By Momina Shehzadi | BS Zoology Graduate, Pakistan In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students. Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus. Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms. The study of fungi is called mycolog...
By Momina Shehzadi | BS Zoology Graduate, Pakistan
In this article, we will learn Bacterial Growth Curve - Lag, Log, Stationary and Death Phase - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students.
INTRODUCTION
Bacterial growth means increase in number of bacteria, not size of individual bacterium. Bacteria reproduce by binary fission where one cell divides into two. When bacteria are inoculated into fresh culture medium and incubated, they show a characteristic pattern of growth which is represented graphically and called Bacterial Growth Curve.
Growth curve is very important to understand bacterial physiology, to determine generation time and to select time for harvesting bacteria for industrial and research purposes.
BACTERIAL GROWTH CURVE
When we plot number of viable bacteria against time of incubation, we get a curve with four distinct phases. The bacterial growth curve has four phases: Lag phase, Log phase, Stationary phase and Death phase.
1. LAG PHASE - Phase of Adaptation
Lag phase is initial phase after inoculation. During this phase bacteria do not divide immediately.
Characteristics:
Bacteria adapt to new environment. They synthesize enzymes, proteins and RNA needed for growth. Cells increase in size but not in number. Metabolism is active. Duration of lag phase depends on type of bacteria, age of inoculum, nutrients and temperature. If old culture is inoculated, lag phase is longer. If actively growing culture is inoculated, lag phase is short or absent.
Importance:
In this phase bacteria prepare for division. Antibiotics are less effective in this phase because cell wall synthesis is slow.
2. LOG PHASE OR EXPONENTIAL PHASE - Phase of Rapid Growth
Log phase is period of rapid growth where bacteria divide at maximum rate.
Characteristics:
Bacteria divide by binary fission at constant rate. Number of bacteria doubles in each generation. Growth is exponential and can be represented by equation. Metabolic activity is maximum. Cells are smallest in size and most active. Cells are most sensitive to antibiotics and environmental factors in this phase because cell wall synthesis is rapid. Generation time is calculated in this phase. Generation time is time required for one cell to divide into two. For E. coli generation time is 20 minutes.
If we plot log of number of cells against time, we get straight line in log phase.
Log phase is used in industry to harvest bacterial products like enzymes and antibiotics because production is maximum.
3. STATIONARY PHASE - Phase of Equilibrium
After log phase, growth rate slows down and eventually becomes equal to death rate.
Characteristics:
In this phase number of new cells formed equals number of cells dying, so total viable count remains constant. Reasons for stationary phase are depletion of nutrients, accumulation of toxic waste products like acids, and decrease in space and oxygen. Bacteria produce secondary metabolites like antibiotics, toxins and spores in stationary phase.
Cells become larger and produce storage materials. Some bacteria like Bacillus produce endospores in this phase to survive harsh conditions.
Stationary phase is important for production of antibiotics like penicillin which are produced as secondary metabolites.
4. DEATH PHASE OR DECLINE PHASE
In death phase, number of viable cells decreases exponentially.
Characteristics:
Death rate exceeds growth rate. Number of dead cells is more than live cells. Reasons are exhaustion of nutrients, accumulation of toxic products and autolysis by enzymes. Cells show abnormal shapes called involution forms.
If we plot total count including dead cells, it remains constant but viable count decreases.
After death phase, if culture is not transferred to fresh medium, all cells die and culture is called dead culture.
MATHEMATICAL EXPRESSION OF GROWTH
Bacterial growth in log phase follows exponential law.
Nt = No x 2 to power n
Where Nt is number of cells after time t, No is initial number, n is number of generations.
Generation time G = t divided by n
MEASUREMENT OF BACTERIAL GROWTH
Bacterial growth can be measured by two methods: Total count and Viable count.
A. TOTAL COUNT METHODS - Count Both Live and Dead Cells
1. Direct Microscopic Count: Bacteria are counted directly under microscope using counting chambers like Petroff Hausser chamber or Neubauer chamber. It is quick method but cannot differentiate live and dead cells.
2. Turbidimetric Method: Growth is measured by turbidity or cloudiness of broth culture using spectrophotometer. More bacteria mean more turbidity and more optical density. OD is measured at 600 nm.
3. Coulter Counter: Electronic device which counts cells passing through small pore.
B. VIABLE COUNT METHODS - Count Only Live Cells
1. Plate Count Method: Most common method. Diluted bacterial sample is plated on agar plates and colonies are counted. Each colony originates from single viable cell and called Colony Forming Unit CFU. Number of bacteria per ml = Number of colonies x Dilution factor divided by Volume plated. For accurate count, plates with 30 to 300 colonies are selected.
2. Membrane Filter Method: Used for bacteria in water where number is very low. Large volume of water is filtered through membrane filter which retains bacteria.
3. Most Probable Number MPN Method: Statistical method used to estimate number of bacteria in water and food.
FACTORS AFFECTING BACTERIAL GROWTH
Growth is affected by temperature, pH, oxygen, nutrients, salt concentration and osmotic pressure. Most pathogenic bacteria grow best at 37 degree Celsius, pH 7.2 to 7.4 and require oxygen.
CONCLUSIONS
Bacterial growth curve shows four phases with different physiological activities. Lag phase is adaptation, log phase is rapid growth, stationary phase is equilibrium and death phase is decline. Measurement of growth by total count and viable count methods is essential in microbiology laboratory.
FAQs for Students - Bacterial Growth Curve
Q1: What are four phases of bacterial growth curve?
Ans: Four phases are Lag phase where bacteria adapt, Log phase where rapid division occurs, Stationary phase where growth equals death, and Death phase where viable cells decrease.
Q2: Why log phase is called exponential phase?
Ans: Because in log phase bacteria divide at constant rate and number doubles each generation, so when plotted on log scale it gives straight line indicating exponential growth.
Q3: What is generation time?
Ans: Generation time is time required for one bacterial cell to divide into two. It is also called doubling time. For E. coli it is 20 minutes.
Q4: What is difference between total count and viable count?
Ans: Total count counts both live and dead cells by direct microscope or turbidity. Viable count counts only live cells capable of forming colonies by plate count method.
Q5: What is CFU?
Ans: CFU means Colony Forming Unit. Each colony on agar plate originates from single viable bacterium, so CFU is used to represent viable bacterial count.
Also Read: Virus - General Characteristics, Structure and Classification
Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.
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