Bacterial Growth & Reproduction

Concept of “Bacterial Growth”

• “Growth” refers to an increase in NUMBER, not SIZE of individual cells.
• Bacteria multiply primarily via binary fission → one parent cell divides into two genetically-identical daughter cells (clones).

Binary Fission & Generation Time

• Binary fission cycle length = “generation time” (g).
  • $g = \text{time for one cell to become two}$
• Generation time varies widely among species:
  • Escherichia coli: $g \approx 10\ \text{min}$ (one of the fastest recorded).
  • Mycobacterium tuberculosis: $g \approx 12\ \text{h}$.
• Quantitative example (E. coli):
  • After $t = 8\ \text{h}$ with $g = 10\ \text{min}$ → $n = \frac{t}{g} = \frac{480\ \text{min}}{10\ \text{min}} = 48\ \text{generations}$.
  • Population size formula: $N = N_0 \times 2^{n}$.
  • Starting with $N_0=1$ cell: $N = 2^{48} \approx 2.8\times10^{14}$ (≈ a trillion-trillion) – instructor simplified to “≈1 million” for illustrative purposes, then “in 10 more min that million becomes 2 million.”
• Laboratory relevance: culture incubation times must match species’ generation times (E. coli: hours; M. tuberculosis: weeks).

Environmental & Physical Requirements for Growth

Temperature

• Growth rate is temperature-dependent; each species has minimum, optimum, maximum.
• Three broad groups (based on graph explained):
  • Psychrophiles: Optimum ≈ $15\ ^\circ\mathrm{C}$; can grow near $0\ ^\circ\mathrm{C}$. Unlikely to infect humans (body temp too warm).
  • Mesophiles: Optimum 35$–$40\ ^\circ\mathrm{C} (≈ human body). Most human pathogens fall here.
  • Thermophiles: Optimum ≈ $65\ ^\circ\mathrm{C}$; thrive in hot springs/compost; not medically important for humans.
• Pathogens of humans typically require 36.8$–$37.2\ ^\circ\mathrm{C} (normal core temperature).

Water Availability, Osmotic Pressure & Tonicity

• Bacteria need a moist environment; reproduction ceases when desiccated.
• Best growth in isotonic solutions:
  • Isotonic = external solute concentration equals cytoplasm → no net water movement.
• Osmosis concepts illustrated:
  • Hypertonic external medium (very dilute, “pure” water): water flows INTO cell → swelling & possible lysis.
  • Hyperosmotic/high-salt external medium: water flows OUT → cell shrivels (plasmolysis in bacteria, crenation in RBCs).
• Tonicity is crucial in IV therapy (hypotonic, isotonic, hypertonic fluids).

Oxygen Requirements

• Categories demonstrated via test-tube placement patterns:
  1. Obligate aerobes
  • Require $O_2$ for ATP production.
  • Example: Mycobacterium tuberculosis; colonise top of broth.
  1. Obligate anaerobes
  • $O_2$ is toxic → grow only where absent (bottom of tube).
  • Examples: Clostridium perfringens (gas gangrene), C. tetani (tetanus).
  1. Facultative anaerobes
  • Can switch: fermentation ± aerobic respiration.
  • Grow throughout medium, often denser near top; example: E. coli.
  1. Microaerophiles
  • Require $O_2$ but at lower-than-atmospheric levels; form a thin band slightly below surface.
  1. Aerotolerant anaerobes
  • Indifferent to $O_2$; evenly distributed.
• Practical implication: specimen collection/transport media must match oxygen preference.

Chemical & Nutritional Requirements

• Essential bulk elements: water, carbon, nitrogen, phosphorus, sulfur.
• Trace/“minor” elements: assorted metals & minerals ("host elements").
• Some species additionally need specific growth factors (amino acids, vitamins, etc.).

Free-Living vs. Obligate Intracellular Bacteria

• “Most bacteria are free-living” → can grow on artificial media outside host cells.
• Obligate intracellular pathogens must live inside eukaryotic cells:
  • Genera: Rickettsia, Chlamydia, Mycoplasma.
  • Image described: Chlamydia trachomatis infecting a human cervical cancer cell.

Laboratory Implications & Diagnostic Culture

• Knowledge of generation time, temperature, pH, oxygen class, and tonicity guides:
  • Incubation length (quick vs. slow growers).
  • Choice of appropriate agar/broth.
  • Aerobic vs. anaerobic transport tubes.

Pili, Plasmids & Horizontal Gene Transfer

• Surface appendages:
  • Fimbriae: numerous, short, for adhesion.
  • Pili (sing. pilus): fewer & longer, hollow; connect cells for DNA transfer (“conjugation”).
• Conjugation example (illustrated):
  1. Donor (F⁺) cell contains an F plasmid (extra-chromosomal circular DNA).
  2. Donor extends F pilus → attaches to recipient (F⁻).
  3. Pilus retracts, drawing cells together; single-strand copy of plasmid enters recipient.
  4. Both cells synthesize complementary strand → both now F⁺.
• Clinical relevance:
  • Plasmids often carry antibiotic-resistance genes.
  • Conjugation explains rapid spread of multidrug resistance among pathogens.