Bacterial Growth Curve and Kinetics
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Direct answer
Bacterial growth in a closed batch culture follows four phases: lag, logarithmic, stationary and death. During the logarithmic phase the population doubles at a fixed rate called the generation time, which is about 20 minutes for Escherichia coli in rich broth. Beta-lactams and fluoroquinolones act best during this phase, when cell wall and DNA synthesis are active, while stationary-phase persisters tolerate the same drugs.
What you must remember
- Lag phase: little net increase in numbers, as the cells synthesise enzymes and ribosomes to adapt to the new medium.
- Logarithmic phase: exponential growth at a constant generation time, when the population is most uniform in its metabolism.
- Generation time: the time taken for the population to double, calculated from the number of doublings.
- Stationary phase: growth rate equals death rate because nutrients are exhausted, and sporulation and secondary metabolites often appear.
- Death phase: viable counts fall exponentially as nutrients run out and toxic products accumulate.
- Measurement: turbidity or optical density counts total cells, while plate counts in colony-forming units count only viable cells.
- Antibiotic phase dependence: beta-lactams act best on growing cells, so they are weaker against slow-growing or stationary bacteria.
- Persisters: a small dormant subpopulation survives killing, which can explain relapse after apparently adequate therapy.
A worked example of generation time
A worked example shows how viable counts convert into a generation time, a calculation that appears often in examinations. The steps below use a culture that grows during a log phase lasting three hours.
- Starting count: the culture begins with 1,000 colony-forming units per millilitre.
- Final count: after 3 hours the count reaches 8,000 colony-forming units per millilitre.
- Number of generations: 8,000 is 1,000 multiplied by 2 to the power 3, so the population has doubled three times.
- Generation time: 180 minutes divided by 3 doublings gives a generation time of 60 minutes.
- Clinical link: a beta-lactam is most effective while the culture is in this log phase, so dosing that keeps drug levels above the minimum inhibitory concentration matters during growth.
Growth is measured by counting viable cells on agar at timed intervals, and the counts are plotted on a logarithmic scale so that the log phase appears as a straight line.
Where the exam tries to catch you
Growth curve questions usually test which phase a given statement describes, or the phase in which an antibiotic works best.
- Trap: taking the lag phase as the period of fastest growth Correct: lag shows no net increase, and the log phase is the period of fastest exponential growth.
- Trap: expecting a stationary-phase culture to be cleared quickly by beta-lactams Correct: non-growing cells are less affected, so killing is slower and often incomplete.
- Trap: equating turbidity with viable count Correct: optical density counts dead cells too, so plate counts are needed to measure viability.
Frequently asked questions
What is the generation time of Escherichia coli in rich broth?
The generation time of Escherichia coli in rich broth at optimal temperature is about 20 minutes. Generation time varies between species and with growth conditions, so it is measured during the logarithmic phase.
Why are beta-lactam antibiotics less effective in the stationary phase of bacterial growth?
Beta-lactam antibiotics are less effective in the stationary phase because they act on the peptidoglycan synthesis of growing cells. Non-dividing bacteria make little new cell wall, so the killing by beta-lactams is slower and often incomplete.
What does the lag phase of bacterial growth represent?
The lag phase of bacterial growth represents a period of adaptation, in which cells synthesise the enzymes, ribosomes and other components needed for the new medium. There is little net increase in cell numbers during this phase.
How is generation time calculated from a bacterial growth curve?
Generation time is calculated by dividing the time spent in the logarithmic phase by the number of generations. The number of generations is found from the logarithm of the final count minus the logarithm of the starting count, divided by the logarithm of two.
What are persister cells in a bacterial population?
Persister cells are a small, dormant subpopulation of bacteria that survive antibiotic exposure without being genetically resistant. They help explain why some infections relapse after an apparently adequate course of treatment.