Bacterial Growth

What is the definition of bacteria growth?

Binary fission is the main method by which bacterial cells reproduce. Binary fission is a simple type of cell division in which the cell duplicates its genetic material, elongates and divides into two new daughter cells that are identical to the parent cell. Thanks to binary fission, bacterial growth can lead to the production of colonies containing millions of bacterial cells!

Fig. 1. Binary fission in bacteria. The process starts with the duplication of the genetic material (the pink bundle gives way to another, green bundle, and the single plasmid gives way to another). The cell then elongates and constricts until it divides into two.

Conditions for bacterial growth

pH for bacterial growth

The pH also plays a critical role in bacterial growth. This is because the pH of a particular environment can affect the activity of enzymes necessary for bacterial growth and metabolism. Therefore, an appropriate pH range is crucial for bacterial growth and survival.

Most bacteria thrive in a neutral pH environment, between 6.5 and 7.5. However, some bacteria have adapted to grow in extremely acidic or basic conditions.

• Acidophiles are bacteria that thrive in acidic environments with a pH as low as 1.0.
• Neutrophiles are bacteria that thrive in neutral environments with a pH between 5 and 8.
• Alkaliphiles are bacteria that can grow in basic environments with a pH as high as 11.0.

Generally, most pathogenic bacteria grow between pH 7.2 and 7.6. However, some bacteria (ex. Lactobacilli) is capable of growth in pH less than 4, while others such as V. cholerae can grow in basic pH (8.2-8.9).

Phases of bacterial growth

Bacteria growth can be divided into four phases:

1. Lag phase

2. Log phase

3. Stationary phase

4. Death phase

It's important to note that the duration of each phase may vary depending on the specific bacteria and the environment they are in. Additionally, not all individual bacteria will pass through all the phases.

Lag phase

Bacterial growth starts at the lag phase. In this stage, the microorganism have just been introduced to the culture medium and are trying to acclimate to it. So, there is metabolic activity going on, but no cell division.

Log phase

Then, we have the log phase. In this phase, we see bacteria dividing exponentially. Here, the bacterium is small in its size, and biochemically active. The number of bacterial cells also starts rising due to cell division.

Stationary phase

The third phase is called the stationary phase. In this stage, bacterial growth stops almost entirely because of the exhaustion of nutrients and elevation of toxin levels. Also, the number of viable cells formed, and the number of dying cells are in balance. Another important thing to know is that, during this phase, the bacterium becomes gram-variable. Bacteria might also produce exotoxins, antibiotics, and bacteriocins at this phase.

Death phase

Lastly, we have the death phase, also known as the decline phase. Here, the bacteria completely stops dividing, and cell death continues due to running out of nutrients and the accumulation of toxin products, so the bacterial population numbers decrease.

Bacterial growth curve

Bacteria need time to grow and reproduce. The time it takes for a bacterial population to double in size is known as the generation time and this varies depending on the species and the growth conditions. Some bacteria have generation times as short as 20 minutes, while others can take several hours or even days. The population of bacteria will continue to grow until the nutrients and other conditions become limiting.

Regardless, the general graph for bacterial growth includes all four phases described above.

Formula for bacterial growth

When dealing with bacterial growth, we might need to determine generation time. By calculating generation time, we can measure the growth rate of a microbial population.

The formula for generation time is as follows:

$$ \text{generation time}(t_{g}) = \frac{\text{time elapsed}(t)}{\text{number of generations}(n)} $$

But, how do we use generation time to find out the number of cells in a certain period of time? We can use the formula below:

$$ n_{t} = n_{0} \times 2^{n} $$

where,

• \( n_{0} \) is the initial number of cells.

• \( n_{t} \) is the final number of cells after a certain period of time.

• \( n \) is the number of new generations over a given amount of time. \( n = \frac{\text{given amount of time}}{\text{generation time}} \)

Let's solve an example!

Methods of measuring bacterial growth

We can use direct and indirect measures of bacterial growth and determine bacterial population size.

First, let's talk about the method of direct microscopic counting for total cell count. In this method, scientists use a specialized slide called the Petroff-Hausser counting chamber (Figure 4). This counting chamber has a grid, facilitating the counting of cells when observed under the microscope.

This method is easy and quick to use. However, there are some limitations. For example, we cannot distinguish between live and dead cells unless special staining techniques are used. Also, if the cells are too small, it can be hard to count them.

Now, a common method used to count viable cells is known as plate count or viable count. Viable cells are referred to as cells that are alive and able to grow on laboratory culture media. Basically, scientists spread the microbes in solid media and then count the colonies. There are two ways to do this (Figure 5):

1. Spread-plate method: adding 0.1 ml or less of sample to the surface of an agar plate, spreading it evenly and then incubating until colonies are seen in the surface. Then, plates containing between 30 and 300 colonies are counted and the number of viable cells is expressed in units of CFU (colony forming units).

2. Pour-plate method: a known volume of culture (0.1 - 1.0 mL) is added to a sterile plate. Then, sterile molten agar medium is added to it, and evenly mixed. After solidification of the agar and incubation, the surface colonies and subsurface colonies are counted.

Preventing bacterial growth

Preventing bacterial growth is important in order to maintain a safe and hygienic environment, as well as to prevent the spread of illness and disease. There are several methods that can be used to prevent bacterial growth:

1. Sanitation: Regularly cleaning and disinfecting surfaces and equipment can help to remove bacteria and prevent them from growing. This is especially important in places where there are people with weak or underdeveloped immune systems (e.g. children, older people and people with autoimmune diseases) or where there is high risk of infection (hospitals, restaurants, etc.).

2. Temperature control: Bacteria are sensitive to temperature changes. Many species cannot survive at high temperatures and cannot reproduce easily at low temperatures. Heating food to the appropriate temperature or refrigerating and freezing food can help to prevent bacterial growth.

3. Humidity control: Bacteria thrive in moist environments, so maintaining low humidity levels can help to prevent bacterial growth.

4. pH control: Many bacteria have a narrow range of pH levels at which they can grow. By controlling the pH of a product, bacteria growth can be prevented.

5. Preservation: Using preservatives such as salt, sugar, vinegar, and alcohol can help to prevent bacterial growth by making the environment inhospitable for them.

6. Personal hygiene: Proper handwashing, showering and other personal hygiene practices can help to prevent the spread of bacteria. This is especially relevant when someone is sick (for example, when someone has diarrhoea).

7. Use of antimicrobial agents: There are several antimicrobial agents that can be used to prevent bacterial growth, such as antibiotics, antiseptics, and disinfectants. Each product has a different use and not all of them work against all bacteria. Thus, especially for antibiotics, we should not use them when it's not recommended by a doctor. They can provide us with the correct type of antimicrobial for each situation.

8. Sterilization: This is the process of destroying or eliminating all forms of life and microorganisms. This is used in the medical, laboratory and food industries. There are different types of sterilization, like temperature- or agent-dependent sterilization methods.

It's important to note that different bacteria have different requirements for growth and not all of these methods will be effective in preventing the growth of all types of bacteria.

Danger Zone for bacterial growth

This range of temperatures is particularly dangerous as it is within the range of temperatures that are commonly used to store and prepare food. Bacteria can double in number every 20 min in the danger zone, which means that even a small amount of bacteria can quickly multiply and reach dangerous levels for human health.

In order to prevent bacterial growth, it is important to keep food out of the danger zone by either keeping it at temperatures above 57°C or below 5°C, or by heating or cooling it quickly through the danger zone.