Microbial Growth
Chapter 6
Biology 1009-Summer 2003
Johnson
Microbial Growth
n Refers
to the number of cells, not the
size of the cells
n Requirements for growth
*Two general types:
– Physical: temp, pH, osmotic pressure
– Chemical: elements, including C, N, S, O,
and P
Physical Requirements
•Temperature
•Psychrophile: cold loving
•Range
0C-20C
•Mesophile:
•Range
moderate temperature loving
20C-40C
•Thermophile
•Range
heat loving
40C-100C
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Physical Requirements (cont.)
n pH
– Most bacteria grow between pH 6.5-7.5
– Very few grow below pH 4
– Some foods (pickles & sauerkraut)
preserved from spoilage by acids made by
bacterial fermentation
Physical Requirements (cont.)
n Osmotic
Pressure
– Microbes obtain most of their nutrients in
solution from surrounding water
– Many foods (honey, salted fish) preserved
this way: high sugar or salt { } draws water
out of bacterial cells, preventing their
growth
Chemical Requirements
n Carbon-50%
of bacteria cell dry wt
– Microbes obtain from organic materials or
CO2
n Iron,
copper, and zinc (trace)
– Used to make enzymes
n Nitrogen,
sulfur, and phosphorus-14%
of bacterial cell dry wt
– Needed for synthesis of cellular material
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Other chemical requirements
n Oxygen
– Bacteria classified based on their oxygen
requirements
• 4 main classifications
Obligate Aerobes
n Require
oxygen to live
Obligate Anaerobes
n No
growth in the presence of oxygen
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Facultative
n Bacterial
growth with or without oxygen
Microaerophilic
n Grow
best in low concentrations of
oxygen (as compared with the normal
21%)
Oxygen Requirements
4
Toxic Forms of Oxygen
n
Singlet oxygen-normal molecular oxygen that
has been boosted into high energy state
n
Superoxide free radicals-formed by all living
organisms during respiration
n
Peroxide anion-found in hydrogen peroxide
Superoxide dismutase
SOD
n O2 +
O2
+2
H+ -----àH2O2
SOD-neutralizes oxygen free radicals
*ok, but hydrogen peroxide also toxic…
Catalase
catalase
n2
H2O2 ----------à 2 H2O + O2
– Obligate anaerobes lack catalase and superoxide
dismutase-they die in the presence of oxygen
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Culture media
n Nutrient
material prepared for the
growth of microbes
– Two main types
• Chemically defined:
– Exact chemical composition is known
– Used to grow very specific organisms
• Complex:
– Exact chemical composition unknown
– Most bacteria and fungi grown on this type
Special Culturing Procedures
n Anaerobic
microbes
– Require reducing media that removes free
oxygen
• Example-fluid thioglycollate media
– Require anaerobic containers
• Example-gas pak jar
– Require agar stab inoculations
Anaerobic Gas Pak
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More special culturing
procedures…
n
Microaerophilic Bacteria
– Grow best at reduced oxygen levels and
increased carbon dioxide
– Utilizes a candle jar
Normal oxygenCandle jar oxygenNormal carbon dioxideCandle jar carbon dioxide-
Candle Jar-used to grow microaerophilic
microbes
Selective Media
n Inhibits
the growth of some bacteria
while selecting for the growth of
other organisms
–Examples:
*Brilliant green: isolates Gram –
Salmonella
*EMB: isolates Gram – GI tract
microbes
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Differential Media
n
Differentiate between different organisms
growing on the same plate
– Example:
• Blood agar plate (TSA with 10% sheep
blood)
–Used to differentiate types of
Streptococci
»Alpha, beta, and gamma
hemolysis
Bacterial Division
n Increases
the number of cells, not size
ØBinary Fission-single cell divides into 2
identical cells
ØMost bacteria:1 to 3 hr generation time
ØE. coli-generation time of 20 minutes
*20 generations (about 7 hours)
• yields 1 million cells
Binary Fission
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Other bacterial division
methods…
n Budding-asexual
form in which an
outgrowth is formed, enlarges, and
breaks free
n Fragmentation-portion of cell breaks off
and generates a new cell
– Filamentous species only (Actinomycetes)
Limiting Environmental Factors
that Affect Bacterial Growth
n Lack
of food and water
n Space
n Accumulation
of waste products
n Lack of oxygen
n Changes in pH
n Temperature fluxuations
Phases of Growth
n Graph
indicating the growth of a
bacterial population over time
n Four phases
– Lag
– Log
– Stationary
– Death
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Lag Phase
n Bacteria
first introduced into
environment or media
n Bacteria get used to their surroundings
n Bacteria very active metabolically
n Little to no change in cell #
n Lasts 1 hr to several days
Log Phase
n Rapid
(exponential) cell growth
population doubles every
generation
n Bacteria sensitive to adverse conditions
n Bacterial
Stationary Phase
n
n
Death Rate=rate of reproduction
Cells begin to encounter environmental stress
– Lack of water
– Lack of nutrients
– pH change
– High amounts of metabolic waste
– Lack of space
Note: Endospores form at this time!
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Death Phase
n Number
of deaths exceeds the number
of new cells formed
n Due to the limiting factors in the
environment
Bacterial Growth Curve
Direct Measurement of Microbial
Growth
1)
Plate counts-most used method/require serial dilutions
*example-pour plates (30-300 colonies countable)
2)
Filtration-passage of liquid through a grid containing small
pores (< .45 um)
3)
Most Probable Number-statistical method used with bacteria
that won’t grow on media
4)
Direct Microscopic Count-known volume of bacterial
suspension placed on special cell counting slide
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Direct Microscopic Count
Indirect Methods of Estimating
Bacterial Numbers
1)
Turbidity-cloudiness produced as media
becomes filled with bacteria
2)
Metabolic Activity-assumes certain amounts
of metabolic wastes are in direct proportion
to # of bacteria
3)
Dry Weight-organisms removed from
media, filtered, and weighed
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