Archaea Flashcards
(17 cards)
3 BROAD GROUPS OF ARCHAEA:
- Methanogens (produce methane)
- Halophiles (salt loving)
- Thermophiles (heat loving)
OPTIMUM TEMPERATURE FOR GROWTH:
Mesophilic microbes: 20 – 42°C
Thermotolerant microbes: up to 50°C
Thermophilic microbes: 40 – 70°C
Extremely thermophilic microbes: 65°C
ARCHAEA
Archaea share many properties that distinguish them from eubacteria, but within the archaea, there are differences in CELL MORPHOLOGY, CELL COMPONENTS and METABOLISM.
Archaeal Cell Envelopes
Differ from bacterial envelopes in the molecular makeup and organization:
– S-layer may be only component outside plasma membrane.
– Some archaea lack a cell wall.
– Capsules and slime layers are rare.
Cell wall chemistry:
No peptidoglycan (i.e. no muramic acid and no Damino acids).
Therefore NOT susceptible to lysozyme, beta-lactam antibiotics or glycopeptide antibiotics
ARCHAEAL MEMBRANES
Have distinctive membrane lipids (different from BOTH bacteria and eukaryotes):
Branched-chain hydrocarbons (alkyl isoprenoids) bonded to glycerol by ETHER bonds, not fatty acids linked by ester bonds.
Also contain diglycerol tetraethers (not known in bacteria or eukaryotes).
May also have polar lipids: phospholipids,
sulpholipids and glycolipids (these also occur in bacteria).
Can mix diethers, tetraethers and other lipids to give membranes of differing rigidity and thickness.
ARCHAEAL MEMBRANES 2
Composed of unique lipids:
– isoprene units (five carbon, branched).
– ether linkages rather than ester linkages to glycerol.
* Some have a monolayer structure instead of a bilayer structure (because of diglycerol
tetraethers).
Archaeal Cell Walls Differ from Bacterial Cell Walls
- Lack peptidoglycan.
- Most common cell wall is an S-layer.
- May have protein sheath external to S layer.
- S-layer may be outside membrane and separated by pseudomurein.
- Pseudomurein may be outermost layer (similar to Gram-positive bacteria)
ARCHAEAL PSEUDOMUREIN SUBUNIT COMPOSITION
(i) L-amino acids rather than Damino acids.
(ii) β(1→3) glycosidic bonds (not β(1→4) glycosidic bonds).
ARCHAEAL MOLECULAR BIOLOGY
The studied archaeal chromosomes are similar to eubacteria: a single, closed DNA circle per cell.
Archaeal genomes generally smaller than in
bacteria.
Mol% G+C: 21 - 68%.
Few plasmids found in archaea, archaeal mRNA similar to bacterial
mRNA: polycistronic mRNAs are produced and there is NO evidence for mRNA splicing.
RIBOSOMES ARE 70S (as in bacteria), but shape is variable and can differ from both bacteria and eukaryotes.
Protein synthesis SENSITIVE TO ANISOMYCIN, resistant to chloramphenicol and kanamycin (i.e. like
eukaryotes).
Archaeal small-subunit rRNA is 16S (not 18S) therefore similar in length to bacterial small-subunit rRNA molecules.
The big surprise was that 16S-rRNA gene sequencing and phylogenetic analysis showed archaea more closely related to eukaryotes than to bacteria.
ARCHAEAL MOLECULAR BIOLOGY 2
The process of ATP generation is believed to involve the formation of a proton gradient across the cell membrane and an ATP synthase (the same as bacteria).
Protoplast and Cytoplasm
- Protoplast is plasma membrane and everything within it.
- Cytoplasm: material bounded by the plasma membrane
The Cytoskeleton
- Homologues of all 3 eukaryotic cytoskeletal elements have been identified in bacteria and 2 in archaea.
- Functions are similar as in eukaryotes (i.e.
roles in cell division, protein localisation, and determination of cell shape).
The Nucleoid
- Irregularly shaped region in bacteria and archaea.
- Not membrane bound (there are a few exceptions).
- Location of chromosome and associated proteins.
- Usually 1 chromosome (closed circular, double-stranded DNA molecule).
- Supercoiling and nucleoid proteins (HU) probably aid in folding (nucleoid proteins differ from histones).
Plasmids
- Extrachromosomal DNA (i.e. not part of the chromosome).
– Found in bacteria, archaea, some fungi.
– Usually small, closed circular DNA molecules. - Exist and replicate independently of chromosome.
– Episomes may integrate into chromosome. - Plasmids contain only a few genes and these are non-essential.
– Confer selective advantage to host (e.g. drug resistance).
Plasmids (continued)
May exist in many copies in cell.
* Inherited stably during cell division.
* Curing is the loss of a plasmid.
* Classification of plasmids based on mode of existence, spread, and function.
External Structures
Extend beyond the cell envelope in bacteria and archaea.
* Functions: protection, attachment to surfaces, horizontal gene transfer, cell movement.
* Pili and fimbriae (attachment; horizontal gene transfer).
* Flagella (cell movement)
