Bacterial Growth Curve

The bacterial growth starts with the inoculation of fresh growth media with a small bacterial population.

Graph with y-axis labelled log of numbers of bacteria and x-axis labelled time. First phase is labelled lag phase where line is flat at a low value. Second phase labelled exponential phase is a linear upwards sloping line. Third phase labelled stationary phase is a flat line at the top of the exponential growth phase. Green dotted line continues on the trajectory of the exponential growth and is lablelled theoretical growth. Fourth phase is labelled death phase and shows bacterial number decreasing along a linear line towards zero.

1) Lag Phase

While there is no measurable increase in the bacterial population, the bacteria are expected to be adapting to the new media and resources, with increased metabolic activity, a potential increase in mass and volume and changes in gene regulation. This is the most poorly understood step in the bacterial growth cycle. The duration of this lag phase varies according to the species along with many different factors, such as the composition of the media, temperature, size of the inoculation sample, etc.

2) Exponential Phase

The exponential phase, or log phase, is the actual rapid rise of the bacterial population described above. This is also where the bacteria are the wealthiest, and the ideal moment to extract samples for inoculating other fresh cultures. In the exponential phase, you can calculate the experimental generation time by measuring the number of viable cells at regular intervals, for example using turbidity.

3) Stationary Phase

In a closed system, such as a laboratory flask or petri dish, growth cannot be sustained indefinitely. The bacterial population will run out of resources, space or produce an overwhelming amount of toxic end products from high metabolic activity. Growth slows down to a negligible rate, but this phase is inadequate for a population count as it is difficult to differentiate between viable and dying cells. Bacteria will also start producing antibiotics and spores, which could affect the estimation of viable cells.

4) Death/Decline Phase4 Death or Decline phase

Due to a shortage of resources or overpopulation, bacteria are badly harmed and start dying at an exponential rate, mirroring the lag phase. Importantly, bacterial samples from the death phase are even unable to start a new culture when transferred to fresh growth media.