Chapter Summary


  • Extremophiles inhabit fringe environments with conditions that do not support human life.
  • The environmental habitat (such as high salt or acidic pH) inhabited by a particular species is defined by the tolerance of that organism’s proteins and other macromolecular structures to the physical conditions within that niche.
  • Global approaches used to study gene expression allow us to view how organisms respond to changes in their environment.


  • Different species exhibit different optimal growth values of temperature, pH, and osmolarity.
  • The Arrhenius equation applies to growth of microorganisms: Growth rate doubles for every 10°C rise in temperature.
  • Membrane fluidity varies with the composition of lipids in a membrane, which in turn dictates the temperature at which an organism can grow.
  • Mesophiles, psychrophiles, and thermophiles are groups of organisms that grow at moderate, low, and high temperatures, respectively.
  • The heat-shock response produces a series of protective proteins in organisms exposed to temperatures near the upper edge of their growth range.


  • Barophiles (piezophiles) can grow at pressures up to 1,000 atm but fail to grow at low pressures.
  • Membrane fluidity can be compromised at high pressures and cold temperatures. Specially designed membranes and protein structures are thought to enable the growth of barophiles.


  • Water activity (aw) is a measure of how much water in a solution is available for a microbe to use.
  • Osmolarity is a measure of the number of solute molecules in a solution and is inversely related to aw.
  • Aquaporins are membrane channel proteins that allow water to move quickly across membranes to equalize internal and external pressures.
  • Compatible solutes are used to minimize pressure differences across the cell membrane.
  • Mechanosensitive channels can leak solutes out of the cell when internal pressure rises.
  • Halophilic organisms grow best at high salt concentration.


  • Hydrogen ion concentration affects protein structure and function. Thus, enzymes have pH optima, minima, and maxima.
  • Microbes use pH homeostasis mechanisms to keep their internal pH near neutral when in acidic or alkaline media.
  • Adding weak acids to certain foods undermines bacterial pH homeostasis mechanisms, thereby preventing food spoilage and killing potential pathogens.
  • Neutralophiles, acidophiles, and alkaliphiles prefer growth under neutral, low, and high pH conditions, respectively.
  • Acid and alkaline stress responses result when a given species is placed under pH conditions that slow its growth. The cell increases the levels of proteins designed to mediate pH homeostasis and protect cell constituents.


  • Oxygen is a benefit to aerobes—organisms that can use it as a terminal electron acceptor to extract energy from nutrients.
  • Oxygen is toxic to all cells that do not have enzymes capable of efficiently destroying the reactive oxygen species—for example, anaerobes.
  • Anaerobic metabolism can be either fermentative or respiratory. Anaerobic respiration requires the organism to possess cytochromes that can use compounds other than oxygen as terminal electron acceptors.
  • Aerotolerant anaerobes grow either in the presence or in the absence of oxygen, but use fermentation as their primary, if not only, means of gathering energy. These microbes also have enzymes that destroy reactive oxygen species, allowing them to grow in oxygen.
  • Facultative anaerobes grow with or without oxygen and have enzymes that destroy reactive oxygen species. In addition, they possess both the ability for fermentative metabolism and the ability to use oxygen as a terminal electron acceptor.


  • Starvation is a stress that can elicit a starvation response in many microbes. Enzymes are produced to increase the efficiency of nutrient gathering and to protect cell macromolecules from damage.
  • The starvation response is usually triggered by the accumulation of small signal molecules such as cyclic AMP or guanosine tetraphosphate.
  • Oligotrophs are organisms that thrive in nutrientpoor conditions.
  • Human activities can cause eutrophication, which damages delicately balanced ecosystems by introducing nutrients that can allow one member of the ecosystem to flourish at the expense of other species.


  • Biocontrol is the use of one microbe to control the growth of another.
  • Probiotics contain certain microbes that, when ingested, aim to restore balance to intestinal flora.
  • Phage therapy offers a possible alternative to antibiotics in the face of rising antibiotic resistance.
  • Disinfection kills pathogens on inanimate objects.