Chapter 27: Bacteria and Archea Flashcards
Prokaryotic shapes
Cocci- spherical shaped
Bacilli- rod-shaped
Spiral- corkscrew-shaped or can resemble commas or loose coils
Prokaryotic cell walls
Bacterial cell walls are composed of peptidoglycan- a polymer composed of modified sugars cross-linked by short polypeptides
- Consist of interlocking chains of identical peptidoglycan monomers N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with a pentapeptide coming off of the NAM
- Pentapeptide amino acids show some slight variation among the bacteria
- Peptidoglycan monomers are synthesized in the cytosol of the bacterium where they attach to a membrane carrier molecule called bactoprenol that escorts them across the cytoplasmic membrane
- Enzymes insert the monomers into existing peptidoglycan network, enabling bacterial growth following binary fission.
- Found in all bacteria except Mycoplasma
Archaeal cell walls contain a variety of polysaccharides and proteins but lack peptidoglycan
Gram stain
Used to classify bacteria by cell wall composition
Gram-positive bacteria have simpler walls with a larger amount of peptidoglycan
- Teichoic acids are found within the cell wall of gram positive bacteria
Gram-negative bacteria are structurally more complex and have less peptidoglycan with an outer membrane composed of lipopolysaccharides
- Lipopolysaccharides of some gram-negative bacteria can be toxic
- Gram-negative bacteria are more likely to be antibiotic resistant
- Outer membrane helps protect it from the body’s defences
Antibiotics target peptidoglycan and damage bacterial cell walls
Capsule
A mucopolysaccharide coating around cell surface
- Termed a capsule if well-defined or a slime layer if not well organized
- Encapsulated bacteria are usually “mucoid”
Enable prokaryotes to adhere to their substrate or other individuals in colony
Virulence factor that blocks phagocytosis
Blocks antigenic sites on the bacterial cell wall from immune responses
Endospores
Allows bacterial cell to survive in harsh conditions such as a lack of water or essential nutrients
Cell forms a spore by replicating its DNA and sequestering it within a mulltilayered structure that forms the endospore
Aids in organism classification based on the position, size, and shape of the spore.
Metabolically inactive endospores, can remain viable in harsh conditions for centuries
Fimbriae
Short and numerous hairlike appendages that allow some prokaryotes to stick to their substrate or other cells
Pili
Appendages that pull two cells together prior to DNA transfer
Sometimes referred to as sex pili
Prokaryotic motility
In a heterogeneous environment many bacteria exhibit taxis- the ability to move toward or away from a stimulus
- Chemotaxis thange their movement pattern in response to chemicals
Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends
Prokaryotic flagellum
Plasmids
Smaller rings of DNA seperate from primary DNA
Facilitate genetic transfer between organisms
Contain genes for antibiotic resistance
Can increase a strain’s pathogenicity
Prokaryotic genetic diversity
Three factors contribute to prokaryotic genetic diversity:
- Rapid reproduction- short generation times allows prokaryotes to evolve quickly
- Mutation- low frequency but because of rapid reproduction mutations can accumulate quickly in a population
- Genetic recombination- the combination of DNA from two sources
Bacterial growth curve
Reproducible growth pattern that results when microorganisms are grown in a closed culture in which no nutrients are added and most waste is not removed
Composed of four phases:
- Lag phase- period after cells are first added to culture medium during which cells are gearing up for the next phase of growth
- Log phase- cells are actively dividing by binary fision and their numbers increase exponentially
- Stationary phase- waste accumulation and depletion of resources contribute to a slowing growth rate that eventually plateaus; rate of cell division is equal to death rate resulting in stagnant growth
- Death phase- cell death exceeds rate of division due to further accumulation of waste and nutrient depletion; some cells can persist and form endospores
Genetic recombination
The combination of DNA from two sources that contributes to genetic diversity
Occurs via three mechanisms:
- Transformation
- Transduction
- Conjugation
Movement of genes among individuals from different species is called horizontal gene transfer
Transformation
The uptake and incorporation of foreign DNA from the surrounding environment by a cell
Many bacteria have cell-surface proteins that recognize DNA from closely related species and transport them into the cell
Once inside the cell the foreign DNA can be incorporated into the genome by homolgous DNA exchange
Transduction
The movement of genes between bacteria by bacteriophages
Usually occurs as an error during the phage replicative cycle when it incorporates a piece of the bacterial chromosome into its capsid
Conjugation
Pocess in which genetic material is transferred between two prokaryotic cells that are temporarily joined
A donor cell attaches to a recipient via a pilus, pulls it closer, and transfers DNA
A piece of DNA called an F factor (fertiilty factor) is required for the production of pili; may exists as a plasmid or be incorporated in the chromosome
- Cells containing the F plasmid function as DNA donors during conjugation
- Cells with the F factor built into its chromosome are called Hfr cells (High Frequency of Recombination)
R plasmids carry genes for antibiotic resistance
Major nutritional modes
Nitrogen metabolism
Nitrogen is essential for the production of amino acids and nucleic acids
Nitrogen fixation is the process by which some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH3)
Individual cells cannot carry out both photosynthesis and nitrogen fixation
Specialized cells called heterocysts perform nitrogen fixation and exchange metabolic products with neighboring cells that perform photosynthesis
