Microbial Growth

Microbial Growth

In bacteria, growth can be defined as an increase in cellular constituents. Growth results in increase of cell number.

When bacteria are cultivated in liquid medium and are grown as batch culture (Growth occurring in a single batch of medium with no fresh medium provided), cell multiplication happens till all the nutrients are exhausted. 

After sometime, nutrient concentrations decline and bacterial cells begin to die. This growth pattern can be plotted in a graph as the logarithm of viable cells versus incubation time (Figure 6.4). The growth curve has four distinct phases.

1.     Lag phase

2.     Logrithmic phase/Exponential phase

3.     Stationary phase

4.     Death phase

1. Lag Phase

When bacteria are introduced into fresh medium, no immediate cell multiplication and increase in cell numbers occur. The cell prepares itself for cell division by synthesizing cell components and increase in cell mass. Since there is a lag in cell division, this phase is called lag phase.

2. Logrithmic Phase/Exponential Phase

During this phase, microorganisms rapidly divide and grow at a maximal rate possible utilizing all the nutrients present in the medium. The growth rate is constant during the exponential phase. The organism divides and doubles in number at regular intervals. The growth curve rises smoothly

3. Stationary Phase

As the nutrients get depleted, the cell growth stops and the growth curve become horizontal. The total number of viable cells remains constant which is due to a balance between cell division and cell death.

4. Death Phase

Nutrient deprivation and build up of wastes lead to the decline in cell numbers. The microbial population dies rapidly and logarithmically and the growth curve also stops down.

Batch culture

It is the growth of microorganisms in a fixed volume of culture medium in which nutrient supply is not renewed and wastes are not removed. It is a closed system. This can be used to study the various growth phases of microorganisms.

Continuous culture

A continuous culture is an open system with constant volume to which fresh medium is added and utilized (spent) medium are removed continuously at a constant rate. 

A microbial culture remains in exponential state for longer periods, for days and even weeks. This enables the researcher to learn about the physiological processes and enzymatic activities of organisms.

There are two ways by which continuous culture is operated.

a)     Chemostat

b)    Turbidostat

Chemostat

The chemostat operates so that the sterile nutrient medium enters the culture vessel at the same rate as the spent medium is removed. The chemostat can control growth rate and cell density simultaneously and independently of each other. Two factors play an important role in achieving this dilution rate and concentration of the limiting nutrient (a carbon or a nitrogen source like sugars or aminoacids). Growth rate can be controlled by adjusting the dilution rate and cell density is controlled by modifying the concentration of the limiting nutrient (Figure 6.5).

Turbidostat

This type of continuous culture system has a photocell that measures the turbidity of the culture vessel. This automatically regulates the flow rate of the culture medium. Turbidostat does not contain limiting nutrients (Figure 6.6).

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

1. Temperature

2. pH

3. Water activity

4. Oxygen

1. Temperature

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.

Temperature classes of microorganisms

Microorganisms are broadly distinguished into four groups in relation to their temperature optima.

·        Psychrophiles

·        Mesophiles

·        Thermophiles

·        Hyperthermophiles

Psychrophiles

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.

Psychrotolerant

Organisms that can grow at 0°C, but have temperature optimum growth temperature range of 20°C-40°C are called psychrotolerant.

Mesophiles

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.

Thermophiles

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.

Hyperthermophiles

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.

2. pH

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.

3. Water Activity and Osmosis

Water activity, (a_w) is the ratio of vapour pressure of the solution to the vapour pressure of pure water (a_w values vary between 0 and 1). Water activity is inversely related to osmotic pressure. Organisms that can grow in low a_w 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.

4. Oxygen

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)