Chapter Summary

4.1

  • Microorganisms require certain essential macroand micronutrients to grow.
  • Microbial genomes evolve in response to nutrient availability.
  • Obligate intracellular bacteria lose metabolic pathways provided by their hosts and develop requirements for growth factors supplied by their hosts.
  • In the global carbon cycle, autotrophs use CO2 as a carbon source, either through photosynthesis or through lithotrophy.
  • Autotrophs make complex organic compounds that are consumed by heterotrophs.
  • Nitrogen fixers, nitrifiers, and denitrifiers contribute to the nitrogen cycle.
  • Bacteria or archaea carry out all of the carbon, nitrogen, and energy reactions just described, whereas eukaryotes carry out only a limited range of heterotrophic and photosynthetic reactions.

4.2

  • Transport systems move nutrients across semipermeable membranes.
  • Facilitated diffusion helps solutes move across a membrane from a region of high concentration to one of lower concentration.
  • Antiporters and symporters are coupled transport systems in which energy released by moving a driving ion (H+ or Na+) from a region of high concentration to one of low concentration is harnessed and used to move a solute against its concentration gradient.
  • ABC transporters use the energy from ATP hydrolysis to move solutes “uphill,” against their concentration gradients.
  • Siderophores are secreted to bind ferric iron and transport it into the cell, where it is reduced to the more useful ferrous form. Siderophore-iron complexes enter cells with the help of ABC transporters.
  • Group translocation systems chemically modify the solute during transport.

4.3

  • Microbes in nature usually exist in complex, multispecies communities, but for detailed studies they must be grown separately in pure culture.
  • Bacteria can be cultured on solid or liquid media.
  • Minimal defined media contain only those nutrients essential for growth.
  • Complex, or rich, media contain many nutrients. Other media exploit specific differences between organisms and can be defined as selective or differential.

4.4

  • Microorganisms in culture may be counted directly under a microscope, with or without staining, or by use of a fluorescence-activated cell sorter.
  • Microorganisms can be counted indirectly, as in viable counts and measurement of dry weight, protein levels, or optical density.
  • A viable bacterial organism is defined as being capable of replicating and forming a colony on a solid-medium surface.

4.5

  • The growth cycle of organisms grown in liquid batch culture consists of lag phase, log phase, stationary phase, and death phase.
  • The physiology of a bacterial population changes with growth phase.
  • Continuous culture can be used to sustain a population of bacteria at a specified growth rate and cell density.

4.6

  • Biofilms are complex, multicellular, surface-attached microbial communities.
  • Chemical signals enable bacteria to communicate (quorum sensing) and in some cases to form biofilms.
  • Biofilm development involves adherence of cells to a substrate, formation of microcolonies, and, ultimately, formation of complex channeled communities that generate new planktonic cells.

4.7

  • Microbial development involves complex changes in cell forms.
  • Endospore development in Bacillus and Clostridium involves production of dormant, stress-resistant endospores.
  • Heterocyst development enables cyanobacteria to fix nitrogen anaerobically while maintaining oxygenic photosynthesis.
  • Multicellular fruiting bodies in Myxococcus and Dictyostelium and mycelia in actinomycetes develop in response to starvation, dispersing dormant cells to new environments.