What you know about Growth curve of microorganisms -Erakina

• A growth curve of microorganisms represents stages in the growth of microbial cells in a population of bacteria some time. 

 close-up of pink spheres embedded on a green surface

•  A growth curve is used to evaluate the variations in the population growth of different microorganisms to understand their growth rate at a periodic time interval. Microorganisms are tiny organisms that are viewed under a microscope: fungus, algae, bacteria, viruses, and protozoa.
• These microorganisms require favourable conditions to increase their population. Therefore, the growth curve is a tool to monitor population growth in different microorganisms. This curve is varied in various microorganisms due to their replicative nature, cell composition, and source of nutrients.
• Understanding the growth factors of microorganisms can aid in utilising this knowledge in research and the medical field. 

Stages of Microbial growth

Under laboratory-controlled conditions, the experimental microbial cultures are studied and divided into four stages based on their appearance. 

growth curve during four different phases

Lag phase

• The lag phase is the initial phase where the microorganisms are not growing or replicating but turning into mature cells. The microbial cells are placed in a petridish containing nutrients required for the cells to grow and mature, preparing for replication.
• There is no cell division because the microorganisms adjust to their new conditions. The cells synthesize proteins and other crucial metabolites lacking in their environment to repair damaged cells.

Exponential phase

• After accumulating all the nutrients from the lag phase, the mature microbial cells replicate by cell division, doubling their population each generation. Their metabolic activity is increased when they meet optimal conditions for their growth; otherwise, the growth rate decreases.
• The microbial cells are at their healthiest stage, so the antibiotics kick into the target microbes by inhibiting their replication. 

Stationary phase

• Once the microbial cell population reaches a certain peak, they stop replicating due to a lack of nutrients. Since the medium eventually runs out of vial nutrients, waste products from the microbial cells start accumulating, thus rendering their optimal conditions essential for their growth. At this point, the cells stop replicating and enter into the stationary phase.
• Here, the number of cells replicated is equal to the number of cells dying. Since these microbial cells starve, they try to adapt to their new environmental conditions. New cells that replicate here adjust to their current surroundings by altering their membrane, making them more adhesive and aggregate.
• This will help the cells to protect their DNA from any damage. The survivability of these newly modified cells is longer under harsh conditions until suitable conditions are met. 

Death phase

• At this stage, the nutrients become depleted, and the waste products in excess lead to harsh conditions for microbial cell growth and replication. Therefore, there is a steep decline in the growth curve during this phase, as the number of living cells is lower than the number of surviving cells.
• As these cells die, their membranes rupture, oozing out all the nutrients, creating a crucial environment for the new cells to feed and grow. However, some cells do not die during these harsh conditions but survive long enough until optimal conditions are met. Here, they stay by lowering their metabolic activities and cellular division until the right conditions.
• Microbial cells do not wholly die since their cells are accustomed to mutations in any conditions.  

Conditions affecting growth

A wide range of microorganisms is found in this world that can survive under extreme conditions. Factors that influence the growth of microorganisms are pH, temperature, humidity, atmospheric pressure, light, and oxygen. 

close-up of two petridishes with microbial culture

pH

• Bacteria are differentiated based on their pH tolerance. Acidophiles can survive in acidic conditions of pH < 7 and are found in hot springs, volcanic vents, etc. Alkaliphiles grow at pH > 8 under alkaline conditions and are found in alkaline soils and extremely saline lakes.
• In contrast, neutrophils thrive near pH 7 and are most commonly found in the stomach where the bacteria secrete enzymes neutralising the surrounding acidic conditions. 

Temperature

• Mesophiles are bacteria found in the human body as they survive under normal (20-45°C) temperatures. However, the growth rate is slow in cold conditions compared to other temperatures, as lower temperatures slow the metabolic activity of the microorganisms.
• Thermophiles are bacterias that grow in hot conditions above 50°C, found in geothermal springs, while psychrophiles thrive in cold temperatures below 25 °C, which are found in deep oceans and glaciers. 

Humidity

• Moisture content in the environment provides a fine breeding ground for various microorganisms to thrive since it provides a warm temperature suitable for the bacteria. Moreover, moisture in the environment contains necessary features crucial for the growth of microorganisms, such as pH, temperature, and oxygen levels. 

Light

• Some bacterias have the characteristics of converting light into chemical energy required for their photosynthesis, such as cyanobacteria. In contrast, other bacterias require light of a shorter wavelength and are found in the deeper part of the oceans. 

Oxygen

• Two types of bacteria that are distinguished based on their oxygen requirement are anaerobes and aerobes. Aerobic bacteria require oxygen for cellular respiration to convert into energy to carry out their metabolic functions. Anaerobic bacteria cannot survive in the presence of oxygen as their metabolic activities are ceased. 

Importance 

• Understanding the growth curve of microorganisms will help research medicines, biotechnology, preparation of dairy products, genetic engineering, preservatives, medical treatments, etc. Knowledge of the growth rate of microorganisms can assist in controlling toxic pollution in water, land, and air.
• Certain bacterias such as Escherichia coli are used in genetic engineering to produce desired DNA (by recombinant DNA technique) utilised in gene therapy. Lactobacillus species of bacteria are used in the production of dairy products to generate lactic acid useful for the human digestive system.
• Additionally, microorganisms that enhance plant characteristics are produced after intensive research in biotechnology. 

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Asha Rachel Thomas

19/04/2022