A bacterial growth curve graphically tracks the number of living bacteria in a population over time. This chart is generated when bacteria are cultivated in a closed system, or batch culture, where resources are finite. This predictable pattern details the life cycle of bacteria under specific conditions. Understanding this curve is a practice in microbiology, with applications in medicine for managing infections and in food science to prevent spoilage.
The Four Phases of a Bacterial Growth Curve
The bacterial growth curve has four phases that reflect the population’s response to its environment: lag, log (or exponential), stationary, and death. Each phase corresponds to specific cellular activities and changes in population size. The transitions between phases are driven by nutrient availability and the accumulation of waste products in the closed system.
Lag Phase
The lag phase is an initial period of adjustment. When introduced to a new medium, cells do not immediately divide but undergo intense metabolic activity. They synthesize the proteins and enzymes needed to use available nutrients. Bacteria may increase in size, but the population number stays constant, creating a flat line on the curve. The duration of this phase varies based on cell health and the change in environment.
Log (Exponential) Phase
Following the lag phase, bacteria enter the log, or exponential, phase. This stage is marked by rapid cell division, where the population doubles at a constant rate. The growth rate depends on the bacterial species and culture conditions. During this phase, bacteria are most metabolically active and vulnerable to antimicrobial agents that disrupt processes like cell wall synthesis. The logarithmic increase in living cells creates a steep upward slope on the curve.
Stationary Phase
Exponential growth stops as nutrients are depleted and toxic byproducts accumulate. This leads to the stationary phase, where the rate of cell division equals the rate of cell death. This equilibrium creates a plateau on the curve, with the viable cell count remaining constant. Cells reduce their metabolic activity to conserve energy and survive the stressful conditions.
Death (Decline) Phase
The final stage is the death, or decline, phase, where the number of dying cells exceeds the number of new cells. This decline is caused by the exhaustion of nutrients and the toxic level of waste products. The hostile environment leads to an exponential decrease in living bacteria. This phase is represented by a downward slope on the growth curve.
Methods for Measuring Bacterial Growth
Plotting a bacterial growth curve requires quantifying the bacteria in a culture over time. This is done using several direct or indirect methods. The choice of method depends on the experiment’s requirements, such as needing to count only live cells or wanting rapid measurements.
A common direct method is the viable plate count. This technique involves diluting a sample from the culture and spreading it onto a solid agar medium. After incubation, each viable bacterium grows into a visible colony that can be counted. The result, reported as colony-forming units (CFUs) per milliliter, provides a count of only living cells.
An indirect method is spectrophotometry, which measures the turbidity, or cloudiness, of a liquid culture. A spectrophotometer passes a beam of light through the sample and measures how much is scattered or absorbed by the bacteria. Higher bacterial concentration results in greater turbidity and a higher absorbance reading. This method is fast but measures both living and dead cells.
Environmental Factors Affecting the Growth Curve
The shape of the bacterial growth curve is not fixed and can be altered by environmental factors. These variables influence the duration of each phase and the overall growth rate. By manipulating these conditions, scientists can control bacterial growth for various applications.
The specific combination of environmental factors dictates the unique shape of the growth curve for a given bacterial population.
- Temperature: Each species has an optimal temperature for growth. Temperatures below this optimum can extend the lag phase and slow overall growth.
- Nutrients: A rich nutrient medium can shorten the lag phase and create a steeper log phase, as the building blocks for division are readily available.
- pH: Most bacteria have a narrow optimal pH range. Deviations can inhibit enzyme function and damage cellular structures, stunting growth.
- Oxygen: The presence or absence of oxygen determines which species can thrive and influences their growth patterns.