The growth curve describes what happens to a bacterial population in a batch culture: a closed container with a fixed amount of nutrient medium, inoculated with a starting population and left to grow. This is the situation in most laboratory experiments, in a flask of broth on a bench, in a blood culture bottle, in a fermentation vessel at the start of a production run before any continuous feeding begins. The pattern that emerges is always the same four phases, though their relative duration changes dramatically depending on the organism, the growth medium, the temperature, and a dozen other variables.
This page explains each phase from the organism's perspective, describes how growth is measured and what each measurement actually tells you, walks through the maths of doubling time and generation time with worked examples, explains how the growth curve changes under different environmental conditions, and connects everything to practical applications in food microbiology, pharmaceutical manufacturing, and research.
The Four Phases of Bacterial Growth
The Lag Phase: Getting Ready to Grow
When bacteria from a culture or an environmental source are transferred into fresh growth medium, they do not immediately start dividing. There is a period of adjustment, the lag phase, during which the population size stays roughly constant but the cells are anything but idle.
During lag phase, bacteria are synthesising the enzymes, transporters, and structural components they need to exploit the new environment. If they have been starved, they need to rebuild their ribosomes and restart protein synthesis machinery. If they have been grown at a different temperature and are now at a new temperature, they need to adjust their membrane lipid composition to maintain the right fluidity. If the new medium contains different carbon sources than what they were previously growing on, they need to synthesise the appropriate catabolic enzymes.
The length of the lag phase is determined by the condition of the inoculating cells and the difference between their previous and current environment. Transferring actively growing log-phase cells into identical fresh medium produces a very short lag phase. Transferring cells from a stationary-phase culture, or from a previously frozen stock, into a new medium with a different carbon source at a different temperature can produce a lag phase lasting many hours.
In practical terms, the lag phase is why you cannot inoculate a pathogen into a food product and expect immediate exponential growth. The food safety window immediately after contamination is real, and food safety systems try to exploit it.
The Log Phase: Exponential Growth
Once the cells have adapted to their environment, they start dividing at the maximum rate that the available nutrients and the environmental conditions support. This is the log phase, also called the exponential phase.
In log phase, every cell in the population is dividing once every generation time, which means the population doubles with each generation. Starting from a single cell with a 20-minute generation time, after 1 hour there are 8 cells, after 2 hours there are 64, after 3 hours there are 512, after 10 hours there are over 1 billion. This exponential relationship is why bacterial contamination of food, water, or a patient can escalate so rapidly from a small initial exposure to a large infective dose.
The generation time (also called the doubling time) varies enormously between species and conditions. Escherichia coli in optimal conditions (37 degrees Celsius, rich LB broth, aerobic) divides every 17 to 20 minutes. Mycobacterium tuberculosis divides every 12 to 24 hours, which is why tuberculosis progresses slowly and requires months of antibiotic treatment. Lactobacillus acidophilus in yogurt fermentation divides every 45 to 120 minutes depending on temperature. Treponema pallidum, the syphilis spirochete, divides approximately every 30 hours, which partly explains why a single large dose of penicillin can cure early syphilis: the slow generation time means few new daughter cells are born during the window of penicillin action.
The Stationary Phase: When Growth Meets its Limits
Exponential growth cannot continue indefinitely in a batch culture. As the population grows and cells consume nutrients, the nutrients that were in excess at the start become limiting. Simultaneously, the population produces metabolic waste products that accumulate in the medium. Acids produced by fermentation lower the pH. Oxygen is depleted for aerobic organisms. Carbon and nitrogen sources fall to concentrations that can no longer support the maximum growth rate.
When the rate of new cell production exactly equals the rate of cell death, the viable cell count plateaus. This is the stationary phase. The total number of cells measured by optical density (which counts live and dead cells) may continue to rise slightly as dead cells contribute to turbidity, but the viable CFU count remains constant.
Stationary phase is not just a waiting room. Bacteria in stationary phase undergo major changes in gene expression, activating stress response regulators like RpoS (the stationary phase sigma factor in E. coli) that increase stress resistance, induce production of secondary metabolites, and in some species, initiate sporulation. Many industrially important products, including antibiotics, pigments, and exopolysaccharides, are produced primarily in stationary phase. Biofilm formation is strongly induced in stationary phase. These stationary-phase cells are also more tolerant of antibiotics than rapidly dividing log-phase cells, which has implications for antibiotic treatment of chronic and persistent infections.
The Death Phase: When the Culture Collapses
Eventually, nutrients are exhausted, waste accumulates to toxic concentrations, and pH or redox potential shifts beyond what the cells can tolerate. The rate of cell death exceeds the rate of new cell production and the viable count begins to fall. In most organisms, this decline is also roughly exponential, though often slower than the rate of growth during log phase.
Some cells survive for extended periods in death phase through sporulation, persister cell formation, or dormancy mechanisms. Spore-forming organisms like Bacillus and Clostridium convert to metabolically inert endospores that can survive death phase conditions indefinitely and germinate when conditions improve. Persister cells are a small fraction of non-spore-forming bacteria that enter a dormant, non-replicating state tolerant to antibiotics, thought to be responsible for relapse of infections after antibiotic treatment is stopped.
How to Measure Bacterial Growth
Optical density (OD600): A spectrophotometer measures how much light at 600 nanometres wavelength is absorbed by a bacterial suspension. As cell density increases, more light is scattered and absorbed, so OD increases. OD600 is fast, non-destructive, and easily automated, making it the standard method for constructing growth curves in research. It measures all particles including dead cells. At high cell densities the relationship between OD and cell number becomes non-linear. Within the linear range (OD600 of 0.1 to 0.8 for most species), there is a consistent relationship between OD and CFU/mL: for E. coli, OD600 of 1.0 corresponds approximately to 8 x 10^8 CFU/mL, though this differs between species and instruments.
Colony forming unit (CFU) counts: Serial dilutions of the culture are plated onto agar and colonies are counted after incubation. Each colony represents one viable cell (or a clump of cells) from the original culture. CFU counting is laborious, takes 24 to 48 hours, and gives a result that is behind the real-time state of the culture. But it counts only living cells and gives the most reliable absolute viable count.
Flow cytometry: Counts and characterises individual cells based on light scattering and fluorescence. Using live/dead staining dyes, it can distinguish viable from dead cells in mixed populations and measure culture dynamics in real time. Essential in research settings where more detailed population information is needed.
Calculating Doubling Time and Generation Time
The doubling time (g) can be calculated from two OD or CFU measurements taken at known time intervals during log phase:
g = t / (log2(N2/N1))
Where t is the time elapsed in minutes between the two measurements, N1 is the cell count or OD reading at the first time point, and N2 is the count or OD at the second time point. The specific growth rate (µ) is related to doubling time by: µ = ln(2) / g = 0.693 / g. The units of µ are per hour or per minute.
For example: OD600 at t = 60 min is 0.15. OD600 at t = 90 min is 0.42. t = 30 minutes. N2/N1 = 0.42/0.15 = 2.8. log2(2.8) = 1.485. g = 30 / 1.485 = 20.2 minutes. This is consistent with E. coli growing at 37 degrees Celsius in rich medium.
What Changes the Growth Rate
Temperature is the single biggest determinant of bacterial growth rate in most practical settings. Every organism has a minimum temperature below which it cannot grow, an optimum temperature at which it grows fastest, and a maximum temperature above which it cannot survive. For human pathogens, the optimum is usually close to 37 degrees Celsius (body temperature). Refrigeration at 4 degrees Celsius does not kill most non-psychrophilic bacteria but dramatically slows growth rate, which is why food stays safe in a refrigerator but not at room temperature.
pH affects enzyme function and membrane integrity. Most pathogens grow best near neutral pH. Acid preservation (pickling, lactic acid fermentation) works by lowering pH below the growth range of pathogens.
Water activity (aw) measures how much water is available for microbial growth. Pure water has aw of 1.0. Most bacteria require aw above 0.90 to grow. Reducing aw through drying, adding salt, or adding sugar is one of the oldest food preservation strategies.
Oxygen availability sorts bacteria into aerobes, anaerobes, facultative anaerobes, and microaerophiles. Providing the correct oxygen level is critical to getting the expected growth curve in laboratory experiments.
Frequently Asked Questions
What are the four phases of bacterial growth?
The four phases are: lag phase (adaptation without growth), log or exponential phase (rapid doubling), stationary phase (growth equals death), and death phase (population declines). The duration of each phase depends on the organism, growth medium, temperature, and other environmental conditions.
What is generation time?
Generation time, also called doubling time, is the time it takes for a bacterial population to double in number during log phase growth. E. coli has a generation time of about 20 minutes at 37 degrees Celsius in rich medium. Mycobacterium tuberculosis has a generation time of 12 to 24 hours, which is why TB infections progress slowly.
How do you calculate bacterial doubling time?
Using two cell count or OD600 measurements taken during log phase: g = t / log2(N2/N1), where t is the elapsed time, N2 is the later measurement, and N1 is the earlier measurement. This gives the doubling time in the same units as t.
What is OD600?
OD600 is the optical density of a bacterial culture measured at 600 nanometres wavelength using a spectrophotometer. It correlates with cell density because bacteria scatter and absorb light. Higher cell density means higher OD600. It is used to monitor bacterial growth in real time. For E. coli, OD600 of 1.0 corresponds roughly to 8 x 10^8 CFU/mL.
Why is the stationary phase important?
In stationary phase, bacteria respond to nutrient limitation and stress by dramatically changing their gene expression. Many secondary metabolites including antibiotics, enzymes, and pigments are produced in stationary phase. Bacteria in stationary phase are more stress-tolerant and antibiotic-tolerant than log-phase cells, which matters for treating persistent and chronic infections.
What is a chemostat?
A chemostat is a type of continuous culture in which fresh medium is added at a constant rate while spent medium and cells are removed at the same rate. This maintains the culture at a fixed volume and, depending on the dilution rate, a fixed growth rate and cell density. It is used in research to study bacterial physiology at defined, steady growth rates, and in industrial fermentation for continuous production.
What is a diauxic growth curve?
Diauxie occurs when bacteria are grown in a medium containing two different carbon sources. The bacteria consume the preferred carbon source (usually glucose) first, growing exponentially. When glucose is exhausted, growth briefly pauses (a second lag phase) while the cells induce the enzymes needed to metabolise the second carbon source. Then growth resumes at a rate appropriate for the second carbon source. Jacques Monod first described this phenomenon with E. coli using glucose and lactose, which led to the discovery of the lac operon.
What does log phase mean for antibiotic treatment?
Antibiotics are generally most effective against bacteria in log phase because most antibiotic targets (cell wall synthesis, DNA replication, ribosomal activity) are most active in rapidly growing cells. Bacteria in stationary phase, growing slowly or not at all, are less susceptible to many antibiotics. This underlies the problem of persistent infections and is why chronic infections are harder to treat than acute ones.
How does temperature affect bacterial growth?
Temperature affects the rate of all enzymatic reactions inside the cell and therefore directly determines the maximum growth rate achievable. Each species has a cardinal temperature range: minimum (below which it cannot grow), optimum (fastest growth), and maximum (above which growth is inhibited or the organism is killed). Psychrophiles grow at low temperatures (0 to 15 degrees Celsius), mesophiles at moderate temperatures (20 to 45 degrees Celsius, includes most human pathogens), thermophiles at high temperatures (45 to 80 degrees Celsius), and hyperthermophiles above 80 degrees Celsius.
What is the countable range for colony counting?
The countable range for a standard spread plate is 30 to 300 colonies. Plates with fewer than 30 colonies are considered too few to count (TFTC) because statistical confidence at very low numbers is poor. Plates with more than 300 colonies are considered too many to count (TMTC) because colonies merge and individual counts become unreliable. Serial dilutions are performed to bring the expected colony count within this range.