Binary fission of bacteria

• E.coli genome size: 4.6 x 106 nucleotides

• Generation time at optimal conditions: 20min

• DNA replication rate: 1000nt/s ≈ 1.2 x 106

nt/20min

• About 25% of genome can be replicated in one generation time

Population growth• Growth rate: change of cell number per unit time• When growth is unlimited, growth rate is low at

first, increases logarithmically• Plotted on semi-log paper, growth is a straight line• Generation time: time required for one cell to grow

into two cells (varies from several minutes to months)

• Specific growth: instantaneous change in relative cell numbers (µ)

• µ = ln(2)/generation time

Logarithmic growth

Batch growth stages

Lag and Exponential

• Lag phase– Longer when inoculum conditions differed– Some inocula may require synthesis– Apparent lag caused by partial death of inoculum

• Exponential phase– Logarithm of cell numbers increases linearly– Specific growth rate is constant—characteristic of

growth conditions– Cell physiology is constant—best source for

biochemical studies is late exponential phase

Stationary and Death

• Stationary phase– Culture is eventually limited by substrate or nutrient

exhaustion or product accumulation

– Metabolism changes to accommodate the limitation

– Lysis of some cells may lead to growth in others

– Cell increase equals decrease (cryptic growth)

• Death phase– Death is generally logarithmic, but slower than growth

– Sometimes death rate changes after hours or days

Calculating specific growth rate

Counting Chamber

Counting cells in a chamber

Quantitative plating

Pour plates

Pour-plating techniques

Optical density