10: BACTERIAL GROWTH CURVE

When bacteria are introduced into a nutrient-rich environment, they undergo population growth that follows a characteristic pattern known as the bacterial growth curve. This curve consists of four distinct phases, each representing a different stage in the life cycle of the population: the lag phase, log phase, stationary phase, and death phase.

During the lag phase, cells are metabolically active but not dividing. They are adjusting to their new environment, synthesizing enzymes, and preparing for population growth. There may be little to no increase in cell number during this time, but critical physiological changes are taking place.

Once cells have adjusted, they enter the log phase (also called the exponential phase), where they begin to divide at a constant and rapid rate. This phase is characterized by exponential growth—each cell divides into two, then four, then eight, and so on. Because of this multiplication pattern, the number of cells increases dramatically in a short period. This is also the phase where cells are most uniform in terms of size, shape, and metabolic activity, making it ideal for many microbiological experiments.

Eventually, nutrients become depleted and waste products begin to accumulate, leading to the stationary phase. In this phase, the rate of cell division slows and is approximately equal to the rate of cell death. The total number of viable cells remains relatively constant, but cellular stress and survival mechanisms become more prominent.

Finally, if environmental conditions continue to deteriorate, the culture enters the death phase (also known as the decline phase), during which the number of dying cells exceeds the number of new cells being produced. This decline continues until nutrients are exhausted or toxic conditions prevent further growth.

To observe and record this growth pattern in real time, we will use a spectrophotometer (an instrument that measures the amount of light absorbed by a sample). The spectrophotometer works by passing a beam of light through a liquid culture and measuring how much light is absorbed by the bacterial cells. The more cells present in the medium, the cloudier (or more turbid) it becomes, and the more light is absorbed.

The instrument emits light at a specific wavelength (the distance between peaks in a light wave), and the optical density (OD) (a measure of how much light is absorbed by a liquid sample) of the sample is determined based on how much light is blocked. Bacteria absorb red light efficiently, so in this exercise, we will use red light to assess culture density.

Two types of readings are possible: transmittance (the amount of light that passes through the sample) and absorbance (the amount that is blocked). We will be using the absorbance scale, which is more commonly used in scientific applications. Be sure to read only from the absorbance (OD) scale for consistency. Switching between transmittance and absorbance mid-experiment will produce invalid results.

Absorbance is measured on a logarithmic scale, which means the values increase in multiples rather than in equal steps. This is appropriate because bacterial populations also increase logarithmically during the log phase of growth. Therefore, optical density is directly proportional to the number of cells per milliliter (cells/mL) in the culture. By taking OD measurements at regular time intervals, you will be able to graph the bacterial growth curve and visually identify each phase based on changes in population density.

Knowing the generation time of a bacterial species is critical in microbiology and allied health fields. It helps predict the speed at which infections can spread, informs decisions in antibiotic treatment timing, and plays a central role in industrial and clinical applications involving microbial growth. Understanding these growth dynamics also aids in the design of experiments and in the development of protocols to control or encourage microbial proliferation.

Knowing the generation time is essential when conducting metabolic testing, which refers to laboratory procedures used to identify microorganisms based on the chemical reactions they perform to acquire energy and nutrients. These tests detect specific metabolic activities—such as sugar fermentation, enzyme production, or utilization of particular substrates—that help distinguish one bacterial species from another. To obtain accurate results, it is important that cultures are in the log phase of growth, when cells are most metabolically active. If testing is performed during the lag, stationary, or death phases, the cells may be too inactive or stressed to express key metabolic traits, increasing the risk of inaccurate or false-negative results.

Accurately monitoring bacterial growth and identifying when a culture is in the log phase are critical for ensuring reliable results in both clinical diagnostics and microbiological research.

 9.1: How Microbes Grow is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.