Chapter 2 — Chapter 2 — Bacteria and Viruses

Bacteria are prokaryotes, 0.5–5 µm. Cell wall of peptidoglycan (murein): chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptide (tetrapeptide) bridges. Gram-positive: thick peptidoglycan (many layers) with teichoic acids, retains crystal violet (purple). Gram-negative: thin peptidoglycan in periplasm plus an outer membrane of lipopolysaccharides (LPS — endotoxin, protective barrier, antigenic) and lipoproteins anchoring outer membrane to peptidoglycan; stains pink. Cell membrane: site of respiration and photosynthesis (no mitochondria). Mesosomes: infoldings of membrane, thought to aid respiration, DNA replication and cell-wall formation during division. Nucleoid: single circular double-stranded DNA. Plasmids: small extra circular DNA carrying genes for antibiotic resistance, toxins, conjugation (F-factor); used as vectors in genetic engineering. 70S ribosomes for protein synthesis. Capsule/slime layer (not in all bacteria): protects from phagocytosis and drying, helps attachment.