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Culture Media

For growing microorganisms in the laboratory, a nutrient solution called culture medium is used. Every microorganism has different nutritional needs and the ability to obtain nutrition from media. Microorganisms have various metabolism pathways that lead to different metabolism products. In that case, the amount and type of nutrient to be added in a medium must be thought carefully to ensure successful growth and production of desirable metabolites. Several parameters that influence microbial growth are nutrient composition, salt ionic concentration, pH, and temperature.

Nutrition

Nutrient is a chemical substance required by the organism for growth. Not all nutrients are required in the same amount. Essential elements required by organism are hydrogen (H), oxygen (O), carbon (C), nitrogen (N), phosphorous (P), and sulfur (S). Chemical formula estimated for a cell is CH1.8O0.5N0.2 which means that C, H, O, and N dominate the nutrient required by organism. Other than C, N, O, and H, microorganisms also require many other elements but in smaller amounts. Phosphorous, generally obtained, in the form of phosphate (PO42-), C H 1.8 O 0.5 N 0.2, which means that C, H, O, and N dominate the nutrient required by organism. Other than C, N, O, and H, microorganisms also require many other elements but in smaller amounts. Phosphorous, generally obtained, in the form of phosphate (P O 4 2 minus), is a key element in nucleic acids and phospholipids. Microorganisms also require trace metals such as copper, iron, zinc, and cobalt inside the culture medium. These trace metals are generally provided in the form of ions that will serve as cofactors in enzymatic reactions. Iron is required for the synthesis of heme-containing compounds such as cytochromes and the electron transport system.

Physical Condition

A graph illustrating the relationship between temperature and enzyme activity. The x-axis represents temperature, and the y-axis represents the enzyme reaction rate. The graph shows a curve starting with a low activity level at low temperatures where membrane gelling occurs. As the temperature increases, the enzyme activity rises. The peak of the curve represents the optimal temperature where the enzyme reaction reaches its maximum rate. Past this point, the curve quickly declines, indicating a sharp decrease in enzyme activity due to protein denaturation at high temperatures.

Figure 1. Graph showing the Effect of Temperature on Microbial Growth.

Growth is defined as an increase in the number of cells. The growth of microorganism is greatly affected by environment conditions such as temperature, pH, water availability, oxygen, pressure, and radiation. To achieve desirable growth of microorganisms, nutrient content of the medium as well as the environment condition must be suitable.

Every microorganism has a minimum and maximum tolerable temperature. There are three critical temperatures for microorganisms that are generally determined by the temperature at which the microorganism’s enzyme function. Those three critical temperatures (also called cardinal temperature) are as follows:

  • Minimum growth temperature: the lowest temperature where cells can still divide
  • Maximum growth temperature: the highest temperature where cells can still divide
  • Optimum growth temperature: the temperature where cells divide most rapidly

Generally, the increase in temperature is affected the increase in cell growth until it reaches the optimum temperature and the cell growth will drop significantly after the optimum temperature until it reaches the maximum temperature. Temperature plays a key role in cell growth by affecting enzyme activity and cell component rigidity. When the temperature rises, enzymatic reactions in cells occur rapidly and growth becomes faster; however, above optimum temperature, cell components become damaged. This cell damage at a high temperature may be irreversible and eventually cause cell death. Near minimum temperature, growth becomes slower. One hypothesis about this phenomenon is that microorganism’s cytoplasmic membrane stiffens to the point that it no longer supports cell transportation function. Yeast cells used in industries are generally grown in the range of 20-40°C. twenty to forty degrees celsius.

Beside temperature, pH, which expresses acidity or alkalinity of an environment also affects microorganism’s cell growth significantly. Every microorganism also has an optimum pH. Optimum pH for most fungi(including yeast) is slightly acidic. The pH affects enzyme and protein condition, and can cause enzyme denaturation and interfere with membrane ion pumping.