Microbial Growth Curves and the Microbiome
A pint of milk left on the counter on a warm day turns sour within a day or two, yet the same milk in the fridge lasts a week. A few bacteria become billions, and then the growth stops. Why does it speed up, why does it stop, and why are some bacteria in your gut helping you rather than harming you? This page answers all three.
Learning outcomes
By the end of this page you should be able to:
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describe the four phases of a microbial growth curve and explain each one;
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calculate the number of cells after a given time, and the generation time;
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explain why growth curves are plotted on a log scale;
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compare batch and continuous culture;
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define the microbiome and discuss its role in health, including the effect of antibiotics.
The bacterial growth curve
Bacteria reproduce by binary fission: one cell becomes two, two become four, and so on. In a closed (batch) culture a fixed volume of growth medium is inoculated with cells, and nothing is added or removed. Plot the population against time and you get a characteristic curve with four phases.
| Phase | What happens | Why |
|---|---|---|
| 1. Lag | Little or no increase in numbers | Cells are adjusting: making enzymes and growing, not yet dividing |
| 2. Exponential (log) | Numbers double at a constant interval | Plenty of nutrients and space; no limits yet |
| 3. Stationary | Numbers level off | Rate of division equals rate of death; nutrients run low, toxic waste builds up, pH changes, oxygen falls |
| 4. Death (decline) | Viable numbers fall | Nutrients exhausted and waste products poison the cells faster than new cells form |
Calculating growth
During the exponential phase each cell divides once per generation time $g$ (the time for the population to double). After $n$ generations, starting from $N_0$ cells:
$$N = N_0 \times 2^n \qquad \text{where } n = \frac{t}{g}$$Why $2^n$? Each division doubles the number, so $n$ divisions multiply it by 2 a total of $n$ times.
Why a log scale?
Bacterial numbers can range from 1,000 to 1,000,000,000 in a single experiment. On an ordinary axis the early phases would be squashed flat against the bottom. Plotting $\log_{10}N$ compresses the range and turns exponential growth into a straight line, which makes it easy to read the generation time. Since $\log_{10}2 \approx 0.30$, each doubling raises the log value by 0.30.
A bacterium divides by binary fission every 20 minutes. Starting with one cell, calculate the number of cells after 2 hours, assuming unrestricted growth.
A bacterium divides by binary fission every 20 minutes. Starting with one cell, calculate the number of cells after 2 hours, assuming unrestricted growth.
A culture begins with $2\times10^3$ bacteria per mL. The generation time is 30 minutes and there is no lag phase. a) Write an expression for the number of cells after…
A culture begins with $2\times10^3$ bacteria per mL. The generation time is 30 minutes and there is no lag phase.
- a) Write an expression for the number of cells after $t$ hours.
- b) Calculate the number of cells per mL after 5 hours.
- c) State two reasons why the real culture would not reach this figure.
Factors affecting microbial growth
The rate of growth depends on the conditions in the growth medium. Temperature: enzymes work fastest at an optimum, so growth slows when it is cold and stops when enzymes denature if it is too hot. pH: each species has an optimum pH. Nutrients: a carbon and nitrogen source is needed to build new cells. Oxygen: aerobes need it, anaerobes are harmed by it. Water availability matters too. This is why milk lasts longer in the fridge.
Batch and continuous culture
Industry grows microbes to make enzymes, antibiotics, insulin and beer. There are two main systems.
| Batch culture | Continuous culture (chemostat) | |
|---|---|---|
| Operation | Fixed volume; nothing added or removed until the end | Fresh medium added steadily; culture and product removed at the same rate |
| Growth curve | All four phases | Held in the exponential phase at a steady state |
| Advantages | Simple, cheap, easy to keep sterile | High output, constant conditions |
| Disadvantages | Downtime to empty, clean and restart | Greater risk of contamination; more complex equipment |
| Best for | Products made in the stationary phase (e.g. penicillin, a secondary metabolite) | Products made during growth (e.g. biomass, many enzymes) |
The microbiome
Your body is home to trillions of microorganisms. The microbiome is the community of all the microorganisms (bacteria, fungi, viruses) living in and on a particular environment, such as the human body, together with their genes.
Different body sites have their own microbiome: the gut (largest), the skin, the mouth and the vagina. Soil also has a microbiome. Its bacteria and fungi act as decomposers, breaking down dead organisms and waste and releasing nutrients that plants can take up again, so they drive the nutrient cycles. Climate warming can alter soil microbiomes and therefore the rate at which nutrients are recycled.
What does the gut microbiome do?
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Digestion: breaks down fibre we cannot digest, releasing short-chain fatty acids as an energy source.
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Vitamins: makes vitamin K (needed for blood clotting) and some B vitamins.
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Immune signalling: trains the immune system to tolerate harmless microbes and respond to harmful ones; the beneficial bacteria also compete with pathogens for space and food.
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Metabolic health: influences how we store fat and control blood glucose.
Disturbing the microbiome
Broad-spectrum antibiotics kill many bacterial species, harmful and helpful. Gaps open up, and surviving or incoming microbes can multiply, sometimes causing diarrhoea. Probiotics (live beneficial bacteria, as in some yoghurts) and a high-fibre diet (a food source for helpful bacteria, a prebiotic effect) may help recovery.
Overuse of antibiotics also exerts selection pressure: bacteria with resistance genes survive, reproduce and pass those genes on, so resistant populations increase.
Infants
A baby's gut is colonised at birth and in the early months. Vaginal birth exposes the baby to the mother's microbes; caesarean birth, and formula rather than breast milk, are associated with a different mix. Breast milk contains sugars that feed helpful bacteria. The early microbiome appears to help develop the immune system.
Define the term microbiome*. Name two parts of the human body that have their own microbiome and state one benefit that gut bacteria provide to the host.
Define the term microbiome. Name two parts of the human body that have their own microbiome and state one benefit that gut bacteria provide to the host.
A person takes a course of broad-spectrum antibiotics for a chest infection and then develops diarrhoea. a) Explain the link between the antibiotics and the gut…
A person takes a course of broad-spectrum antibiotics for a chest infection and then develops diarrhoea.
- a) Explain the link between the antibiotics and the gut microbiome.
- b) Describe how probiotics or a diet rich in fibre might help restore the microbiome.
- c) Explain why overuse of antibiotics may also encourage antibiotic-resistant bacteria.
Research suggests that the gut microbiome of infants differs depending on whether they are born vaginally or by caesarean section, and whether they are breastfed. a)…
Research suggests that the gut microbiome of infants differs depending on whether they are born vaginally or by caesarean section, and whether they are breastfed.
- a) Suggest how birth method and feeding could influence the infant microbiome.
- b) Explain the role of the gut microbiome in immune development and in vitamin K production.
- c) A news article claims that a probiotic yoghurt "cures" allergies. Evaluate the claim and state what evidence would be needed.
Back to the start: milk sours quickly on a warm day because the bacteria are in the exponential phase, doubling every few tens of minutes; the cold of the fridge lengthens the generation time enormously. Eventually, waste products such as lactic acid slow growth, and the culture reaches the stationary phase.
Summary
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A closed culture shows four phases: lag, exponential, stationary and death.
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In the exponential phase $N = N_0 \times 2^n$, where $n = t/g$ and $g$ is the generation time.
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Log plots turn exponential growth into a straight line; one doubling adds 0.30 to $\log_{10}N$.
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Growth stops because of nutrient depletion, toxic waste and changes in pH or oxygen.
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Batch culture goes through all phases; a chemostat holds the culture in exponential growth.
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The microbiome is the community of microbes in an environment. The gut microbiome aids digestion, makes vitamin K and supports immunity.
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Antibiotics disturb the microbiome and select for resistant bacteria; evaluate health claims by the quality of the evidence.