Bacterial Growth Curve

What is Bacterial Growth Curve?

A bacterial growth curve represents the changes in the number of bacterial cells in a population over time when grown in a closed system like a culture medium. It is usually plotted on a graph with time on the x-axis and logarithm of cell number on the y-axis. The curve shows distinct phases that reflect how bacteria adapt, multiply, and decline in a given environment.

Phases of Bacterial Growth Curve

Bacterial growth curve is divided into four different phases of growth that is, Lag phase, Log Phase, Stationary phase and Death phase.

Lag Phase

Bacteria adapt to grow in the new environment. It stays for 1-4 hours. This is the time when individual bacteria have matured and are not yet able to multiply. The slope of this line is the organism's specific growth rate, measured as the number of cell divisions per unit time. Actual growing conditions vary depending on growing conditions.

Log Phase

Log phase is also known as exponential phase because it is characterized by doubling of cells. It lasts for 6-8 hours. The number of newly emerging bacteria per unit time is proportional to the current population.

When growth is unrestrained, doubling continues at a constant rate, so both cell number and population percentage increase with each successive period.

The slope of this line is the organism's specific growth rate, a measure of the number of cell divisions per unit time. This actual rate depends on the frequency of cell division events and growth conditions that affect the chances of both daughter cells to survive.

Stationary Phase

The stationary phase occurs due to limiting factors. This is most often due to nutrient depletion or inhibitor formation such as organic acid. This stage may last for hours and even days. Cell death as a function of time is rather unpredictable and very difficult to explain. Another explanation for the stationary phase is not enough space in the cell.
Newly formed cells per minute = dying cells per minute The

Death Phase

Bacteria run out of nutrients and die, but cell numbers remain constant. The depletion phase is brought about by nutrient depletion, toxic build-up and autolytic enzymes. A few surviving cells may persist for months after most of the cells have died. These few viable cells may grow at the expense of released nutrients.

Factors Affecting Bacterial Growth Curve

The bacterial growth curve is influenced by environmental and nutritional factors that determine how fast bacteria multiply, adapt, and survive.

• Nutrient availability: Rich nutrients promote rapid growth, while limited nutrients slow down or stop multiplication.

• Oxygen concentration: Proper oxygen content should be present, aerobic bacteria need oxygen while anaerobic bacteria grow without it.

• Temperature: Each species has an optimum range, extremes slow or stop growth.

• pH levels: Most bacteria prefer near neutral pH, acidic or alkaline environments inhibit growth.

• Water content: Water is essential for metabolic activities, lack of moisture reduces bacterial activity.

• Accumulation of waste products: Toxins and acids produced during metabolism can limit growth and push bacteria into the stationary phase.

Bacterial Growth Curve NEET MCQs

Q1. Which of the following phases sees a population's organisms multiplying more quickly?

1. Deceleration phase

2. Stationary phase

3. Exponential phase

4. Lag phase

Correct answer: 4) Exponential phase

Explanation:

In the log phase, organisms proliferate more rapidly. It is the second stage of the sigmoid growth curve (logistic). And at this stage, the population is growing the fastest.

The growth rate gradually slows down during this phase as a result of nutritional availability restrictions and a buildup of metabolic products.

When a population experiences exponential growth, the growth rate per capita (per person) remains constant regardless of population size, resulting in exponential population expansion.

In nature, populations may grow exponentially for a while, but ultimately, the availability of resources will limit their expansion.

It is a stage of a cell cluster's proliferation in a culture medium. The number represented by a portion of the growth curve increases exponentially during this period. When the logarithm of the integers is plotted against time, this piece is a segment of a straight line.

One of the phases seen in a bacterial growth curve is the log phase, also known as the exponential phase or logarithmic phase.

Hence the correct answer is option (4) Log phase.

Q2. Which of the following statements is true regarding the relationship between exponential growth and the environment?

1. Exponential growth always leads to a population crash due to the depletion of resources in the environment.

2. Exponential growth can occur indefinitely in the absence of limiting factors in the environment.

3. Exponential growth can only occur in environments with unlimited resources.

4. Exponential growth is unaffected by changes in the environment.

Correct answer: 2) Exponential growth can occur indefinitely in the absence of limiting factors in the environment.

Explanation:

The term "exponential growth" describes a situation in which there are no growth restrictions and a population expands at a pace proportional to its current size. In other words, as time goes on, the population becomes exponentially larger since it is doing so at a faster rate.

Exponential growth and the environment have a complicated relationship. Exponential development is limitless in the absence of limiting conditions like scarcity of resources, predators, illness, and rivalry. This is because the population has unrestricted access to resources and can grow in size at any time.

However, in reality, there are always environmental constraints that can have an impact on population increase.

For Example, the growth rate will slow down or halt altogether if the population grows too high in comparison to the resources available, which will result in a population crash or fall.

Population expansion and survival can also be significantly impacted by environmental changes like habitat degradation and climate change.

Hence, the correct answer is option 2) Exponential growth can occur indefinitely in the absence of limiting factors in the environment.

Q3. Choose the correct statements.

I. A population's period of acclimatization is depicted by the lag phase in a population growth rate curve.
II. On a population growth rate curve, the steepest portion of the growth phase denotes the period during which a population is expanding exponentially.
III. The population is in dynamic equilibrium during the stationary phase of a population growth rate curve.

IV. Humans have altered the earth's carrying capacity.

1. All are correct

2. I, II, III

3. II, III

4. I, IV

Correct answer: 1) All are correct

Explanation:

Because the population's logistic growth curve is also known as its sigmoid growth curve.

The first phase of a bacterial population's existence when cells are adapting to a new environment before beginning exponential growth is known as lag time.

The growth rate per capita (per person) of a population remains constant regardless of population size with exponential growth, leading the population to expand exponentially as it becomes larger.

In nature, people could grow exponentially for a while, but eventually, the resources available will limit their expansion.

Due to an increase in environmental stress, the growth rate decreases from the exponential phase.
There are several causes of environmental stress, including fierce competition for resources, a lack of available space, and higher mortality.

With migration, agriculture, and technological advancements in medicine, and communication, humans have enhanced the world's carrying capacity. We can forecast population increase using a population's age distribution.

The repercussions of unchecked human population expansion on the environment might be catastrophic in the long run.

Hence, the correct option is 1) All are correct.

Also Read:

• MCQs on Models of Population Growth

• Organisms and Populations

• Abiotic and Biotic

FAQs on Bacterial Growth Curve

What are the four phases of bacterial growth?

The bacterial growth curve has four main phases:

1. Lag phase – Cells are metabolically active but not dividing.

2. Log (exponential) phase – Rapid cell division with maximum growth rate.

3. Stationary phase – Growth rate slows as nutrients deplete and waste accumulates; cell division balances cell death.

4. Death phase – Cells die at an exponential rate due to lack of nutrients and toxic conditions.

Why does bacterial growth stop in the stationary phase?

The bacterial growth stops in the stationary phase because the nutrients in the medium become limited and toxic by-products accumulate. Oxygen availability also decreases, and the pH changes. As a result, the number of new cells formed is equal to the number of cells dying, leading to a plateau in population size.

What happens in the bacterial death phase?

In the death phase, the death rate exceeds the growth rate. Nutrient becomes limited, accumulation of toxic wastes, and unfavorable environmental conditions cause massive cell death. The population decreases exponentially.

Why is the log phase also called the exponential phase?

The log phase is called the exponential phase because cells divide at a constant and maximum rate, doubling their population at regular intervals (generation time). The increase in cell number follows an exponential pattern (2ⁿ), where “n” is the number of generations. This is the most favorable phase for studying bacterial physiology and testing antibiotics.