Environmental factors for microbial growth

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▶ENVIRONMENTAL FACTORS

  • What do real estate agents always say? Location, location, location! It’s all about where you live, or at least adapting to where you live. At least it is for microbes.
  • Competition is fierce out in the microbial world (non-microbial world, too!) and resources can be scarce.
  • those microbes that are willing and able to adapt to what might be considered a harsh environment, it can certainly mean less competition.
  • So what environmental conditions can affect microbial growth? Temperature, oxygen, pH, water activity, pressure, radiation, lack of nutrients…these are the primary ones.
  • We will cover more about metabolism (i.e. what type of food can they eat?) later, so let us focus now on the physical characteristics of the environment and the adaptations of microbes.

▶OSMOLARITY

  • Cells are subject to changes in osmotic pressure, due to the fact that the plasma membrane is freely permeable to water (a process known as passive diffusion).
  • Water will generally move in the direction necessary to try and equilibrate the cell’s solute concentration to the solute concentration of the surrounding environment.
  • If the solute concentration of the environment is lower than the solute concentration found inside the cell, the environment is said to be hypotonic.
  • In this situation water will pass into the cell, causing the cell to swell and increasing internal pressure.
  • If the situation is not rectified then the cell will eventually burst from lysis of the plasma membrane.
  • Conversely, if the solute concentration of the environment is higher than the solute concentration found inside the cell, the environment is said to be hypertonic.
  • In this situation water will leave the cell, causing the cell to dehydrate.
  • Extended periods of dehydration will cause permanent damage to the plasma membrane.

▶PH

  • pH is defined as the negative logarithm of the hydrogen ion concentration of a solution, expressed in molarity.
  • The pH scale ranges from 0 to 14, with 0 representing an extremely acidic solution (1.0 M H+) and 14 representing an extremely alkaline solution (1.0 x 10-14 M H+).
  • Each pH units 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.
  • Typically cells would prefer a pH that is similar to their internal environment, with cytoplasm having a pH of 7.2.
  • That means that most microbes are neutrophiles (“neutral lovers”), preferring a pH in the range of 5.5 to 8.0. There are some microbes, however, that have evolved to live in the extreme pH environments.

▶TEMPERATURE

  • Microbes have no way to regulate their internal temperature so they must evolve adaptations for the environment they would like to live in.
  • Changes in temperature have the biggest effect on enzymes and their activity, with an optimal temperature that leads to the fastest metabolism and resulting growth rate.
  • Temperatures below optimal will lead to a decrease in 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 growth rate peaks, as well as minimum and maximum temperatures where growth continues but is not as robust.
  • A bacterium the growth range is typically around 30 degrees.

▶OXYGEN CONCENTRATION

  • The oxygen requirement of an organism relates to the type of metabolism that it is using, Energy generation is tied to the movement of electrons through the electron transport chain (ETC).
  • where the final electron acceptor can be oxygen or a non-oxygen molecule.
  • Organisms that use oxygen as the final electron acceptor are engaging in aerobic respiration for their metabolism.
  • If they require the presence of atmospheric oxygen (20%) for their metabolism then they are referred to as obligate aerobes.
  • Microaerophiles require oxygen, but at a lower level than normal atmospheric levels they only grow at levels of 2-10%.
  • Organisms that can grow in the absence of oxygen are referred to as anaerobes, with several different categories existing.
  • The facultative anaerobes are the most versatile,
  • being able to grow in the presence or absence of oxygen by switching their metabolism to match their environment.
  • Pressure The vast majority of microbes, living on land or water surface, are exposed to a pressure of approximately 1 atmosphere.
  • But there are microbes that live on the bottom of the ocean, where the hydrostatic pressure can reach 600-1,000 atm.
  • These microbes are the barophiles (“pressure lovers”), microbes that have adapted to prefer and even require the high pressures.
  • These microbes have increased unsaturated fatty acids in their plasma membrane, as well as shortened fatty acid tails.

▶RADIATION

  • All cells are susceptible to the damage cause by radiation, which adversely affects DNA with its short wavelength and high energy.
  • Ionizing radiation, such as x-rays and gamma rays, causes mutations and destruction of the cell’s DNA.
  • While bacterial endospores are extremely resistant to the harmful effects of ionizing radiation, vegetative cells were thought to be quite susceptible to its impact.