Prokaryotes

What are prokaryotes?

Prokaryotes are surrounded by a plasma membrane, and are unicellular living organisms which lack membrane-bound organelles and a nuclear membrane. All prokaryotes have a cell wall surrounding the plasma membrane that protects the organism from drying out (desiccation) and other challenging environmental conditions. Prokaryotes are similar to eukaryotes because they both contain DNA, RNA, proteins and small molecules in their cytoplasm.

What prokaryotes are found in the environment?

1. Bacteria, found ubiquitously
2. Eubacteria, found in the soil
3. Archeabacteria, found in bogs and hot acid springs

What are three morphologic classifications of bacteria?

The three main morphologic classes of bacteria are $bacilli$ (rod-shaped), $spirilli$ (spiral-shaped) and $cocci$ (spherical-shaped). See Figure 6.1 on page .

Figure 6.1: Three morphologic classes of bacteria.

What are three unique characteristics of bacterial reproduction?

1. Bacterial DNA is circular and is located in the membrane-lacing nucleiod region.
2. Some bacteria contain extragenomic information called plasmids which are smaller, circular genetic sequences. Plasmids help the bacteria survive in adverse environments (e.g. by conferring antibiotic resistance), and replicate independently of the bacterial chromosome.
   • Bacteriophages are viruses that infect only bacteria; they can transmit plasmids or bacterial DNA from one bacteria to another.
3. Bacteria can incorporate DNA fragments from the external environment into their genomes, i.e. transformation.

How do bacteria reproduce?

By binary fission. This simple form of reproduction allows bacteria - given enough food and absence of environmental threats - to replicate quickly and with a particular growth pattern (see Figure 6.2 on page ):

• Lag phase (a): bacteria adapt to a new environment
• Log (Exponential) phase (b): Constant rate of cell doubling results in an exponential population growth
• Stationary phase (c): Balance of bacterial growth and death
• Death phase (d): Accumulation of toxic metabolites or lack of nutrients resulting in bacterial cell death and population decline

Figure 6.2: Bacterial survival curve: (a) lag phase, (b) log (exponential) phase, (c) stationary phase, and (d) death phase.

What four mechanisms do bacteria use to increase genetic variability since binary fission, an asexual process, is used to replicate?

1. Fast reproduction - Bacteria reproduce very quickly and, with natural selection of the ``fittest'' organism, the stronger strain will survive and overcome weaker strains.
2. Transduction - The transfer of bacterial genes following phage-mediated lysis of a host cell:
   1. Generalized transduction occurs when a random piece of DNA is packaged into a phage capsid. After bacterial cell lysis, this phage can transfer the DNA to another bacterium.
   2. Specialized transduction occurs when a phage integrates into a bacterial chromosome. When the lytic cycle takes place, the phage can take nearby bacterial genes with it that can be transferred to a new host when the phage infects a new bacterium.
3. Transformation - Incorporation of genetic information into a bacterium by the bacterial uptake of DNA from the environment.
4. Conjugation - As close to sex as bacteria can have. A ``male" bacteria projects a cytoplasmic extension (a sex pili) towards a recipient ``female" bacteria.

Note: These mechanisms can be used by bacteria to obtain or confer antibiotic resistance. Other adaptive mechanisms to environmental stressors can be transferred this way as well.

What is unique about bacterial ribosomes?

Bacterial ribosomes have $30S$ and $50S$ subunits, are not bound to a membrane organelle and are free floating in the cytosol.

What metabolic pathways do bacteria employ?

Anaerobic (without O$_{2}$) metabolic pathways, i.e. glycolysis or fermentation, are found in the oldest strains of bacteria and are an adaptive remnant of living in the primordial earth's atmosphere which lacked O$_{2}$. With increasing atmospheric concentrations of O$_{2}$ over time, strains capable of harnessing O$_{2}$ (aerobic) evolved.

How do bacteria exist in the environment?

Most bacteria are free-living in the environment outside of other organisms. Still, some bacteria such as $Helicobacter \ pylori$ in the stomach exist as parasites while others live in harmony (symbiotically) with host organisms; e.g. Escherichia coli in the large intestine of humans.


When bacteria from the environment are introduced into new, favorable environments, e.g. the gastrointestinal tract, colonization takes place and the host gets sick.