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9.1 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.
  • Cells can be counted by direct viable cell count. The pour plate and spread plate methods are used to plate serial dilutions into or onto, respectively, agar to allow counting of viable cells that give rise to colony-forming units. Membrane filtration is used to count live cells in dilute solutions. The most probable cell number (MPN) method allows estimation of cell numbers in cultures without using solid media.
  • Indirect methods can be used to estimate culture density by measuring turbidity of a culture or live cell density by measuring metabolic activity.
  • Other patterns of cell division include multiple nucleoid formation in cells; asymmetric division, as in budding; and the formation of hyphae and terminal spores.
  • 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.

9.2 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.
  • Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are, respectively, the lowest and the highest oxygen levels that the organism will tolerate.
  • Peroxidase, superoxide dismutase, and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen. All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS.
  • A capnophile is an organism that requires a higher than atmospheric concentration of CO2 to grow.

9.3 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.

9.4 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.

9.5 Other Environmental Conditions that Affect Growth

  • Halophiles require high salt concentration in the medium, whereas halotolerant organisms can grow and multiply in the presence of high salt but do not require it for growth.
  • Halotolerant pathogens are an important source of foodborne illnesses because they contaminate foods preserved in salt.
  • Photosynthetic bacteria depend on visible light for energy.
  • Most bacteria, with few exceptions, require high moisture to grow.

9.6 Media Used for Bacterial Growth

  • Chemically defined media contain only chemically known components.
  • Selective media favor the growth of some microorganisms while inhibiting others.
  • Enriched media contain added essential nutrients a specific organism needs to grow
  • Differential media help distinguish bacteria by the color of the colonies or the change in the medium.
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