26.1 Why Do Biologists Study Bacteria and Archaea?

• Phyla refers to major lineages

• Microbiology is the study of organisms at can be seen only with the aid of a microscope

• Bacteria or archaea that live in high-salt, high-temperature, low temperature, or high-pressure habitats are extremophiles

• Bacteria that cause disease are said to be pathogens (“disease-producers”).

• Pathogenic bacteria have been responsible for some of the most devastating epidemics in human history.

• The germ theory of disease states that infectious diseases are caused by specific microbes in the body

• Infectious diseases are spread in three main ways:

  • some are passed from person to person

  • some are transmitted by bites from insects or animals

  • some are acquired by ingesting contaminated food or water, or being exposed to microbes in the surrounding environment.

• Endospores are tough, thick-walled, dormant structures formed during times of environmental stress, often in response to a lack of nutrients.

• Antibiotics are molecules that kill bacteria or stop them from growing.

• Biofilms are dense bacterial colonies enmeshed in a polysaccharide-rich matrix that helps shield the bacteria from antibiotics.

• In fact, microbes play an important role in wastewater treatment efforts, and researchers are using them to clean up sites polluted with organic solvents-an effort called bioremediation.

26.2 How Do Biologists Study Bacteria and Archaea?

• One classical technique for isolating new types of bacteria and archaea is called enrichment culture.

• Enrichment cultures are based on establishing a specified set of growing conditions like temperature, lighting, substrate, types of available food, and so on.

• Organisms that only grow when incubated at 45 degrees Celsius to 75 degrees Celsius are called thermophiles.

• Metagenomics is being used to catalog all the genes present in a mixed community of prokaryotes.

• Direct sequencing is a technique based on isolating and sequencing a specific gene from organisms found in a particular habitat

• Tree of life forms the framework for the phylogeny of bacteria and archaea

26.3 What Themes Occur in the Diversification of Bacteria and Archaea?

• Through this process, prokaryotes can acquire diverse traits-such as an biotic resistance- that would otherwise not be available when cells divide by binary fission

• Gene transfer occurs in three ways:

  • Transformation-when bacteria or archaea naturally take up DNA from the environment that has been released by cell lysis or secreted

  • Transduction-when viruses pick up DNA from one prokaryotic cell and transfer it to another cell

  • Conjugation-when genetic information is transferred by direct cell -to cell contact

• Conjugation can also result in genetic recombination.

• Within bacteria, biologists distinguish between two general types of cell wall by using a dyeing system called the Gram stain.

• At the molecular level, most cells that are Gram-positive have a plasma membrane surrounded by a cell wall with extensive peptidoglycan

• Most cells that are Gram-negative, in contrast, have a plasma membrane surrounded by a cell wall at has two components-a thin gelatinous layer containing peptidoglycan and an outer phospholipid bilayer

• Members of the Bacteria and the Archaea are remarkably diverse in their overall size, shape, and motility as well as in the composition of their cell walls and plasma membranes.

• Bacteria and archaea acquire energy to produce ATP in three ways:

  • Phototrophs (“light-feeders”) use light energy to excite electrons.

  • Chemoorganotrophs (“chemical-carbon-feeders”) oxidize organic molecules with high potential energy, such as sugars.

  • Chemolithotrophs (“chemical-rock-feeders”) oxidize inorganic molecules with high potential energy, such as ammonia (NH3) or hydrogen sulfide (H2S).

• Bacteria and archaea fulfill their second nutritional needs-obtaining building-block compounds with carbon-carbon bonds-in two ways:

  • Autotrophs （“self-feeders”） synthesize their own compounds from simple starting materials such as C02 and methane ( CH4).

  • Heterotrophs （“ other-feeders”） absorb ready-to-use organic compounds-called building-block compounds-produced by other organisms in their environment.

• During cellular respiration in both prokaryotes and eukaryotes, enzymes strip electrons from organic molecules that have high potential energy and then transfer these high-energy electrons to the electron carriers NADH and FADH2

• The energy that is released allows components of the ETC to generate a proton gradient across the plasma membrane

• One strategy for making ATP that does not involve electron transport chains is called fermentation

• Instead of using molecules as a source of high-energy electrons, phototrophs pursue a radically different strategy: photophosphorylation

• Cyanobacteria is a lineage of photosynthetic bacteria.

• The steps in the process, referred to as nitrogen fixation, are highly endergonic reduction-oxidation (redox) reactions

• Because of their abundance, ubiquity, and ability to do sophisticated chemistry, prokaryotes have an enormous influence on the global environment.

26.4 Key Lineages of Bacteria and Archaea

• Some species form stalked cells, while others form aggregates of cells organized as spore-forming fruiting bodies.