。 6.Microbial Grow factors on growth 121 ls inferus even seem able to live over 1.5 miles below the nt to use i rth's surface. in d causes a rise in cause of the effec called extrem ntal emphasis will be iven to solutes and water acti H.temp of their response to these factors Solutes and Water Activity The Turbidostat Because a selectively croorganisms from their environment,they can be affected by changes in the os sel is auto ed to maintain a predet ined tur ause it to burs atically from the chem h veryusefulbecause they pro Most bacteria.agac. and fungi have cell walls tha isms with rigid cell walls are placed ina hypertonic envir water leaves and the plasn ne:the cell usually become en microbial TCnlalconditionsre e and e ke in food and industrial microbioloy. their protopla se o 1.How does an open system differ fromaclosed culture systemo compatible with metabolism and growth when a sis or uptake o ctaine,proline,glu e and fune mploy su and polyols for exar Po e,arabitol,glycer 6.4 The Influence of Environmental solutes for this funeton be pt e Factors on Growth and functio A fo As we have seen(pp.114-15),microorganisms must be able to sium ions (se m ions also eles ed but no salt s fo surroundings see section20.3 protozoa do no have a cel wall.the water when ivin in hy ents of microorganisms. tective function of the cell wall (p.61) Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 6. Microbial Growth © The McGraw−Hill Companies, 2002 completely depleted under these balanced conditions. If the dilu￾tion rate rises too high, the microorganisms can actually be washed out of the culture vessel before reproducing because the dilution rate is greater than the maximum growth rate. The limit￾ing nutrient concentration rises at higher dilution rates because fewer microorganisms are present to use it. At very low dilution rates, an increase in D causes a rise in both cell density and the growth rate. This is because of the effect of nutrient concentration on the growth rate, sometimes called the Monod relationship (figure 6.2b). Only a limited supply of nutri￾ent is available at low dilution rates. Much of the available energy must be used for cell maintenance, not for growth and reproduc￾tion. As the dilution rate increases, the amount of nutrients and the resulting cell density rise because energy is available for both maintenance and growth. The growth rate increases when the to￾tal available energy exceeds the maintenance energy. The Turbidostat The second type of continuous culture system, the turbidostat, has a photocell that measures the absorbance or turbidity of the culture in the growth vessel. The flow rate of media through the vessel is automatically regulated to maintain a predetermined tur￾bidity or cell density. The turbidostat differs from the chemostat in several ways. The dilution rate in a turbidostat varies rather than remaining constant, and its culture medium lacks a limiting nutrient. The turbidostat operates best at high dilution rates; the chemostat is most stable and effective at lower dilution rates. Continuous culture systems are very useful because they pro￾vide a constant supply of cells in exponential phase and growing at a known rate. They make possible the study of microbial growth at very low nutrient levels, concentrations close to those present in natural environments. These systems are essential for research in many areas—for example, in studies on interactions between microbial species under environmental conditions re￾sembling those in a freshwater lake or pond. Continuous systems also are used in food and industrial microbiology. 1. How does an open system differ from a closed culture system or batch culture? 2. Describe how the two different kinds of continuous culture systems, the chemostat and turbidostat, operate. 3. What is the dilution rate? What is maintenance energy? 6.4 The Influence of Environmental Factors on Growth As we have seen (pp. 114–15), microorganisms must be able to respond to variations in nutrient levels, and particularly to nutri￾ent limitation. The growth of microorganisms also is greatly af￾fected by the chemical and physical nature of their surroundings. An understanding of environmental influences aids in the control of microbial growth and the study of the ecological distribution of microorganisms. The ability of some microorganisms to adapt to extreme and inhospitable environments is truly remarkable. Procaryotes are present anywhere life can exist. Many habitats in which procary￾otes thrive would kill most other organisms. Procaryotes such as Bacillus infernus even seem able to live over 1.5 miles below the Earth’s surface, without oxygen and at temperatures above 60°C. Microorganisms that grow in such harsh conditions are often called extremophiles. In this section we shall briefly review how some of the most important environmental factors affect microbial growth. Major emphasis will be given to solutes and water activity, pH, temper￾ature, oxygen level, pressure, and radiation. Table 6.3 summa￾rizes the way in which microorganisms are categorized in terms of their response to these factors. Solutes and Water Activity Because a selectively permeable plasma membrane separates mi￾croorganisms from their environment, they can be affected by changes in the osmotic concentration of their surroundings. If a mi￾croorganism is placed in a hypotonic solution (one with a lower os￾motic concentration), water will enter the cell and cause it to burst unless something is done to prevent the influx. The osmotic con￾centration of the cytoplasm can be reduced by use of inclusion bod￾ies (see pp. 49–52). Procaryotes also can contain pressure-sensitive channels that open to allow solute escape when the osmolarity of the environment becomes much lower than that of the cytoplasm. Most bacteria, algae, and fungi have rigid cell walls that maintain the shape and integrity of the cell. When microorgan￾isms with rigid cell walls are placed in a hypertonic environment, water leaves and the plasma membrane shrinks away from the wall, a process known as plasmolysis. This dehydrates the cell and may damage the plasma membrane; the cell usually becomes metabolically inactive and ceases to grow. Many microorganisms keep the osmotic concentration of their protoplasm somewhat above that of the habitat by the use of compatible solutes, so that the plasma membrane is always pressed firmly against their cell wall. Compatible solutes are solutes that are compatible with metabolism and growth when at high intracellular concentrations. Most procaryotes increase their internal osmotic concentration in a hypertonic environment through the synthesis or uptake of choline, betaine, proline, glu￾tamic acid, and other amino acids; elevated levels of potassium ions are also involved to some extent. Algae and fungi employ su￾crose and polyols—for example, arabitol, glycerol, and manni￾tol—for the same purpose. Polyols and amino acids are ideal solutes for this function because they normally do not disrupt en￾zyme structure and function. A few procaryotes like Halobac￾terium salinarium raise their osmotic concentration with potas￾sium ions (sodium ions are also elevated but not as much as potassium). Halobacterium’s enzymes have been altered so that they actually require high salt concentrations for normal activity (see section 20.3). Since protozoa do not have a cell wall, they must use contractile vacuoles (see figure 27.3) to eliminate excess water when living in hypotonic environments. Osmosis and the pro￾tective function of the cell wall (p. 61) 6.4 The Influence of Environmental Factors on Growth 121