Bacterial growth
What is Growth in Bacterial Physiology ?
Growth is the orderly increase in the sum of all the components of an organism. Bacterial growth involves both an increase in the size of organisms and an increase in their number.
What is Death in Bacterial Physiology ?
Death means the irreversible loss of the ability to reproduce.
Bacterial Reproduction
Bacteria reproduce by binary fission, a process by which one parent cell divides to form two progeny cells. Because one cell gives rise to two progeny cells, bacteria are said to undergo exponential growth (logarithmic growth).
Theoretically a bacterium can give rise to 1021 progeny in 24 hours.
What is Generation time (doubling time)?
It is the time interval required for a bacterial cell to double under optimum condition.
The doubling (generation) time of most of the bacteria like Escherichia coli is 18 to 20 minutes. Exception: Mycobacterium tuberculosis (18 hours) & Mycobacterium leprae (18 days).
The doubling time varies not only with the species, but also with the amount of nutrients, the temperature, the pH, and other environmental factors.
Bacterial requirements for growth:
A. Nutritional requirements:
1. Essential (major) elements:
They are essential components of proteins, carbohydrates, lipids & nucleic acids. Such as, hydrogen, oxygen, carbon, nitrogen, phosphorus, sulphur, calcium, iron, magnesium, potassium etc. These elements are found in the form of water, inorganic salt, small molecules & macromolecules.
2. Micronutrients:
Needed in very small amount. Such as, zinc, Mn, Cu etc.
3. Organic growth factors:
Some bacteria require organic growth factors which can not be synthesized by themselves. Such as, purines, pyrimidines & amino acids required for synthesis of nucleic acid & some vitamins needed as coenzymes & enzymes.
Nutritional types of bacteria (on the basis of carbon & energy source):
• Autotrophs: use CO2 as a sole source of carbon for growth.
• Heterotrophs: use organic form of carbon for energy.
• Lithotrophs: oxidize inorganic compounds for energy.
• Phototrophs: use radiant energy (light).
Fastidious organisms
Bacteria that require extra nutrients for their growth. e.g. Neisseria, Haemophilus, Lactobacillus.
B. Environmental factors in Bacterial Physiology
• Water
• Oxygen concentration
• Carbon dioxide
• pH
• Temperature
• Light
• Osmotic pressure
• Salt concentration
Effects of pH in Bacterial physiology
• Optimum pH for growth is 7.2 to 7.6
• Neutralophiles grow best at a pH of 6.0– 8.0 (neutral pH)
• Acidophiles: grow at acidic pH (optimum pH 3 to 4.5). e.g. Lactobacilli.
• Alkaliphiles: grow at alkaline pH (pH 9 to 10.5). e.g. V. cholereae.
Effect of temperature in Bacterial physiology
• Optimum temperature for growth is 35 C to 37 C.
•Psychrophilic: grow between 0-20 C. e.g. soil & water saprophytes.
•Mesophilic: grow between 20-40 C. e.g. most of the bacteria.
•Thermophilic: grow between 55-80 C. e.g. B. stearothermophilus.
•Hyperthermophilic can grow above the temperature of boiling water, which exists under high pressure in the depths of the ocean.
Effect of oxygen in Bacterial physiology
According to oxygen requirement, there are 5 groups of bacteria:
(1) Obligate (strict) aerobes: they require oxygen to grow. Such as M. tuberculosis, Pseudomonas.
(2) Facultative anaerobes: can grow in presence or absence of oxygen. Such as E. coli, S. aureus etc.
(3) Obligate (strict) anaerobes: which cannot grow in the presence of oxygen. Such as Clostridium tetani, Clostridium botulinum etc.
(4) Aerotolerant anaerobes: can tolerate low concentration of oxygen (3-5%). e.g. Clostridium perfringens.
(5) Microaerophilic: grow best at low concentration of oxygen (2-10%). e.g. H. pylori.
Why anaerobic bacteria can not grow in presence of oxygen?
• The use of oxygen by bacteria generates two toxic molecules, hydrogen peroxide and the free radical superoxide. These are toxic for bacteria.
• Bacteria require three enzymes to neutralize these toxic molecules. Superoxide dismutase, Catalase & peroxidase.
• Anaerobic bacteria lack all of these enzymes. So, they can not grow in presence of oxygen.
Distribution of superoxide dismutase, catalase & peroxidase in bacteria with different O2 tolerance.
• Organisms requiring high salt concentrations are called halophilic.
• Mild halophilic (require 1-6% salt)
• Moderate halophilic (require 6-15% salt)
• Extreme halophilic (require 15-30% salt)
• Organisms requiring high osmotic pressures are called osmophilic.
• Organisms requiring high concentration of carbon dioxide (5-10%) are called capnophilic. e.g. Neisseria spp, H. influenza etc.
Bacterial growth curve : Bacterial Physiology
If a fixed volume of liquid medium is inoculated with microbial cells taken from a culture that has previously been grown to saturation and the number of viable cells per milliliter is determined periodically and plotted, a curve is usually obtained, called bacterial growth curve.
Phases of growth curvev
1. Lag phase
2. Log phase
3. Stationary phase
4. Decline phase
Lag phase
• The first phase is the lag phase, which represents a period during which cells adapt to their new environment.
• In this phase vigorous metabolic activity occurs but cells do not divide.
• The cells may be growing in volume or mass, synthesizing enzymes, proteins, RNA etc.
• This phase can last for a few minutes up to many hours.
• Surface acting agents (detergent, soap etc) act better in this phase.
• Protein synthesis inhibitor antibiotics can act in this phase
Log phase
• The log (logarithmic) phase is when rapid cell division occurs.
• Cell number increases exponentially with time at a logarithmic rate. So, the log phase is also known as the exponential phase.
• Average duration is usually 8 hours.
• The morphology of the bacteria is best developed in this phase & organism manifests typical biochemical characters.
• This period continues until one of the following things happen:
1. One or more nutrients in the medium become exhausted.
2. Toxic metabolic products accumulate and inhibit growth.
3. Exhaustion of biological space.
4. Lack of oxygen (for aerobic bacteria).
Importance of log phase
• Cell wall inhibiting antibiotics such as β-lactam drugs, act better during this phase because the drugs are effective when cells are making peptidoglycan (i.e., when they are dividing).
• At the end of this phase spore forming bacteria starts to form spores. Continuous culture/maintenance of exponential phase
• Cells can be maintained in the exponential phase by transferring them repeatedly into fresh medium of identical composition while they are still growing exponentially. This is referred to as continuous culture.
• The most common type of continuous culture devices used are chemostat & turbidostat, into which fresh nutrients are added and from which waste products are removed continuously, can remain in the log phase and do not enter into the stationary phase.
• Continuous culture is needed in industrial & research purpose, e.g. in recombinant DNA technology.
Stationary phase
• The stationary phase occurs when nutrient exhausted or toxic products accumulated which cause growth to slow until the number of new cells produced balances the number of cells that die, resulting in a steady state.
• In this phase cell death & multiplication is equal.
• When this occurs, the total cell count slowly increases, although the viable count stays constant.
• This phase can last for few hours to few days.
Importance of stationary phase
• Bacteria produce secondary metabolites such as exotoxins (except diphtheria toxin) & antibiotics.
• Spore forming bacteria undergo sporulation.
Death/decline phase
• The final phase is the death phase. After a period of time in the stationary phase, which varies with the organism and with the culture conditions, the death rate increases until it reaches a steady level, which is marked by a decline in the number of viable bacteria.
• This phase occurs due to exhaustion of nutrients, accumulation of toxic products & autolytic enzymes.
Importance of decline phase
• In this phase, endotoxin release after death of bacteria.
• Diphtheria toxin liberates.
Synchrony of growth : Bacterial Physiology
• All cells in the same stage of all cell cycle.
• Only persists for 1-4 cycles.
• In ordinary culture – growing non synchronously.
• At any moment cells are present in every possible stage of division cycle.
