- If the growth of microorganisms in a fermenter is measured over time, the population growth looks like the graph below
A typical growth curve for a population in an enclosed environment
- The shape of this curve ( a little like an ‘S’), gives it its name - a sigmoid growth curve
- The curve has four distinct phases:
- Lag phase - organisms are adapting to the environment before they are able to reproduce; in addition, at this stage there are very few organisms and so reproduction is not producing larger numbers of offspring
- Log phase (aka exponential phase) - food supply is abundant, birth rate is rapid and death rate is low; growth is exponential and only limited by the number of new individuals that can be produced
- Stationary phase - population levels out due to a factor in the environment, such as a nutrient, becoming limited as it is not being replenished; birth rate and death rate are equal and will remain so until either the nutrient is replenished or becomes severely limited
- Death phase - population decreases as death rate is now greater than birth rate; this is usually because food supply is short or metabolic wastes produced by the population have built up to toxic levels
- Organisms in a natural environment are unlikely to show population growth like a sigmoid growth curve because they are affected by many other factors, including:
- changing temperature or light
- predators
- disease
- immigration (individuals moving into the area)
- emigration (individuals moving out of the area)