Sec 7: Environmental Factors That Influence Microbes Growth

Environmental factors that influence Microbes growth Osmolarity Cells are subject to changes in osmotic pressure, due to the fact that the plasma membrane is permeable to water (a process known as passive diffusion). Water will generally move in the direction necessary to try and balance the solute concentration of the cell with the solute concentration of the surrounding environment. If the concentration of environmental solutes is lower than the concentration of solutes found inside the cells, the environment is said to be hypotonic. In this situation water will enter the cells, causing the cells to swell and increasing If the situation is not corrected then the cell will eventually explode from plasma membrane lysis. Conversely, if the concentration of environmental solutes is higher than the concentration of solutes found inside the cells, the environment is said to be hypertonic. In this situation water will leave the cells, causing Prolonged dehydration will cause permanent damage to the plasma membrane. Cells in hypotonic solutions need to reduce the osmotic concentration of the cytoplasm. Sometimes cells can use inclusions to chemically change the solute, reducing molarity.In a real emergency they can utilize what are known as mechanosensitive (MS) channels, located in their plasma membrane. Cells in hypertonic solutions that need to increase the osmotic concentration of the cytoplasm, cells can take up additional solutes from the environment. However, cells must be careful about what solutes they take up, as some solutes can interfere with cellular function and metabolism. There are some microbes that have evolved extreme hypertonic environments, particularly high salt concentrations, to the point where they require the presence of high levels of sodium chloride to grow. Halophiles, which require NaCl concentrations above 0.2 M, take up potassium and chloride ions as a way to offset the effects of a hypertonic environment in which to live. pH pH is defined as the negative logarithm of ion concentration hydrogen of a solution, expressed in molarity. The pH scale ranges from 0 to 14, with 0 representing a very acidic solution (1.0 M H+) and 14 representing a very basic solution (1.0 x 10-14 M h+). Each pH unit represents a tenfold change in hydrogen ion concentration, meaning a solution with a pH of 3 is 10x more acidic than a solution with a pH of 4. Acidophiles ("acid lovers"), preferring an environmental pH range of 0 various mechanisms to maintain their internal pH within an acceptable the stability of their plasma membranes.These organisms transport potassium ions) into the cells, thereby reducing the movement of H+ into use proton pumps that actively pump H+ out. to 5.5, must use range and maintain cations (such as the cells. They also Alkaliphiles ("alkaline lovers"), preferring an environmental pH range of 8.0 to 11.5, must pump protons, to maintain the pH of their cytoplasm. They usually use antiporters, which pump protons in and sodium ions out. Temperature Microbes have no way to regulate their internal temperature so they have had to develop adaptations for the environment in which they want to live. Changes in temperature have the greatest effect on enzymes and their activity, with optimal temperatures leading to the fastest metabolism and resulting growth rates. Suboptimal temperatures will lead to decreased enzyme activity and slower metabolism, while higher temperatures can actually denature proteins such as enzymes and carrier proteins, leading to cell death. As a result, microbes have a growth curve in relation to temperature with an optimal temperature at which the growth rate reaches its peak, as well as minimum and maximum temperatures at which growth continues but For bacteria the growth range is usually around 30 degrees. Psychrophiles are cold lovers, with an optimum of 15 C or lower and a growth range of -20 C to 20 C.Most of these microbes are found in the ocean, where temperatures are often 5 C or colder. They can also be found in the Arctic and Antarctic, living on ice wherever they can find pockets of liquid water. Adaptation to cold requires the evolution of specific proteins, especially enzymes, that can still function at low temperatures. Humans are most familiar with mesophylls, microbes with a growth optimum of 37 C and a range of 20-45 C. Almost all human microflora falls into this category, as well as almost all human pathogens. Mesophylls occupy the same environments as humans, in terms of the food we eat, the surfaces we touch, and the water we drink and swim in. On the warmer end of the spectrum is where we find thermophiles ("heat lovers"), microbes that prefer high temperatures. Thermophiles usually have a range of 45-80 C, and optimal growth is 60 C. There are also hyperthermophiles, microbes that like extra spicy things. These microbes have growth optima of 88-106 C, a minimum of 65 C and a maximum of 120 C. Both thermophiles and hyperthermophiles require special heat-stable enzymes that are resistant to denaturation and progress, partly due to the presence companion protein. Oxygen Concentration An organism's oxygen needs are related to the type of metabolism it uses. Energy Generation is related to the movement of electrons through the electron transport chain (ETC), where the final electron acceptor can be oxygen or nonoxygen molecules. Organisms that use oxygen as an electron acceptor ultimately engage in aerobic respiration for their metabolism. If they require the presence of atmospheric oxygen (20%) for their metabolism then they are called as obligate aerobes. Microaerophiles require oxygen, but at levels lower than normal atmospheric levels – they only grow at a rate of 2-10%. Organisms that can grow in the absence of oxygen are referred to as anaerobes, with several different categories existing.Facultative anaerobes are the most versatile, able to grow in the presence or absence of oxygen by changing their metabolism to suit their environment.They prefer to grow in the presence of oxygen, as aerobic respiration produces the greatest amount of energy and allows for faster growth. Aerotolerant anaerobes can also grow in the presence or absence of oxygen, showing no preference. Obligate anaerobes can only grow in the absence of oxygen and find oxygenated environments to be toxic. Pressure Most microbes, living on land or water surfaces, are exposed to a pressure of about 1 atmosphere. But there are microbes that live at the bottom of the ocean, where hydrostatic pressure can reach 600-1,000 atm. These microbes are barophils ("pressure lovers"), microbes that have adapted to prefer and even require high pressure. These microbes have increased unsaturated fatty acids in their plasma membranes, as well as shortened fatty acid tails. Radiation All cells are susceptible to damage caused by radiation, which adversely affects DNA with its short wavelengths and high energy. Ionizing radiation, such as X-rays and gamma rays, causes mutation and destruction of a cell's DNA. While bacterial endospores are highly resistant to the harmful effects of ionizing radiation, vegetative cells are considered quite susceptible to its effects. That is, until the discovery of Deinococcus radiodurans, a bacterium capable of completely reassembling its DNA after.