Cell Architecture

1. The Plasma Membrane

A 4–5 nm phospholipid bilayer of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin. Unlike eukaryotes, bacteria have no sterols (mycoplasmas are an exception, scavenging cholesterol from hosts) and instead use hopanoids for membrane stiffness. Hopanoids preserve as biomarkers in 2.7–1.6 Gyr-old shales — among our earliest direct evidence of bacterial life.

The membrane carries the entire respiratory electron-transport chain (no mitochondria) and the F₁F_O ATP synthase. Proton-motive force (\(\Delta p \approx -180\) mV in E. coli) drives ATP synthesis, flagellar rotation, and active solute transport.

2. The Nucleoid

Bacterial chromosomes are typically a single circular dsDNA molecule, ~4×10⁶bp in E. coli, occupying a poorly demarcated region of cytoplasm called the nucleoid. John Cairns’ 1963 autoradiograph of an E. coli chromosome stretched out at 1.6 mm long — ~1000× the cell length — demonstrated the topological problem the cell must solve every generation.

Compaction is achieved by negative supercoiling (DNA gyrase introduces ~−0.06 superhelical density), nucleoid-associated proteins (HU, H-NS, IHF, Fis), and macrodomain organisation. Replication initiates at oriC, two replisomes proceed bidirectionally at ~1000 bp/s, and termination occurs atter sites with the help of Tus protein.

3. The 70S Ribosome

The bacterial ribosome is a 2.5 MDa ribonucleoprotein machine of two subunits: 50S (5S + 23S rRNA, 33 proteins) and 30S (16S rRNA, 21 proteins). Bacteria carry ~10⁴ ribosomes per cell during fast growth — ~25% of the total dry mass of the cell.

Translation differs from eukaryotes in several ways exploited by antibiotics: a formyl-methionyl initiator tRNA, a Shine-Dalgarno mRNA-rRNA pairing for initiation site selection, and the ability to translate polycistronic mRNAs. See the Ribosome course for the structural and mechanistic detail.

4. Plasmids and the Mobile Genome

In addition to the main chromosome, most bacteria carry one or more plasmids — circular dsDNA elements that replicate independently. Plasmids range from ~1 kb cryptic miniplasmids to >500 kb megaplasmids and carry a remarkable variety of accessory genes:

• Resistance plasmids (R-plasmids): most clinical antibiotic-resistance genes are plasmid-encoded.
• Virulence plasmids: Shigella, Yersinia pestis, Bacillus anthracis all carry their pathogenicity on plasmids.
• Conjugative (F) plasmids: encode the type-IV secretion machinery for cell-to-cell DNA transfer.
• Metabolic plasmids: nitrogen fixation in Rhizobium, hydrocarbon degradation in Pseudomonas.

Horizontal gene transfer via plasmids, transduction (phage-mediated), and natural transformation makes bacterial “evolution” far more reticulate than the tree-of-life metaphor suggests. A pan-genome may dwarf any single isolate’s genome.

5. Surface Appendages

• Capsule — a thick polysaccharide layer external to the cell wall. Major virulence factor in Streptococcus pneumoniae and Haemophilus influenzae; targeted by polysaccharide vaccines.
• Fimbriae (pili) — short, numerous adhesion organelles. Type 1 pili tip-bind mannose-containing receptors on uroepithelial cells (E. coli UTIs).
• Sex pili (F-pili) — longer, fewer, mediate conjugative DNA transfer.
• Flagella — the rotary motor for swimming (Module 4).
• Endospores — in Bacillus and Clostridium, dormant survival structures resistant to heat, UV, desiccation, and most disinfectants. Anthrax spores can survive decades in soil.

6. Compartments? The Bacterial Microcompartment

Bacteria were thought to be entirely lacking in internal compartments. We now know they have bacterial microcompartments (BMCs)— protein-shelled organelles ~100 nm across. The carboxysome in cyanobacteria concentrates CO₂ around RuBisCO; metabolosomes in Salmonella and others sequester toxic intermediates of propanediol and ethanolamine catabolism. They are the bacterial answer to membrane compartmentalisation, built from polyhedral protein cages instead of lipid bilayers.