BACTERIAL GROWTH
Growth of Bacteria is the orderly increase of all the chemical constituents of the bacteria. Multiplication is the consequence of growth. Death of bacteria is the irreversible loss of ability to reproduce.
Bacteria are composed of proteins, carbohydrates, lipids, water and trace elements.
Bacteria are composed of proteins, carbohydrates, lipids, water and trace elements.
Factors Required for Bacterial Growth
The requirements for bacterial growth are:
The requirements for bacterial growth are:
(A)) Environmental factors affecting growth, and
(B) Sources of metabolic energy.A. Environmental Factors affecting Growth:
1. Nutrients. Nutrients in growth media must contain all the elements necessary for the synthesis of new organisms. In general the following must be provided : (a) Hydrogen donors and acceptors,
(b) Carbon source, (c) Nitrogen source, (d) Minerals : sulphur and phosphorus, (e) Growth factors: amino acids, purines, pyrimidines; vitamins, (f) Trace elements: Mg, Fe, Mn.
Growth Factors:
A growth factor is an organic compound which a cell must contain in order to grow but which it is unable to synthesize. These substances are essential for the organism and are to be supplied as nutrients. Thiamine, nicotinic acid, folic acid and para-aminobenzoic acid are examples of growth factors.
Essential Metabolites: These metabolites are essential for growth of bacterium. These must be synthesized by the bacterium, or be provided in the medium. Mg, Fe and Mn are essential trace elements.
Autotrophs live only on inorganic substances, i.e. do not require organic nutrients for growth. They are not of medical importance.
Heterotrophs require organic materials for growth, e.g. proteins, carbohydrates, lipids as source of energy. All bacteria of medical importance belong to heterotrophs. Parasites may depend on the host for certain foods. Saprophytes grow, on dead organic matter.
2. pH of the medium. Most pathogenic bacteria grow best in pH 7.2-7.4. Vibno cholerae can grow in pH 8.2-9.0.
3. Gaseous Requirement
(a) Role of Oxygen. Bacteria may be classified into four groups on oxygen requirement :
(i) Aerobes. They cannot grow without oxygen, e.g. Mycobacterium tuberculosis.
(ii) Facultative anaerobes. These grow under both aerobic and anaerobic conditions. Most bacteria are facultative anaerobes, e.g. Enterobacteriaceae.
(iii)Anaerobes. They only grow in absence of free oxygen, e.g. Clostridium, Bacteroides.
(iv) Microaerophils grow best in oxygen less than that present in the air, e.g. Campylobacter.
Aerobes and facultative anaerobes have the metabolic systems: (1) cytochrome systems for the metabolism of oxygen, (2) Superoxide dismutase, (3) catalase.
Anaerobic bacteria do not grow in the presence of oxygen. They do not use oxygen for growth and metabolism but obtain their energy from fermentation reactions. Anaerobic bacteria are killed by oxygen or toxic oxygen radicals. Multiple mechanisms play role for oxygen toxicity : (1) They do not have cytochrome systems for oxygen metabolism, (2) They may have low levels of superoxide dismutase, and (3) They may or may not have catalase.
(b) Carbon dioxide. All bacteria require CO2 for their growth. Most bacteria produce CO2. N. gonorrhoeae and N. meningitides and Br abortus grow better in presence of 5 per cent CO2.
4. Temperature. Most bacteria are mesophilic. Mesophilic bacteria grow best at 30-37°C. Optimum temperature for growth of common pathogenic bacteria is 37°C. Bacteria of a species will not grow but may remain alive at a maximum and a minimum temperature.
5. Ionic strength and osmotic pressure.
6. Light. Optimum condition for growth is darkness.
B.Sources of Metabolic Energy
Mainly three mechanisms generate metabolic energy. These are fermentation, respiration and photosynthesis. An organism to grow, at least one of these mechanisms must be used.
REPRODUCTION
Bacteria reproduce by binary fission. Multiplication takes place in geometric progression. The nucleus (chromosome) undergoes duplication prior to cell division. When the cell grows about twice its size, the nuclear material divides, and a transverse septum originates from plasma membrane and cell wall and divides the cell into two parts. The two daughter cells receive an identical set of chromosomes. The daughter cells separate and may be arranged singly, in pairs, clumps, or chains.
GROWTH CURVE
The growth curve indicates multiplication and death of bacteria. When a bacterium is inoculated in a medium, it passes through four growth phases which will be evident in a growth curve drawn by plotting the logarithm of the number of bacteria against time.
Number of bacteria in the culture at different periods may be :
(1) Total count. It includes both living and dead bacteria, or
(2) Viable count. It includes only the living bacteria.
Microbial concentration can be measured in terms of cell concentration, i.e. the number of viable cells per unit volume of culture, or of biomass concentration, i.e. dry weight of cells per unit volume of culture.
(a) Role of Oxygen. Bacteria may be classified into four groups on oxygen requirement :
(i) Aerobes. They cannot grow without oxygen, e.g. Mycobacterium tuberculosis.
(ii) Facultative anaerobes. These grow under both aerobic and anaerobic conditions. Most bacteria are facultative anaerobes, e.g. Enterobacteriaceae.
(iii)Anaerobes. They only grow in absence of free oxygen, e.g. Clostridium, Bacteroides.
(iv) Microaerophils grow best in oxygen less than that present in the air, e.g. Campylobacter.
Aerobes and facultative anaerobes have the metabolic systems: (1) cytochrome systems for the metabolism of oxygen, (2) Superoxide dismutase, (3) catalase.
Anaerobic bacteria do not grow in the presence of oxygen. They do not use oxygen for growth and metabolism but obtain their energy from fermentation reactions. Anaerobic bacteria are killed by oxygen or toxic oxygen radicals. Multiple mechanisms play role for oxygen toxicity : (1) They do not have cytochrome systems for oxygen metabolism, (2) They may have low levels of superoxide dismutase, and (3) They may or may not have catalase.
(b) Carbon dioxide. All bacteria require CO2 for their growth. Most bacteria produce CO2. N. gonorrhoeae and N. meningitides and Br abortus grow better in presence of 5 per cent CO2.
4. Temperature. Most bacteria are mesophilic. Mesophilic bacteria grow best at 30-37°C. Optimum temperature for growth of common pathogenic bacteria is 37°C. Bacteria of a species will not grow but may remain alive at a maximum and a minimum temperature.
5. Ionic strength and osmotic pressure.
6. Light. Optimum condition for growth is darkness.
B.Sources of Metabolic Energy
Mainly three mechanisms generate metabolic energy. These are fermentation, respiration and photosynthesis. An organism to grow, at least one of these mechanisms must be used.
REPRODUCTION
Bacteria reproduce by binary fission. Multiplication takes place in geometric progression. The nucleus (chromosome) undergoes duplication prior to cell division. When the cell grows about twice its size, the nuclear material divides, and a transverse septum originates from plasma membrane and cell wall and divides the cell into two parts. The two daughter cells receive an identical set of chromosomes. The daughter cells separate and may be arranged singly, in pairs, clumps, or chains.
GROWTH CURVE
The growth curve indicates multiplication and death of bacteria. When a bacterium is inoculated in a medium, it passes through four growth phases which will be evident in a growth curve drawn by plotting the logarithm of the number of bacteria against time.
Number of bacteria in the culture at different periods may be :
(1) Total count. It includes both living and dead bacteria, or
(2) Viable count. It includes only the living bacteria.
Microbial concentration can be measured in terms of cell concentration, i.e. the number of viable cells per unit volume of culture, or of biomass concentration, i.e. dry weight of cells per unit volume of culture.
Growth Phases
1. Lag Phase. In this phase there is increase in cell size but not multiplication.
Time is required for adaptation (synthesis of new enzymes) to new environment.
During this phase vigorous metabolic activity occurs but cells do not divide.
Enzymes and intermediates are formed and accumulate until they are present in concentration that permits growth to start.
Antibiotics have little effect at this stage.
2. Exponential Phase or Logarithmic (Log) Phase.
The cells multiply at the maximum rate in this exponential phase, i.e. there is linear relationship between time and logarithm of the number of cells.
Mass increases in an exponential manner.
This continues until one of two things happens: either one or more nutrients in the medium become exhausted, or toxic metabolic products, accumulate and inhibit growth.
Nutrient oxygen becomes limited for aerobic organisms.
In exponential phase, the biomass increases exponentially with respect to time, i.e. the biomass doubles with each doubling time.
The average time required for the population, or the biomass, to double is known as the generation time or doubling time.
Linear plots of exponential growth can be produced by plotting the logarithm of biomass concentration as a function of time.
Importance : Antibiotics act better at this phase.
3. Maximal Stationary Phase.
Due to exhaustion of nutrients or accumulation of toxic products death of bacteria starts and the growth cease completely.
The count remains stationary due to balance between multiplication and death rate.
Importance: Production of exotoxins, antibiotics, metachromatic granules, and spore formation takes place in this phase.
4. Decline phase or death phase. In this phase there is progressive death of cells.
However, some living bacteria use the breakdown products of dead bacteria as nutrient and remain as persister
Time is required for adaptation (synthesis of new enzymes) to new environment.
During this phase vigorous metabolic activity occurs but cells do not divide.
Enzymes and intermediates are formed and accumulate until they are present in concentration that permits growth to start.
Antibiotics have little effect at this stage.
2. Exponential Phase or Logarithmic (Log) Phase.
The cells multiply at the maximum rate in this exponential phase, i.e. there is linear relationship between time and logarithm of the number of cells.
Mass increases in an exponential manner.
This continues until one of two things happens: either one or more nutrients in the medium become exhausted, or toxic metabolic products, accumulate and inhibit growth.
Nutrient oxygen becomes limited for aerobic organisms.
In exponential phase, the biomass increases exponentially with respect to time, i.e. the biomass doubles with each doubling time.
The average time required for the population, or the biomass, to double is known as the generation time or doubling time.
Linear plots of exponential growth can be produced by plotting the logarithm of biomass concentration as a function of time.
Importance : Antibiotics act better at this phase.
3. Maximal Stationary Phase.
Due to exhaustion of nutrients or accumulation of toxic products death of bacteria starts and the growth cease completely.
The count remains stationary due to balance between multiplication and death rate.
Importance: Production of exotoxins, antibiotics, metachromatic granules, and spore formation takes place in this phase.
4. Decline phase or death phase. In this phase there is progressive death of cells.
However, some living bacteria use the breakdown products of dead bacteria as nutrient and remain as persister
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