Bacterial growth
Assist. Prof. Emrah Ruh
NEU Faculty of Medicine
Department of Medical Microbiology
Bacterial growth
 Microbial growth:
 Increase in the number of microbial cells in a
population
 Growth rate:
 The change in cell number or cell mass per unit
time
 During the cell division cycle, all the structural
components of the cell double
Bacterial growth
 Generation:
 The interval for the formation of two cells from
one cell
 Generation time (doubling time):
 The time required for the cell population to
double
 Vary widely among organisms (many bacteria:
1-3 hr; few bacteria: 10 min or several days!)
Bacterial growth
 The requirements:
 Physical
 Temperature, pH, salt
 Chemical
 Carbon, nitrogen, oxygen
Bacterial growth
Physical requirements – Temperature
 Psychrophiles: can grow at 0°C but optimum is
about 15°C
 Psychotrophs: can grow at 0°C also but optimum
is 20 - 30°C
 Mesophiles: grow best at moderate around 37°C
(many pathogenic microorganisms)
 Thermophiles: have a growth optimum at around
60°C
 Hyperthermophiles: have growth optimum of
80°C or higher (archaea)
Bacterial growth
Physical requirements – Temperature
Bacterial growth
Physical requirements – Temperature
Bacterial growth
Physical requirements – Temperature
 Effect of
temperature on
growth rate and
the molecular
consequences for
the cell
Bacterial growth
Physical requirements – pH
 Most bacteria grow best at pH value between
6.5 and 7.5
 Acidophiles: organisms that grow best at low pH
(as low as pH 1)
 Alkaliphiles: organisms that grow best at high pH
(as high as pH 10) (archaebacteria)
Bacterial growth
Physical requirements – Salt
 In a hypertonic solution, most microbes
undergo plasmolysis
Isotonic
Hypertonic
Bacterial growth
Physical requirements – Salt
 Halophiles: specific requirement for the
sodium ion (microorganisms found in the sea)
 Mild halophile  1-6% NaCl
 Moderate halophile  6-15% NaCl
 Extreme halophile  15-30% NaCl
(archaebacteria)
 Halotolerant: grow best in the absence of
NaCl, but can tolerate some NaCl
Bacterial growth
Physical requirements
 Osmophiles: are able to live in environments
high in sugar
 Xerophiles: are able to grow in very dry
environments
Bacterial growth
Chemical requirements
 Macronutrients
 Micronutrients (trace elements)
 Growth factors
Bacterial growth
Chemical requirements
 Macronutrients
 Required in large amounts
 Micronutrients (trace elements)
 Required in tiny (trace) amounts
 Metals; enzyme structure
Bacterial growth
Chemical requirements
 Growth factors (organic compounds)
 Vitamins, amino acids, purines and pyrimidines
 Required in tiny amounts; only by some bacteria
 Most microorganisms are able to synthesize all of
these compounds; some microorganisms require
one or more of them performed from the
environment
 Vitamins are the most commonly needed growth
factors
Bacterial growth
Chemical requirements
Macronutrients
Micronutrients
Vitamins
Carbon (C)
Chromium (Cr)
p-Aminobenzoic acid
Hydrogen (H)
Cobalt (Co)
Folic acid
Oxygen (O)
Copper (Cu)
Biotin
Nitrogen (N)
Manganese (Mn)
Cobalamin
Phosphorus (P)
Molybdenum (Mn)
Lipoic acid
Sulfur (S)
Nickel (Ni)
Nicotinic acid (niacin)
Potassium (P)
Selenium (Se)
Pantothenic acid
Magnesium (Mg)
Tungsten (Tu)
Riboflavin
Sodium (Na)
Vanadium (V)
Thiamine
Calcium (Ca)
Zinc (Zn)
Vitamins (B6)
Iron (Fe)
Iron (Fe)
Vitamin K group; quinones
Hydroxamates
Bacterial growth
Chemical requirements – Carbon
 The major element in all classes of
macromolecules
 A typical cell is about 50% carbon
 All organisms require a carbon source;
chemoheterotrophs use an organic molecule
and autotrophs typically use carbon dioxide
Bacterial growth
Chemical requirements – Nitrogen
 A typical bacterial cell is about 12% nitrogen
 Important element in proteins, nucleic acids,...
 Bacteria obtain nitrogen from ammonia (NH3) or
nitrate (NO3-); a few bacteria (nitrogen-fixing
bacteria) obtain from nitrogen gas (N2)
Bacterial growth
Chemical requirements – Oxygen
 Oxygen requirement:
 Obligate aerobes
 Microaerophiles
 Facultative anaerobes
 Aerotolerant anaerobes
 Obligate anaerobes
Requirement
for oxygen
Bacterial growth
Chemical requirements – Oxygen
Bacterial growth
Chemical requirements – Oxygen
 Aerobic bacteria: grow on the top of the tube in
order to absorb maximal amount of O2 for survival.
 Microaerophilic bacteria: grow just below surface as
they require O2 at low concentration. They are
sensitive to O2 -derived free radicals.
 Facultative anaerobic bacteria: grow throughout the
tube. These bacteria can use O2. But in the absence
of O2, they have anaerobic methods of energy
production, so they can live all along the test tube.
Bacterial growth
Chemical requirements – Oxygen
 Aerotolerant anaerobic bacteria: cannot use O2
for growth but also are not affected by it and
able to survive in the presence of some O2. So,
they can spread a little bit upward from the
bottom of the test tube.
 Anaerobic bacteria: cannot use O2 for growth
and are even harmed by it. So, they grow at the
bottom in order to avoid O2.
Bacterial growth
Chemical requirements – Oxygen
 Aerobes, facultative anaerobes, and
aerotolerant anaerobes must have the following
enzymes to avoid toxic O2 radicals:
 Superoxide dismutase (2O2- + 2H+
 Catalase (2H2O2
2H2O + O2)
or peroxidase (H2O2 + 2H+
2H2O)
O2 + H2O2)
Bacterial growth
Media
 Medium: is any material prepared for the growth
of bacteria in a laboratory
 Culture: Microbes that grow and multiply in a
medium
Bacterial growth
Media
 Media used in microbiology labs are divided into
four groups according to their agar content
 Agar is a biological substance extracted from sea
algae
 Agar solidifies the media
 > 90oC: liquid phase
 < 40oC: solid phase
Bacterial growth
Media
• Liquid media; do not include agar
• Semi-liquid media; include 0,05-0,2 % agar
• Semi-solid media; include 0,3-0,5 % agar
• Solid media; include 1,5-2 % agar
Bacterial growth
Media
Liquid
medium
Semi-liquid
medium
Semi-solid
medium
Solid
medium
Bacterial growth
Media
Bacterial growth in solid culture is observed as
colony formation
Colony
Bacterial growth
Media
Bacterial growth in liquid culture is observed as
turbidity
Turbidity
Medium (no bacteria)
Culture (bacteria)
Bacterial growth
Measurement of bacterial growth
 Counts are reported as the number of colony-
forming units (CFU) instead of the number of
cells
 If the concentration of bacteria is too great,
the colonies will grow into each other and the
plate will be uncountable
 To insure a countable culture plate, a series of
dilutions should be prepared
Bacterial growth
Measurement of bacterial growth
Plate no. 4  32 colonies X 10,000 = 320,000 cfu/ml
Bacterial growth
Media - Ingredients
 Simple growth requirements:
 Nutrient broth ( + agar: Nutrient agar)
Constituent
Peptone (provides vitamins and minerals)
Beef extract (supplies energy, C, N and S)
Sodium chloride
Agar
Water
Bacterial growth
Media – General purpose media
 General purpose media:
 Simple growth requirements
Nutrient broth
Nutrient agar
Bacterial growth
Media – General purpose media
 General purpose media:
 Simple growth requirements + blood: Blood agar
 The most common media in microbiology!
Bacterial growth
Media – Special purpose media
 Special purpose media:
 Selective media: supress unwanted microbes;
encourage desired microbes
 Differential media: differentiate the colonies of
desired microbes from others
 Enrichment media: increase the numbers of
desired microbes to detectable levels
Bacterial growth
Media – Special purpose media
 Special purpose media:
 Endo agar, EMB agar and MacConkey agar:
Selective and differentiative media
 Selective for Gram-negative bacteria (suppress
Gram-positive bacteria)
 Differentiative for lactose-fermenting bacteria
(Lactose (+) bacteria produce colourful colonies)
Bacterial growth
Media – Special purpose media
Lactose fermentation test
Lactose (+) bacteria
produce
colourful colonies
Bacterial growth
Bacterial division
 Binary fission:
 The normal reproductive method of bacteria
 A single cell divides into two identical cells
Bacterial growth
Bacterial division
Bacterial growth
Logarithmic representation
 Logarithmic representation of bacterial
populations:
 Bacterial division occurs according to a
logarithmic progression (two cells, four
cells, eight cells,...)
Bacterial growth
Logarithmic representation
 Logarithmic representation of bacterial
populations:
Bacterial growth
Phases of growth
 Lag phase
 Log (exponential) phase
 Stationary phase
 Death phase
Bacterial growth
Phases of growth
Bacterial growth
Phases of growth
 Lag phase:
 No change in the number of cells; metabolic
activity is high
 DNA and enzyme synthesis occurs
Bacterial growth
Phases of growth
 Log (exponential) phase:
 Bacteria multiply at the fastest rate (under
the optimal conditions)
 Logarithmic or exponential increase in
population
Bacterial growth
Phases of growth
 Stationary phase:
 Equilibrium between cell division and death
 Nutrients are exhausted and waste
products build up; pH increases
Bacterial growth
Phases of growth
 Death phase:
 The number of deaths exceeds the number
of new cells formed
 Population decreases at a logarithmic rate