BIOS 2150 Microbiology Thursday 9/20/18
1. Temperature (Section 9.4)
Psychrophiles: prefer cooler temperatures (below 15 degrees C). Do not
live in humans. (Psychorotrophs grow at higher temps and can cause
spoilage of refrigerated foods).
Mesophiles: prefer 25-40 degrees C. Live in humans as pathogens and non-
pathogens.
Thermophiles: prefer 50-60 degrees C. Found in compost piles and hot
springs.
o Often domain archaea
o Often not pathogens to humans
Temperature Considerations:
Minimum growth temperature
o Doesn’t necessarily kill them… they just stop metabolizing
Should your refrigerator be set above or below this temperature? Below 40.
Optimum growth temperature
o Where they are going to do best. Human pathogens typically like 37
degree Celcius
Maximum growth temperature
o The highest they can withstand
o As you get closer to the minimum and maximum the growth slows
Should buffet food lines hold food above or below this temperature? Above.
2. pH (Section 9.3)
Most bacteria prefer a pH that is near neutral (6.5-7.5)
o pH is a measure of the hydrogen ion (H+) concentration
o Scale runs from 0-14. At 7 it is a one-to-one ratio. H+OH-
As you go down the scale there is a excess of H+
Up the scale you have an excess of OH-
o Hydrogen ions binds to bacterial enzymes and cause them to fail
(preventing the bacteria from metabolizing)
Fungi can tolerate more acidic conditions
Microbes called Acidophiles grow in acidic environments
3. Osmotic Pressure (Section 9.5 page 391)
(Recall the function of the bacterial cell wall from Unit 1) The cell wall
protects the cell from blowing up with an excess of water
Plasmolysis: loss of water from a cell
o Stop metabolizing due to loss of water. Water loss slows them down.
High salt and sugar concentration can limit the growth of bacteria
Fungi can withstand changes in osmotic pressure better than bacterial cells
and typically require less water
o See the theme? Fungi hold up better in more diverse environments
Hypertonic environments (higher is solutes than inside the cell) cause
plasmolysis due to high osmotic pressure
1
1. Temperature (Section 9.4)
Psychrophiles: prefer cooler temperatures (below 15 degrees C). Do not
live in humans. (Psychorotrophs grow at higher temps and can cause
spoilage of refrigerated foods).
Mesophiles: prefer 25-40 degrees C. Live in humans as pathogens and non-
pathogens.
Thermophiles: prefer 50-60 degrees C. Found in compost piles and hot
springs.
o Often domain archaea
o Often not pathogens to humans
Temperature Considerations:
Minimum growth temperature
o Doesn’t necessarily kill them… they just stop metabolizing
Should your refrigerator be set above or below this temperature? Below 40.
Optimum growth temperature
o Where they are going to do best. Human pathogens typically like 37
degree Celcius
Maximum growth temperature
o The highest they can withstand
o As you get closer to the minimum and maximum the growth slows
Should buffet food lines hold food above or below this temperature? Above.
2. pH (Section 9.3)
Most bacteria prefer a pH that is near neutral (6.5-7.5)
o pH is a measure of the hydrogen ion (H+) concentration
o Scale runs from 0-14. At 7 it is a one-to-one ratio. H+OH-
As you go down the scale there is a excess of H+
Up the scale you have an excess of OH-
o Hydrogen ions binds to bacterial enzymes and cause them to fail
(preventing the bacteria from metabolizing)
Fungi can tolerate more acidic conditions
Microbes called Acidophiles grow in acidic environments
3. Osmotic Pressure (Section 9.5 page 391)
(Recall the function of the bacterial cell wall from Unit 1) The cell wall
protects the cell from blowing up with an excess of water
Plasmolysis: loss of water from a cell
o Stop metabolizing due to loss of water. Water loss slows them down.
High salt and sugar concentration can limit the growth of bacteria
Fungi can withstand changes in osmotic pressure better than bacterial cells
and typically require less water
o See the theme? Fungi hold up better in more diverse environments
Hypertonic environments (higher is solutes than inside the cell) cause
plasmolysis due to high osmotic pressure
1
