) dissolved in water that is present. Solutes bind water, making it less available to organisms. Water availability is expressed in terms of Water activity (aw): varies from zero (no free water) to one (pure water) the more water we have, the more we have growth Table 1. Water activity of Several Substances  13. Osmolarity and Microbial Growth from low {} of solutes to high {} Osmosis: Water diffuses from high to low concentrations. of water Typically, the cytoplasm has a higher solute concentration than the surrounding environment; thus, the tendency is for water to move into the cell (positive water balance). it is the normal state of the cell. When a cell is in an environment with a higher external solute concentration, water will flow out unless the cell has a mechanism to prevent this.  13. Osmolarity and Microbial Growth Halophiles and Related Organisms Halophiles: organisms that grow best at aw = 0.98 (seawater); Although halophiles require at least some have a specific requirement for NaCl (Figure 1) NaCl for growth, the NaCl optimum varies with the organism and is habitat dependent. Halotolerant: organisms that can tolerate some additional dissolved solutes but generally grow best in the absence of the halotolerant organisms can live and grow without salt. They do not require salt (NaCl) to survive or thrive. However, they can tolerate the presence of salt if it is in their environment. In general, they grow best in conditions without added salt, but they have the ability to survive and even grow in environments added solute with moderate salt concentrations. This is different from halophiles, which need salt for their growth. Extreme halophiles: organisms that require very high levels (15 percent to 30 percent) of NaCl; often unable to grow at lower concentrations Figure 1. Effect of NaCl concentration on growth of microorganisms of different salt tolerances or requirements pure water  13. Osmolarity and Microbial Growth Osmophiles: organisms that live in environments high in sugar as solute Xerophiles: organisms able to grow in very dry environments  14. Oxygen and Microbial Growth Aerobes: require oxygen (respiration) and grow at full oxygen tension (air is 21% O2) Obligate aerobe: require oxygen Microaerophiles: can use oxygen only when it is present at levels reduced from that in air due to limited respiration or This is because of the limited capacity of these organisms to respire or because they contain some O2-sensitive molecule such as an oxygen sensitivity O2-labile enzyme. Facultative organisms: O2 is not required, but they grow better with the presence of O2 (is can live with or without O2)  14. Oxygen and Microbial Growth Anaerobes: cannot respire oxygen Two types: Aerotolerant anaerobes: tolerate oxygen and grow normally in its presence even though they cannot respire Obligate anaerobes: inhibited or killed by oxygen, (e.g., some Bacteria and Archaea, few fungi, and few protozoa) Table 1. Oxygen Relationships of Microorganisms  14. Oxygen and Microbial Growth Culture Techniques for Aerobes and Anaerobes Special techniques are needed to grow aerobic and anaerobic microorganisms. This is because the O2 that is consumed by the organisms during growth is not replaced fast enough by diffusion from the air. Aerobes need extensive aeration (e.g., shaking, bubbling). Anaerobes need oxygen excluded. reducing agents: chemicals that may be added to culture media to reduce oxygen (e.g., thioglycolate broth) (Figure 1) complex medium that separates microbes based on oxygen requirements Oxygen can penetrate only the top of the tube. Microbes grow at different heights based on oxygen exposure.  Figure 1. Growth versus O2 concentration Obligate aerobes grow only at the top of such tubes. Facultative organisms grow throughout the tube but grow best near the top. Microaerophiles grow near the top but not right at the top. Anaerobes grow only near the bottom of the tube, where O2 cannot penetrate. aerobic anaerobic facultative microaerophilic aerotolerant anaerobe oxygen decrease gradually  14. Oxygen and Microbial Growth Why is oxygen toxic? Molecular oxygen (O2) is not toxic. Exposure to oxygen yields toxic byproducts. superoxide anion (O2-) hydrogen peroxide (H2O2) hydroxyl radical (OH·)  14. Oxygen and Microbial Growth Enzymes are present to neutralize most of these toxic oxygen species. Catalase and peroxidase convert H2O2 to O2 and H2O. Superoxide dismutase converts 2 O2- to H2O2 and O2. Superoxide reductase in some strict anaerobes converts O2- to H2O2 without producing O2. THE END