Microbial Growth
0 In microbiology growth is defined as an increase in
the number of cells.
0 Knowledge of how microbial populations expand is
useful when designing methods to control microbial
growth.
Factors that Affect Microbial
Growth
0 Physical Factors
0 Temperature
0 pH
0 Osmotic pressure
0 Chemical Factors
Physical Requirements
0 A. Temperature
0 Optimal growth temperature
0 Permissible range
0 human pathogens optimal = 37°C
Physical Requirements
0 Temperature
0 Psychrophile: cold loving
0 Range: 0C-20C
0 Mesophile: moderate temp. loving
0 Range: 20C-40C
0 Thermophile: heat loving
0 Range: 40C-100C
Physical Requirements
0 B. pH
0 Acidophiles- “acid loving”
0 Acidity inhibits most microbial growth and is used frequently for food
preservation (Ex: pickling)
0 Certain bacteria, such as those in sauerkraut and yogurt, prefer acidic
environments of 6.0 or below.
0 Fungi tend to live in slightly acidic environments pH 5-6.
0 Molds and yeast grow in wider pH range, but prefer pH between
5 and 6.
0 Neutrophiles0 most organisms optimal pH 6.5-7.5 (near neutral)
0 Since the pH of most human tissue is 7.0 to 7.2, these neutrophilic
bacteria usually grow well in the body. (Most human pathogens)
0 Alkaliphiles- “alkali loving”
0 Alkalinity inhibits microbial growth, but not commonly used for
food preservation.
0 Grow at pH of 7-12 or higher
0 Example: Vibrio cholerae optimal pH= 9
Physical Requirements
0 C. Osmotic pressure- Cells are 80-90 % water
0 Most bacteria require isotonic solutions (no net flow of water in
either direction of cell)
0 human blood = 0.9% NaCl, isotonic
0 human skin = ~3-6% NaCl, hypertonic
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High Osmotic pressure
In extremely salty solution,
bacteria lose water through
osmosis causing plamolysis
(shrinking of cell mb.
•
Bacteria are in equilibrium
with their environment
•
•
Bacteria gain water and cell
may burst
This is an example of how
antibiotics work
Chemical Requirements
0 A. Carbon- structural backbone of all organic compounds
0 B. Nitrogen, Sulfur and Phosphorus
0 Needed for synthesis of cellular material
0 (all above: to build organic molecules)
0 E. Trace elements: K+, Mg2+, Ca2+ , Fe2+ ...
0 Used to make enzymes
0 F. Oxygen- can classify mo’s based on O2 requirements
Aerobe-requires O2
Pseudomonas- common
nosocomial pathogen
Anaerobe-can’t use O2
Clostridium bacteria
Facultative-with or
w/o O2
E. coli, staphylococcus, yeast,
many intestinal bacteria
Microbial Growth
0 Growth of Bacterial Cultures
0 Logarithmic representation of Bacterial Growth
0 Phases of growth
0 Bacterial Growth Curve
0 When bacteria are inoculated into a liquid growth medium,
we can plot of the number of cells in the population over
time.
Bacterial Growth Curve
Lag phase
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•
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•
Little to no growth
Pop. doesn’t
increase
Bacteria acclimate
to new environment
Intense metabolic
activity= growth in
size
Log phase
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Period of
exponential growth
with constant
generation time
Period of most rapid
growth
Cells are more
susceptible to
adverse
environmental
factors
Example: antibiotics
Stationary phase
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•
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Cell growth= cell
death, stability
Slow microbial
growth influenced
by limited nutrients
Low O2
Accumulation of
toxic waste
Death phase
•
Cell death exceeds
cell growth