Summary

7.1 How Microbes Grow

  • Most bacterial cells divide by binary fission. Generation time in bacterial growth is defined as the doubling time of the population.
  • Cells in a closed system follow a pattern of growth with four phases: lag, logarithmic (exponential), stationary, and death.
  • Biofilms are communities of microorganisms enmeshed in a matrix of extracellular polymeric substance. The formation of a biofilm occurs when planktonic cells attach to a substrate and become sessile. Cells in biofilms coordinate their activity by communicating through quorum sensing.
  • Biofilms are commonly found on surfaces in nature and in the human body, where they may be beneficial or cause severe infections. Pathogens associated with biofilms are often more resistant to antibiotics and disinfectants.

7.2 Oxygen Requirements for Microbial Growth

  • Aerobic and anaerobic environments can be found in diverse niches throughout nature, including different sites within and on the human body.
  • Microorganisms vary in their requirements for molecular oxygen. Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen.
  • Obligate anaerobes cannot grow in the presence of oxygen. They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen.
  • Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.
  • Although aerotolerant anaerobes do not perform aerobic respiration, they can grow in the presence of oxygen. Most aerotolerant anaerobes test negative for the enzyme catalase.
  • Microaerophiles need oxygen to grow, albeit at a lower concentration than 21% oxygen in air.

7.3 The Effects of pH on Microbial Growth

  • Bacteria are generally neutrophiles. They grow best at neutral pH close to 7.0.
  • Acidophiles grow optimally at a pH near 3.0. Alkaliphiles are organisms that grow optimally between a pH of 8 and 10.5. Extreme acidophiles and alkaliphiles grow slowly or not at all near neutral pH.
  • Microorganisms grow best at their optimum growth pH. Growth occurs slowly or not at all below the minimum growth pH and above the maximum growth pH.

7.4 Temperature and Microbial Growth

  • Microorganisms thrive at a wide range of temperatures; they have colonized different natural environments and have adapted to extreme temperatures. Both extreme cold and hot temperatures require evolutionary adjustments to macromolecules and biological processes.
  • Psychrophiles grow best in the temperature range of 0–15 °C whereas psychrotrophs thrive between 4°C and 25 °C.
  • Mesophiles grow best at moderate temperatures in the range of 20 °C to about 45 °C. Pathogens are usually mesophiles.
  • Thermophiles and hyperthemophiles are adapted to life at temperatures above 50 °C.
  • Adaptations to cold and hot temperatures require changes in the composition of membrane lipids and proteins.

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