1. Microbial Growth

2. Growth
When we speak of bacterial growth, we are talking about an increase in population size not an increase in the size of a single bacterium
Animation: Overview of Bacterial Growth
Binary Fission
Bacterial cells replicate by a form of asexual reproduction called binary fission
Animation: Binary Fission

3. Generation Time
Generation Time – time required to complete fission cycle from parent cell to 2 daughter cells. (Doubling time). In terms of a population it is the amount of time needed to double the population.
The length of the generation time is a measure of the Growth Rate of the microbe.
 It varies depending on environmental conditions.
Different microbes have different generation times.
Escherichia coli minutes
Mycobacterium tuberculosis – 18 hours
Mycobacterium leprae days

4. Phases of Microbial Growth Cycle
Lag phase
The bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide.
Log phase or Exponential phase (Doubling period)
Cells in this phase are typically in the healthiest state. Growth is at maximal rate.
Stationary phase
Growth rate of population is zero.
Growth is equal to death
Either an essential nutrient is used up or waste product of the organism accumulates in the medium
Death phase
Lack of nutrients and increasing accumulation of wastes lead to number of cell deaths so the number of new divisions decreases.

5. The Growth Curve

6. Batch culture
Batch culture: a closed-system microbial culture of fixed volume
Typical growth curve for population of cells grown in a closed system is characterized by four phases
Lag phase
Exponential phase
Stationary phase
Death phase
Animation: Bacterial Growth Curve

7. Continuous Culture:
Continuous culture is an open process in which microbial cultures also grow continuously in log phase. One of the nutrients of culture medium is kept limited. Hence, at log phase the cell growth stops as the nutrients of limited quantity is exhausted. In continuous culture, fermented medium is continuously removed from the fermentor.
Chemostat: It controls both population density (i.e. cell density) as well as growth rate of the culture. The chemostat is controlled by two elements, the dilution rate and the concentration of limiting nutrient e.g. carbon or nitrogen sources.

8. Chemostat

9. Turbidostat
It is another type of continuous culture system. It consists of a photocell which measures the turbidity of the culture. The flow rate of medium through vessel is automatically regulated to maintain turbidity.

10. Effects of Temperature on Microbial Growth
Microorganisms can be classified into groups by their growth temperature optima
Psychrophile: low temperature
Psychrotolerant
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile (extreme thermophile): very high temperature

11. Temperature and Growth Relations in Different Classes

12. Effects of Temperature on Microbial Growth
Mesophiles: organisms that have midrange temperature optima; found in
Warm-blooded animals
Terrestrial and aquatic environments
Temperate and tropical latitudes

13. Microbial Growth at Cold Temperatures
Extremophiles
Organisms that have evolved to grow optimally under very hot or very cold conditions
Psychrophiles
Organisms with cold temperature optima; the most extreme representatives inhabit permanently cold environments
Psychrotolerant
Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC; more widely distributed in nature than psychrophiles

14. Microbial Growth at HotTemperatures
Thermophiles
Hyperthermophiles
produce enzymes widely used in industrial microbiology
(E.g., Taq polymerase; used to automate the repetitive steps in the polymerase chain reaction (PCR) technique)

15. pH

16. pH
Most bacteria and protozoa grow best in a narrow range around neutral pH ( ) – these organisms are called neutrophiles
Other bacteria and fungi are acidophiles – grow best in acidic habitats
Acidic waste products can help preserve foods by preventing further microbial growth
Alkalinophiles live in alkaline soils and water up to pH 11.5

17. Physical Effects of Water
Microbes require water to dissolve enzymes and nutrients required in metabolism
Water is important reactant in many metabolic reactions
Most cells die in absence of water
Some have cell walls that retain water
Endospores and cysts cease most metabolic activity in a dry environment for years

18. Oxygen and Microbial Growth
Aerobes: require oxygen to live
Anaerobes: do not require oxygen and may even be killed by exposure
Facultative organisms: can live with or without oxygen
Aerotolerant anaerobes: can tolerate oxygen and grow in its presence even though they cannot use it
Microaerophiles: can use oxygen only when it is present at levels reduced from that in air