Bacterial growth curve :
If a small number of bacteria is inoculated into a liquid nutrient medium and bacteria is counted of frequent intervals and a graphical demonstration is produced by putting log number of cells along the Y-axis and time along the X-axis, then it is known as bacterial growth curve.
Phases of bacterial growth curve:
1. Lag phase or preparatory phase
2. Log phase or exponential phase
3. Stationary phase
4. Decline phase/Death phase
Description of each phase:
1.Lag Phase:
This is the time or period required for bacteria for adjustment in human body.
Criteria:
- No appreciable multiplication of bacteria occurs in this phase.
- Metabolic activity increases.
- Cell size may increase
- Time requires 1-4 hours
Importance:
Membrane acting antibiotics such as polymyxin, amphotericin-B can be used in this phase. Detergents, soaps and other surface acting agents act better in the lag phase.
2. Log Phase:
Criteria:
- Rapid cell multiplication & cell number increases in geometric process.
- Active synthesis of the cell wall occurs.
- Metabolic activity increase at a very high rate
- Time requires 1-4 hours.
Importance:
- Antibiotics act better at this phase as cell wall growth is very active during this phase.
- At this phase disease-producing capability of bacteria is highest, and if not treated the disease properly at this stage, dreadful conditions like septicaemia can result.
3. Stationary phase:
Criteria:
- At this stage, multiplication rate & death rate of bacteria are equal, so net growth rate is zero.
- Cell dies because of exhaustion of nutrients in the bacteria & accumulation of toxic products in themedium.
- Exotoxin production starts at this stage.
- Time requires – few hours to days.
Importance:
- Release of exotoxin starts.
- Spore forming bacteria starts formation of spores.
- Cell wall acting antibiotics may be used.
- Gram positive bacteria may be transformed into Gram negative bacteria by erosion of
peptidoglycan layer.
4. Decline phase/Death phase:
Criteria:
- The death rate is greater than the multiplication rate.
- Accumulation of toxic products occurs.
- Time requires a few hours to days.
Importance:
- Exotoxin of C. diphtheria is produced in this phase.
- Bacteria may develop L-forms, which are resistant to antibiotics.
- Sporulation starts in some bacteria.