English

Bacterial Growth

Middle schoolBiologyLogistic virtual culture after a lag, with seeded dilution-plate counting

Run the culture, then click the orange sampling bulb to plate a virtual dilution. Drag the thermometer to start a new temperature trial.

3D

Drag the apparatus to adjust it. Drag the background to rotate. Scroll to zoom.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Compare temperature, nutrient capacity and elapsed time in a virtual bacterial culture. Take independent virtual dilution plates, count colonies and recover an estimate of the original culture population.

Learning goals

  • Separate the growth rate from the carrying capacity.
  • Explain how dilution changes a plate count but not the original culture.
  • Compare a noisy colony-count estimate with the model population.

How to use it

  1. 1Run the default culture beyond its lag and save a population measurement.
  2. 2Plate a virtual sample, then change dilution without replacing that plate.
  3. 3Plate repeated samples at a fixed time and compare count variability.
  4. 4Compare a nutrient-limited culture with a temperature-limited growth trial.

The model

The culture remains at its initial population during the lag.
A colony count is corrected for dilution and plated volume.

Assumptions

  • The 10 mL virtual culture has a constant effective carrying capacity equal to full-nutrient capacity times nutrient availability.
  • Growth begins only after the selected lag and follows a deterministic logistic curve. The Gaussian temperature response has an illustrative optimum at 37 °C; it is not calibrated to a particular strain.
  • Each viable culture unit is assumed to yield one colony. Expected colonies equal population times plated volume divided by culture volume and the tenfold dilution factor.
  • Counts use exact seeded Poisson sampling for expectations below 50 and a nonnegative rounded normal approximation above that threshold. Sampling does not remove culture material.
  • The usual countable range of 30 to 300 colonies is a teaching guide. Colony overlap above 300 is represented by a confluent display; numerical counts continue.

Try this in class

  1. Q1Why does a more dilute plate give fewer colonies but a similar corrected population estimate?
  2. Q2Why can two plates taken at the same culture time differ?

Frequently asked questions

Does changing dilution alter the existing plate?

No. A plate stores its sampling time, dilution, volume and count. New settings apply only to the next sample.

Are these conditions suitable for culturing real bacteria?

The controls describe a mathematical teaching model, not a practical culture protocol or a measured organism.