Microbial Growth

1
Chapter 6

• Microbial Growth

2
Microbial Growth

• Microbial growth is the increase in number of
  cells, not cell size

3
The Requirements for Growth Physical
Requirements

• Temperature
• Minimum growth temperature
• Optimum growth temperature
• Maximum growth temperature

4
Temperature
Figure 6.1
5
Psychrotrophs

• Grow between 0C and 20-30C
• Cause food spoilage

6
Psychrotrophs
Figure 6.2
7
The Requirements for Growth Physical
Requirements

• pH
• Most bacteria grow between pH 6.5 and 7.5
• Molds and yeasts grow between pH 5 and 6
• Acidophiles grow in acidic environments

8
The Requirements for Growth Physical
Requirements

• Osmotic pressure
• Hypertonic environments, increase salt or sugar,
  cause plasmolysis
• Extreme or obligate halophiles require high
  osmotic pressure
• Facultative halophiles tolerate high osmotic
  pressure

9
The Requirements for Growth Physical
Requirements
Figure 6.4
10
The Requirements for Growth Chemical
Requirements

• Carbon
• Structural organic molecules, energy source
• Chemoheterotrophs use organic carbon sources
• Autotrophs use CO2

11
The Requirements for Growth Chemical
Requirements

• Nitrogen
• In amino acids and proteins
• Most bacteria decompose proteins
• Some bacteria use NH4 or NO3
• A few bacteria use N2 in nitrogen fixation
• Sulfur
• In amino acids, thiamine and biotin
• Most bacteria decompose proteins
• Some bacteria use SO42 or H2S
• Phosphorus
• In DNA, RNA, ATP, and membranes
• PO43 is a source of phosphorus

12
The Requirements for Growth Chemical
Requirements

• Trace elements
• Inorganic elements required in small amounts
• Usually as enzyme cofactors

13
The Requirements for Growth Chemical
Requirements

• Oxygen (O2)

Table 6.1
14
Toxic Forms of Oxygen

• Singlet oxygen O2 boosted to a higher-energy
  state
• Superoxide free radicals O2
• Peroxide anion O22
• Hydroxyl radical (OH?)

15
The Requirements for Growth Chemical
Requirements

• Organic growth factors
• Organic compounds obtained from the environment
• Vitamins, amino acids, purines, and pyrimidines

16
Culture Media

• Culture medium Nutrients prepared for microbial
  growth
• Sterile No living microbes
• Inoculum Introduction of microbes into medium
• Culture Microbes growing in/on culture medium

17
Agar

• Complex polysaccharide
• Used as solidifying agent for culture media in
  Petri plates, slants, and deeps
• Generally not metabolized by microbes
• Liquefies at 100C
• Solidifies 40C

18
Culture Media

• Chemically defined media Exact chemical
  composition is known
• Complex media Extracts and digests of yeasts,
  meat, or plants
• Nutrient broth
• Nutrient agar

19
Culture Media
Tables 6.2, 6.4
20
Anaerobic Culture Methods

• Reducing media
• Contain chemicals (thioglycollate or oxyrase)
  that combine O2
• Heated to drive off O2

21
Anaerobic Culture Methods

• Anaerobic jar

Figure 6.5
22
Anaerobic Culture Methods

• Anaerobic chamber

Figure 6.6
23
Capnophiles Require High CO2

• Candle jar
• CO2-packet

Figure 6.7
24
Selective Media

• Suppress unwanted microbes and encourage desired
  microbes.

Figure 6.9bc
25
Differential Media

• Make it easy to distinguish colonies of different
  microbes.

Figure 6.9a
26
Enrichment Media

• Encourages growth of desired microbe
• Assume a soil sample contains a few
  phenol-degrading bacteria and thousands of other
  bacteria
• Inoculate phenol-containing culture medium with
  the soil and incubate
• Transfer 1 ml to another flask of the phenol
  medium and incubate
• Transfer 1 ml to another flask of the phenol
  medium and incubate
• Only phenol-metabolizing bacteria will be growing

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• A pure culture contains only one species or
  strain.
• A colony is a population of cells arising from a
  single cell or spore or from a group of attached
  cells.
• A colony is often called a colony-forming unit
  (CFU).

28
Streak Plate
Figure 6.10ab
29
Preserving Bacteria Cultures

• Deep-freezing 50to 95C
• Lyophilization (freeze-drying) Frozen (54 to
  72C) and dehydrated in a vacuum

30
Reproduction in Prokaryotes

• Binary fission
• Budding
• Conidiospores (actinomycetes)
• Fragmentation of filaments

31
Binary Fission
Figure 6.11
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Figure 6.12b
33

• If 100 cells growing for 5 hours produced
  1,720,320 cells

34
Figure 6.13
35
Figure 6.14
36
Direct Measurements of Microbial Growth

• Plate counts Perform serial dilutions of a sample

Figure 6.15, step 1
37
Plate Count

• Inoculate Petri plates from serial dilutions

Figure 6.16
38
Plate Count

• After incubation, count colonies on plates that
  have 25-250 colonies (CFUs)

Figure 6.15
39
Direct Measurements of Microbial Growth

• Filtration

Figure 6.17
40
Direct Measurements of Microbial Growth

• Multiple tube MPN test.
• Count positive tubes and compare to statistical
  MPN table.

Figure 6.18b
41
Direct Measurements of Microbial Growth

• Direct microscopic count

42
Direct Measurements of Microbial Growth
Figure 6.19, steps 1, 3
43
Estimating Bacterial Numbers by Indirect Methods

• Turbidity

Figure 6.20
44
Measuring Microbial Growth

• Direct methods
• Plate counts
• Filtration
• MPN
• Direct microscopic count
• Dry weight

• Indirect methods
• Turbidity
• Metabolic activity
• Dry weight