Bacterial Growth

Bacterial cells need to grow before they reproduce. We will discuss bacterial reproduction on another page. When we culture bacteria, often on an agar plate we see the large number of bacteria that have grown as colonies on the agar.

The best conditions for bacteria to grow are:

• warmth

• a food source with a range of nutrients (such as nutrient agar, food or body tissues)

• moisture (bacteria need to absorb water before they can grow and reproduce)

Environmental conditions, such as temperature, pH, oxygen availability, water availability, nutrient availability all influence the rate of growth of bacterial populations.

Nutrients

Bacteria need the correct nutrients in order to be able to grow and reproduce. Glucose is the most obvious nutrient that bacteria require for respiration in order to make energy for the other life processes. Bacteria also need to make proteins for their structures, enzymes etc. Therefore, they need amino acids as nutrients, since amino acids are the building blocks of proteins.

Temperature

Bacterial growth slows down at low temperatures because the chemical reactions that are part of bacterial growth take place much more slowly at low temperatures. This is why we put food in the fridge or freezer to keep food fresh for longer or store food and keep it safe to eat. At really high temperatures most bacteria are killed, because the enzymes that control the bacterial chemical reactions (e.g. respiration) are denatured (destroyed). However, there are some bacteria that are resistant to high temperatures and these can survive at very high temperatures. These bacteria are known as thermophiles. In New Zealand you might find thermophile bacteria living in thermal pools in places, such as Rotorua. Most bacteria grow best at mid-range temperatures, whilst human pathogenic bacteria grow best at body temperature.

pH

Specific bacteria grow best at specific ranges of pH. Many grow best a neutral pH values (pH 7). This means they will not be found at pH values outside this range. If the pH is too low (acidic) or too high (basic) then the enzymes that control the bacterial metabolism will be denatured (destroyed). There are some bacteria that are specially adapted to live at lower or higher pH values. For example, lactic acid bacteria grow best in acidic conditions.

Oxygen

Aerobic bacteria that require oxygen in order to respire will die when the oxygen levels are too low. Some bacteria can respire aerobically and anaerobically. These bacteria will be found in a wider range of environments and a wider range of oxygen levels.

Water

The chemical reactions that keep bacteria alive take place in the cytoplasm of the cells. Cytoplasm is a watery substance. Many substances require water in order to dissolve and be transported in and out of bacterial cells and water is needed for many of the chemical reactions to proceed. If water is scarce then the chemical reactions may take place too slowly or not at all, and the bacterial cells might dry out and die.

Stages of the Bacterial Growth Curve

The lag phase:

The bacteria are adjusting to their new environment and getting 'ready' to replicate their DNA and reproduce by binary fission. There is plenty of cellular activity in the bacteria, making the proteins and other molecules they need for cell replication. No cell division occurs at this stage.

The log phase or exponential growth phase:

The conditions are ideal for bacterial growth and reproduction. Bacterial cells can reproduce by binary fission every 20 minutes. Each replication they double in number. This is the phase of rapid reproduction where numbers of bacteria rise very quickly. Bacterial metabolism is high at this stage and new DNA, cell wall components and all other substances needed for growth are made. If you were to treat a bacterial disease with antibiotics, the antibiotics would be most effective during this log phase, as antibiotics act to disrupt bacterial cell walls and protein synthesis.

Towards the end of this phase bacteria are starting to compete for food, nutrients and space and their growth and reproduction slows down. The bacteria are also producing many waste products that if allowed to accumulate are harmful to bacterial cells. There are still more new bacteria bring replicated than the number of cells dying, but the growth rate is starting to tail off.

The stationary phase:

Once the nutrients and space have been depleted and too many waste products have accumulated the growth curve enters the stationary phase and a plateau forms. The number of dying cells equals the number of new cells being produced. There is no bacterial population growth at this stage. Bacterial cell competition is at its highest and the cells are at their least active. It is at this stage that some bacteria start to generate substances that help them survive extreme environments and some bacteria form spores. These spores and other substances can cause disease. We will mention some bacteria that forms spores later on when we look at specific bacterial diseases.

The death phase:

The number of cells that are dying continues to rise at this stage and the number of living cells decreases rapidly. As the cells die, they break open (or lyse) and release their contents into the environment. The nutrients from the dying bacterial cells can now be used by spore forming bacteria as nutrients, to help them survive for longer in the harsh conditions.