Factors Influencing Growth
The growth and activities of microorganisms are greatly influenced by the physical and chemical conditions of their environment. Among all factors, four key factors play major roles in controlling the growth of microorganisms. They are
3. Water activity
Temperature is one of the most important environmental factor affecting the growth and survival of microorganisms. Temperature can affect microorganisms because the enzyme catalysed reactions are sensitive to fluctuations in temperature.
For every microorganism, there is a minimum temperature below which no growth occurs, an optimum temperature at which growth is most rapid, and a maximum temperature above which no growth occur. These three temperatures are called cardinal temperatures.
Microorganisms are broadly distinguished into four groups in relation to their temperature optima.
A psychrophile can be defined as an organism with an optimal growth temperature of 15°C, maximum growth temperature of 20°C and a minimum growth temperature at 0°C. These organisms are found in polar regions like Arctic and Antarctic oceans. They are rapidly killed as the temperature rises because the cellular constituents start to leak due to cell membrane disruption. Some examples of psychrophiles are Moritella, Photobacterium and Pseudomonas.
Organisms that can grow at 0°C, but have temperature optimum growth temperature range of 20°C-40°C are called psychrotolerant.
These are microorganisms that grow in optimum temperature between 20-45°C, they have a temperature minimum of 15-20°C and a maximum temperature of 45°C. All human pathogens are mesophiles.
Organisms whose growth temperature optimum is between 55-65°C are called thermophiles. They have minimum growth temperature of 45°C. These organisms are found in compost stacks, hot water lines and hot springs. They contain enzymes that are heat stable and protein synthesis systems function well at high temperature.
Organisms whose growth optimum temperature is above 80°C are called hyperthermophiles. These are mostly bacteria and archaebacteria. They are found in boiling hot springs and hydrothermal vents on seafloor.
pH is defined as the negative logarithm of the hydrogen ion concentration. pH scale extends from pH 0.0 to pH 14.0 and each exchange of 1 pH unit represents a 10 fold change in hydrogen ion concentration. pH greatly influences microbial growth. Each organism has a definite pH range and well defined pH growth optimum. Most natural environments have pH values between 5 and 9.
Organisms are classified into Acidophiles, Neutrophiles and Alkalophiles based on their optimum growth pH.
Acidophiles are organisms that grow best at low pH (0.0–5.5) Example: Most fungi, bacteria like Acidithiobacillus, Archaebacteria like Sulfolobus and Thermoplasma.
Neutrophiles are organisms that grow well at an optimum pH between 5.5 and 8.0. Most bacteria and protozoa are neutrophiles.
Organisms that prefer to grow at pH between 8.5-11.5 are called alkalophiles. These microorganisms are typically found in soda lakes and high carbonate soils. Example: Bacillus firmus.
Water activity, (aw) is the ratio of vapour pressure of the solution to the vapour pressure of pure water (aw values vary between 0 and 1). Water activity is inversely related to osmotic pressure. Organisms that can grow in low aw values are called osmotolerant. Example: Staphylococus aureus.
Only a few organisms are capable of tolerating high salt concentration and still growing optimally in low water activity. Such organisms are called halophiles. Halophiles can grow in 1-15% Sodium chloride (NaCl) concentrations. Organisms that can grow in very salty environments are called extreme halophiles. (They can grow in 15-30%) NaCl concentration. Example: Halobacterium.
Most of the microorganisms require oxygen for their optimal growth but some of them survive very well in total absence of oxygen and are killed when exposed to air.
Based on their need and tolerance for oxygen, microorganisms are classified into the following types.
1. Obligate aerobes exhibit growth only at full oxygen level (21% O2 on air) because O2 is needed for their respiration and metabolic activities Example: Micrococcus, most Algae, Fungi and Protozoa.
2. Microaerophiles are aerobes that require oxygen at levels lower than that of air. Example: Azospirillum, Campylobacter, Treponema
3. Obligate anaerobes does not require oxygen for their respiration and growth. This group cannot tolerate O2 and are killed in its presence. Example: Methanogens, Clostridium.
4. Aerotolerant anaerobes can grow in the presence of oxygen though O2 is not required for their growth. Example: Streptococcus pyogenes.
5. Facultative anaerobes can grow either under oxic or anoxic conditions: Example: Escherichia coli. (Figure 6.7)
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