Growth of Bacteria | Microbiology | Pharmacy

Growth and Multiplication of Bacteria

Bacterial cell division- binary fission

  • Most bacteria divide by binary fission in which the bacteria undergo cell division to produce two daughter cells identical to the parent cell.Bacterial DNA/chromosome- circular double stranded DNA
  • Nuclear division followed by cell division.
  • During binary fission, Two identical double strand DNA are separated and distributed to the daughter cells.
  • Transverse septum grows across the cell from the cell membrane, followed by the deposition of cell wall material.
  • Finally to daughter cell separates.
Binary Fission in Bacteria
Binary Fission: exponential Growth in Bacteria

Bacterial growth curve

Bacteria growth follows a defined course .i.e. growth curve obtained by plotting bacterial count vs time. The bacterial growth curve has four phases.

  • 1. Lag phase
  • 2. Log/logarithmic/exponential phase
  • 3. Stationary phase
  • 4. Decline phase
Lag PhaseLog phaseStationary phaseDecline phase
-The period between inoculation and the beginning of multiplication is known as the lag phase.
-In this phase, the organism adapts to the new environment.
-During the lag phase, necessary enzymes, intermediate metabolites are built up in adequate quantity for multiplication to proceed/start.
-Increase in size of a cell
-Number of cells- not increase
-Duration of phase depends on species, culture type and temperature.
-At the end of the lag phase, the bacterium has maximum cell size.
-Cell division starts
-A number of organism increase exponentially or by geometric progression with time.
The straight curve between logarithm viable count versus time.
-Cells are smaller and strain uniformly.
-After low phase bacterial growth ceases due to exhaustion of nutrient and accumulation of toxic products.
-Number of progeny cell form is just enough to replace the number of cell that die.
-Number of viable cells remain stationary as there is is almost balance between dying cell and new formed cell.
-Cells are gram variable and show irregularity straining due to intracellular storage of granules.
-Sporulation occurs
-Many bacteria produce exotoxin this phase
-The stationary phase decline phase start due to exhaustion of nutrients accumulation of the toxic product and autolytic enzymes.
-Bacterial population decline due to the death of cells.
-Decline in viable count of the bacteria.
-Involution forms are common.
The bacterial growth curve phases
Bacterial Growth Curve: Lag, Log, Stationary & Decline Phase

1. Lag phase

  • Period between inoculation and the beginning of multiplication is known as the lag phase.
  • In this phase, the organism adapts to the new environment.
  • During lag phase, necessary enzymes, intermediate metabolites are built up in adequate quantity for multiplication to proceed/start.
  • Increase in size of a cell
  • Number of cells- not increase
  • Duration of phase depends on species, culture type and temperature.
  • At the end of the lag phase, the bacterium has maximum cell size.

2. Log/logarithmic/exponential phase

  • Cell division starts
  • A number of organism increase exponentially or by geometric progression with time.
  • The straight curve between logarithm viable count versus time.
  • Cells are smaller and strain uniformly.

3. Stationary Phase

  • After low phase bacterial growth ceases due to exhaustion of nutrient and accumulation of toxic products.
  • Number of progeny cell form is just enough to replace the number of cell that die.
  • Number of viable cells remain stationary as there is is almost balance between dying cell and new formed cell.
  • Cells are gram variable and show irregularity straining due to intracellular storage of granules.
  • Sporulation occurs
  • Many bacteria produce exotoxin this phase

4. Decline phase

  • Stationary phase decline phase start due to exhaustion of nutrients accumulation of the toxic product and autolytic enzymes.
  • Bacterial population decline due to the death of cells.
  • Decline in viable count of the bacteria.
  • Involution forms are common.

Bacteria count

Total Count

  • Total number of bacteria whether they are living or dead.
  • Total count is obtained by using counting chamber under microscope

Viable count

  • Total number of living/viable cells
  • The viable count is done by
    • Dilution method
    • plating method

Generation time

  • Time required for a bacterium to give rise to two daughter cells under optimum condition is known Generation time.
  • Generation time for most bacterium is 20 minutes
  • Mycobacterium tuberculosis 20 hrs
  • Mycobacterium lepre – 20 days

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