Prokaryotes, viruses and infectious disease

5.1 · Free notes

Prokaryotic cells and binary fission

Key points

• Prokaryotes (bacteria) have no membrane-bound nucleus and no membrane-bound organelles (no mitochondria, ER or Golgi). They are typically 0.5–5 µm long, much smaller than eukaryotic cells (10–100 µm).
• The genetic material is a single circular DNA molecule lying free in the cytoplasm in a region called the nucleoid. It is not associated with histone proteins, unlike the linear DNA of eukaryotes.
• Many bacteria also contain plasmids: small, circular, double-stranded DNA loops that often carry antibiotic resistance genes. They can be passed to other bacteria and are not essential for basic survival.
• Bacterial ribosomes are 70S (smaller). Eukaryotic cytoplasmic ribosomes are 80S. Both carry out translation.
• The cell wall is made of peptidoglycan (murein), a polymer of sugars cross-linked by short peptides. It supports the cell and prevents osmotic lysis. Plant cell walls are cellulose and fungal walls are chitin.
• Optional structures are a capsule (slime layer that protects against phagocytosis and drying out), flagella (motility) and pili (attachment and conjugation). All of these lie outside the cell surface membrane.
• Gram staining: Gram-positive bacteria have a thick peptidoglycan wall and stain purple. Gram-negative bacteria have a thin wall plus an outer membrane and stain pink/red.
• Binary fission: the circular DNA and plasmids replicate, the cell elongates and the DNA copies move apart. The membrane pinches in and a new cell wall (septum) forms. This gives two genetically identical daughter cells. No spindle forms, so it is not mitosis.
• Under ideal conditions growth is exponential: N = N₀ × 2ⁿ, where n = number of divisions = total time ÷ generation time. A growth curve has lag, log (exponential), stationary and death phases.
• Aseptic technique prevents contamination and growth of human pathogens. Flame loops, work near a Bunsen flame, use sterile equipment, tape lids and incubate at ≤25 °C in schools. Grow cultures on agar or in nutrient broth.
• Required practical (antimicrobial agents): measure the zone of inhibition around discs on a bacterial lawn. Area = πr², and a larger clear zone means a more effective agent. Include a control disc (sterile water).
• Viable counts use serial dilution and colony counting: cells per cm³ = colonies ÷ volume plated (cm³) × dilution factor.

In more detail

• Gram-negative walls: a thin peptidoglycan layer with an outer lipid membrane, so they stain pink/red with the counterstain and are often less affected by penicillin.
• Worked growth: a generation time of 20 minutes for 3 hours gives n = 9 divisions; starting from 50 cells, N = 50 × 2⁹ = 25 600.
• Growth curve phases: lag (adjusting, making enzymes), log/exponential (fastest division), stationary (births = deaths as nutrients run out), death (deaths exceed births).
• Worked viable count: 42 colonies from 0.1 cm³ of a 10⁻⁴ dilution → 42 ÷ 0.1 × 10⁴ = 4.2 × 10⁶ cells per cm³.