This document provides an overview of microbial growth and the growth cycle of bacteria. It discusses binary fission as the process by which bacteria divide, as well as doubling time and exponential growth. The four phases of bacterial growth are described as lag phase, exponential/log phase, stationary phase, and death phase. The key requirements for bacterial growth are also outlined, including temperature, pH, osmotic pressure, carbon, nitrogen, sulfur, phosphorus, oxygen, and organic growth factors. Various microorganisms are classified based on their temperature, pH, salt, and oxygen requirements. The roles of these different factors and requirements in microbial growth are explained.

1. WELCOME
Dr. Hamima Hasnat
Assistant Professor
Department of Microbiology
Eastern Medical College, Cumilla

2. Microbial growth

3. Learning objectives :
• What is binary fission?
• What is doubling time or generation time?
• In details about growth cycle of bacteria.
• What are the components required for bacterial growth?

4. Binary fission
• Bacteria divide by a Process called Binary Fission.
• Binary Fission is a type of asexual reproduction in which a cell
divides to produce two nearly equal-sized progeny cells.

5. Binary fision involves three processes:
1. Increase in cell size (cell elongation),
2. DNA replication and
3. Cell division

7. Doubling time/ Generation time
• The interval of time between two cell divisions, or time
required for the bacterium to double under optimum condition
is known as the generation time.

8. • E.g., doubling (generation) time 20 minutes for E. coli, 18
hours for Mycobacterium tuberculosis, 24 days for Lepra
bacilli .
• An ordinary bacterium doubles every 20-30 minutes.

9. • Logarithmic or Exponential Growth: The population of
bacterial cells divide at a constant rate so that the total number
of cells doubles with each division.

10. Bacteria
Undergo
Exponential
Growth

11. • The growth cycle of bacteria has four major phases:
lag,
exponential (log),
stationary, and
death

12. 1. Lag phase: It is the period where the individual bacteria are
maturing and not yet able to divide.
During the lag phase of the bacterial growth cycle, synthesis
of RNA, enzymes and other molecules occurs
vigorous metabolic activity occurs but cells do not divide.
Last for a few minutes up to many hours.

13. The length of lag phase depends upon :
a. Type of bacteria.
b. Better the medium, shorter the lag phase.
c. The phase of culture from which inoculation is taken.
d. Size or volume of inoculum.
e. Environmental factors like temperature.

14. 2. The log (logarithmic) phase : In logarithmic phase the
bacterial cell start dividing.
rapid cell division occurs.
their number increase by geometric progression with time
Continue as long as cells have adequate nutrients & good
environment
Many antibiotics, such as penicillin, are most efficacious
during this phase

15. 3. The stationary phase: Reproduction rate is balanced by the
death rate and population remains constant.
There is exhaustion of some nutrients and accumulation of
some toxic materials
In this stage microbial death is equal to microbial growth
Metabolic activity of surviving cells slows down which
stabilizes the population

16. The insufficient supply of nutrients also causes some
bacteria to form spores during this phase.
Cells frequently are gram variable and show irregular
staining due to the presence of intracellular storage granules.

17. Why enter the stationary phase:
• Nutrient limitation (slow growth)
• Oxygen limitation
• Accumulation of toxic waste products
• Critical population density reached

18. 4. Death or Decline phase: It is final phase which is marked by
a decline in the number of viable bacteria.
death exceeds division.
Sometimes a small numbers of survivors may persist for
month even after death of majority of cells these few surviving
cells probably grow at expence of nutrients released

19. The factors responsible are:
a. Nutritional exhaustion
b. Toxic accumulation
c. Autolytic enzymes

20. • The growth curve: It indicates logarithm of the number of
viable cells versus the incubation time.

22. • Requirement for microbial growth:
Physical requirements Chemical requirements
Temperature Carbon
pH Nitrogen, Sulfur and
phosphorus
Osmotic pressure oxygen
Organic growth factors

23. • Temperature: Different microbial species vary widely in their
optimal temperature ranges for growth.
1. Psychrophilic: grow best at low temperatures (–5 to 15°C).
e.g: Yersinia enterocolitica , Listeria monocytogens.
2. Psychrotrophs: have a temperature optimum between 20°C
and 30°C but grow well at lower temperatures.
3. Mesophilic: grow best at 30–37°C, Most organisms are
mesophilic. E.g. Escherichia coli , staphylococcus aureus , etc.

24. 4. Thermophilic: grow best at 50–60°C. E.g.Bacillus
sterothermophilus.
5. Hyperthermophilic : can grow at well above the temperature
of boiling water, which exists under high pressure in the
depths of the ocean.

26. • Hydrogen Ion Concentration (pH): Most organisms have a
fairly narrow optimal pH range.
1. Neutralophiles : Most organisms grow best at a pH of 6.0–
8.0,
2. Acidophiles : have optima as low as pH 3.0, (e.g,
Lactobacillus spp.)
3. Alkaliphiles: have optima as high as pH 10.5. (e.g, Vibrio
cholera)

27. Hydrogen Ion Concentration (pH) for
bacterial growth

28. • Osmotic Pressure :Most bacteria require isotonic solutions. If
the cells are placed in a high salt or sugar solution the cells
lose water and plasmolysed.
1. Halophiles: Require moderate to large salt concentrations.
E.g, Most bacteria in oceans.
2. Extreme or Obligate Halophiles: Require very high salt
concentrations (20 to 30%). E.g, Halobacterium.

29. 3. Facultative Halophiles: Do not require high salt concentrations
for growth, but tolerate 2% salt or more. E.g, Vibrio
parahaemolyticus.

30. Isotonic Versus Hypertonic Solution
Plasmolysis

31. Effects of Osmosis on Bacterial Cells

32. • Carbon:- Structural organic molecules, energy source .
1. Heterotrophs : Obtain carbon from their energy source:
lipids, proteins, and carbohydrates.
2. Autotrophs : Obtain carbon from carbon dioxide.

33. • Nitrogen, Sulfur, and Phosphorus:
1. Nitrogen: Makes up 5% of dry cell weight. Used to form
amino acids, DNA, and RNA.
2. Sulfur: Sulfur is a component of many organic cell
substances. It forms part of the structure of several
coenzymes and some vitamins (thiamin and biotin).

34. 3. Phosphorus: Used to form DNA, RNA, ATP, and
phospholipids, coenzymes like NAD, NADP, and flavins. In
addition, many metabolites, lipids (phospholipids, lipid A), cell
wall components (teichoic acid), some capsular
polysaccharides, and some proteins are phosphorylated.

35. • Oxygen :
1. Obligate aerobes: Require oxygen to live. Example:
Pseudomonas spp.
2. Facultative anaerobes: able to live aerobically or
anaerobically; Examples: E. coli, Staphylococcus.
3. Obligate anaerobes : Cannot use oxygen and are harmed by
the presence of toxic forms of oxygen.
Examples: Clostridium bacteria that cause tetanus and
botulism.

36. 4. Microaerophiles: which require small amounts of oxygen (2–
10%) for aerobic respiration (higher concentrations are
inhibitory). Example: Campylobacter.
5. Aerotolerant anaerobes: Can’t use oxygen, but tolerate its
presence. Can break down toxic forms of oxygen. Example:
Lactobacillus carries out fermentation regardless of oxygen
presence.

37. • Toxic Forms of Oxygen:
1. Superoxide Free Radicals (O2-.): Extremely toxic and reactive
form of oxygen. All organisms growing in atmospheric
oxygen must produce an enzyme superoxide dismutase
(SOD), to get rid of them. SOD is made by aerobes,
facultative anaerobes, and aerotolerant anaerobes, but not by
anaerobes or microaerophiles.
Reaction:
O2-. + O2-. + 2H+ ----------------> H2O2 + O2
SOD
Superoxide
free radicals
Hydrogn
peroxide

38. 2. Hydrogen Peroxide (H2O2): Peroxide ion is toxic .There
are two different enzymes that break down hydrogen
peroxide:
A. Catalase: Breaks hydrogen peroxide into water and O2.
2H2O2----------> 2H2O + O2
Gas
Bubbles
Hydrogen
peroxide
catalase

39. • B. Peroxidase: Converts hydrogen peroxide into water.
H2O2 + 2H+---------------> H2O
• All strict anaerobes lack both superoxide dismutase and
catalase.
Hydrogen
peroxide
Peroxidase

40. Other Elements:
• Potassium, magnesium, and calcium are often required as
enzyme cofactors.
• Calcium is required for cell wall synthesis in Gram positive
bacteria.
Trace Elements:
• Many are used as enzyme cofactors. Commonly found in tap
water.
• Iron, Copper, Molybdenum Zinc.

41. Organic growth factors:
• Organic compounds are required by bacteria for their growth
and maintenance.
• Such as vitamins, amino acids, purines, and pyrimidines.
• A growth factor may be essential for some bacteria and
accessory for other bacteria, and totally unrequired by some
others.
• E.g. H.influenza requires accessory growth factors X & V.

42. hamimahasnat@yahoo.com

43. Thank you