1. General Composition of All Bacteria
Bacteria are single-celled prokaryotic organisms — meaning they lack a nucleus and membrane-bound organelles — but they are still complex, highly organized chemical systems.
Chemical Composition (average for an E. coli–like bacterium)
By dry weight:
- Proteins~55% — structural proteins, enzymes, ribosomes
- RNA~20% — rRNA, tRNA, mRNA
- DNA~3% — bacterial chromosome and plasmids
- Lipids~9% — phospholipid membranes
- Polysaccharides~5% — peptidoglycan cell wall, glycogen storage
- Other small molecules & ions~8% — metabolites, cofactors, salts
By element:
- Carbon (C)~50% of dry weight
- Oxygen (O)~20%
- Nitrogen (N)~14%
- Hydrogen (H)~8%
- Phosphorus (P)~3%
- Sulfur (S), Potassium (K), Magnesium (Mg), Calcium (Ca), Iron (Fe), trace metals<2% combined
Structural Components
- Cell Envelope:
- Plasma membrane (phospholipid bilayer with embedded proteins)
- Cell wall (peptidoglycan; Gram-positive has thick layer, Gram-negative has thin layer plus outer membrane)
- Some have capsule (polysaccharide coat)
- Cytoplasm:
- Ribosomes (70S type: 50S + 30S subunits)
- Enzymes and metabolites
- Nucleoid (single circular chromosome)
- Plasmids (extra DNA circles)
- Surface Appendages:
- Flagella (for motility)
- Pili/fimbriae (for adhesion, conjugation)
2. Specific Examples from Our Selenocysteine / Pyrrolysine Bacteria
Escherichia coli
- Gram status: Gram-negative
- Shape: Rod (bacillus)
- Special features:
- Outer membrane with lipopolysaccharide (LPS)
- Periplasm between inner and outer membranes
- Can use UGA as Selenocysteine codon in formate dehydrogenase
- Chemical note: High proportion of membrane proteins for nutrient transport
Clostridium spp.
- Gram status: Gram-positive
- Shape: Rod
- Special features:
- Thick peptidoglycan layer, no outer membrane
- Spore-forming
- Anaerobic metabolism
- Uses selenoenzymes in anaerobic respiration
- Chemical note: Endospores have unique coat proteins with high cysteine content
Treponema pallidum
- Gram status: Gram-negative-like (but atypical)
- Shape: Spirochete
- Special features:
- Very thin peptidoglycan layer
- Axial filaments (endoflagella) in periplasm
- Requires host environment — cannot be cultured long-term in vitro
- Uses selenoenzymes in energy metabolism
- Chemical note: High lipid content in outer membrane for immune evasion
Desulfitobacterium hafniense
- Gram status: Gram-positive
- Shape: Rod
- Special features:
- Anaerobic
- Capable of reductive dehalogenation of organohalides
- Uses Pyrrolysine via UAG reassignment in methyltransferases
- Chemical note: Contains unique corrinoid cofactors in enzymes
Methanosarcina spp. (archaea, not bacteria)
- Mentioned for completeness — Pyrrolysine users.
- Composition differs slightly from bacteria: ether lipids instead of ester lipids in membranes.
3. Simplified Table of Bacterial Makeup
| Organism | Gram Type | Shape | Special Feature | Code Reassignment |
|---|---|---|---|---|
| E. coli | Negative | Rod | Outer membrane, LPS, periplasm | UGA → Sec |
| Clostridium spp. | Positive | Rod | Spore-forming, anaerobic | UGA → Sec |
| Treponema pallidum | Negative-like | Spirochete | Axial filaments, high lipid outer layer | UGA → Sec |
| Desulfitobacterium hafniense | Positive | Rod | Dehalogenation enzymes with Pyrrolysine | UAG → Pyl |
Key terms in plain language
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