MBIO 3401 LAB EXAM 1 QUESTIONS AND ANSWERS
2026
What does ubiquitous mean? - answers - occuring everywhere
What all are considered to be microorganisms? - answers - eukaryotes (fungi and
protozoans)
Prokaryotes (bacteria)
Archaea
Why are microbes important to our environment? - answers - 1. They can cause
disease in plants and animals
2. Can be used to produce antibiotics and other pharmaceuticals
3. Can be used to produce a wide range of food products
4. Can be used in industry
5. Are important in nutrient cycling
Media: pda - answers - potato dextrose agar
-used for fungal isolation from the air
- incubate agar side down (right side up)
Media: bhi - answers - brain-heart infusion plate
-used to grow microorganisms from the body's surface
-incubate agar side up (upside down)
Media: tsa - answers - tryptic soy agar
-used for salt-tolerant bacteria
-incubated agar side up (upside down)
What types of media are incubated at 30c? And why? - answers - tsa and pda plates
Because 30c is the optimal temperature for microbes that grow in the air
What type of media is incubated at 37c? And why? - answers - bhi plates
Because 37c is the optimal body temperature
Which types of media are incubated agar side down? Agar side up?
What is the reasoning for this? - answers - agar side up (upside down) - tsa and bhi -
bacterial isolation and growth
--to prevent condensation from falling on the medium and contaminating
Agar side down (right side up) - pda - fungal isolation and growth
--because the fungi produce large number of spores and they can fall on the lid in an
inverted position, and a lower chance for water condensation at room temp
,The two general types of microbiological media based on composition: - answers - 1.
Chemically defined medium
2. Complex or rich medium
Chemically defined medium - answers - one where the exact identity and concentration
of each component is known
-usually made up of inorganic salts and simple organic compounds
Complex or rich medium - answers - made up of natural products, like meat, plant, or
yeast extract
-the exact amount of each element is not known
What is a third type of microbiological media? - answers - living medium
Living medium - answers - used for cultivating viruses and bacteria that only grow inside
a eukaryotic cell
-tissue culture is covered with a nutrient broth and incubated at the appropriate
temperature
Why do we sterilize culture media and practice aseptic techniques? - answers - to
prevent undesired microorganisms from contaminating a pure culture and also prevent
pure cultures from contaminating the surrounding environment
Autoclaves - answers - - how wet sterilization is done
-designed to achieve an operating pressure of 15 psi above the normal atmospheric
pressure at sea level causing water to boil at 121c (instead of 100c)
-kills all forms of life (cells and endospores) when maintained for 20 min
Define aseptic technique - answers - - to prevent contamination of cultures and media
from microbes in the environment.
- to transfer cultures from one medium by inoculating another medium
Standard plate count method - answers - counting only living cells
What does high bacterial numbers in milk suggest? - answers - it can suggest a
diseased cow, poor sanitation of milking devices and milk cans, poor sanitation of milk
handlers, contamination of milk during transport to the pasteurization plant,
contaminated pasteurization machinery, and even insufficient pasteurization
Laws state that pasteurized milk must have what kind of bacterial count? Raw milk? -
answers - - less than 20,000 bacteria per ml and 10 coliforms per ml
-no more than 30,000 bacteria per ml
What can happen if milk is not refrigerated properly during transportation? - answers -
natural flora in the milk can grow to potentially dangerous levels
, What is the purpose of pasteurization? - answers - to eliminate any potentially
pathogenic bacteria in raw milk
***how to perform serial dilutions - answers - 1:10, 1:100, 1:1000 and so on
-inoculate one-tenth ml of the desired dilution into the tube of melted agar and pour the
agar into the plate. Doing this with each dilution will allow you to eventually reach a
point where the original inoculum is sufficiently diluted in order to obtain isolated
colonies
Pour plates - answers - a way in which you can obtain isolated colonies by mechanically
diluting a culture
How does the pour plate differ from the streak plate method? - answers - in the pour
plate method the agar medium is inoculated while it is still liquid therefor colonies will
develop on the surface as well as throughout the medium
-on the surface of the agar, the colonies are circular and flat
-the colonies embedded or trapped in the medium are smaller and spindle shaped
Water blanks (dilution blanks) - answers - sterile tubes or bottles containing a measured
amount of sterile water
Plating - answers - the act of pipetting from the dilutions to the empty, sterile petri
dishes
Serial dilutions - answers - dilutions of the original specimen in increments of 10 and/or
100 with sterile water
Plating medium - answers - agar medium to be added to the plated specimens and
dilutions of the specimens
Plated specimens - answers - well-mixed plating medium and specimens or dilutions
that are allowed to solidify
Cfu - answers - colony forming units
To calculate:
Colonies x positive dilution factor = cfu/ml
Calculate cfu if you counted 40 colonies on an 10^-6 plate - answers - 40 cfu * 10^6/ml
= 40,000,000 cfu/ml
3 types of growth patterns that are more commonly used to describe a bacterium: -
answers - 1. An isolated colony on the surface of a nutrient agar plate
2. A nutrient broth culture
3. An agar slant
2026
What does ubiquitous mean? - answers - occuring everywhere
What all are considered to be microorganisms? - answers - eukaryotes (fungi and
protozoans)
Prokaryotes (bacteria)
Archaea
Why are microbes important to our environment? - answers - 1. They can cause
disease in plants and animals
2. Can be used to produce antibiotics and other pharmaceuticals
3. Can be used to produce a wide range of food products
4. Can be used in industry
5. Are important in nutrient cycling
Media: pda - answers - potato dextrose agar
-used for fungal isolation from the air
- incubate agar side down (right side up)
Media: bhi - answers - brain-heart infusion plate
-used to grow microorganisms from the body's surface
-incubate agar side up (upside down)
Media: tsa - answers - tryptic soy agar
-used for salt-tolerant bacteria
-incubated agar side up (upside down)
What types of media are incubated at 30c? And why? - answers - tsa and pda plates
Because 30c is the optimal temperature for microbes that grow in the air
What type of media is incubated at 37c? And why? - answers - bhi plates
Because 37c is the optimal body temperature
Which types of media are incubated agar side down? Agar side up?
What is the reasoning for this? - answers - agar side up (upside down) - tsa and bhi -
bacterial isolation and growth
--to prevent condensation from falling on the medium and contaminating
Agar side down (right side up) - pda - fungal isolation and growth
--because the fungi produce large number of spores and they can fall on the lid in an
inverted position, and a lower chance for water condensation at room temp
,The two general types of microbiological media based on composition: - answers - 1.
Chemically defined medium
2. Complex or rich medium
Chemically defined medium - answers - one where the exact identity and concentration
of each component is known
-usually made up of inorganic salts and simple organic compounds
Complex or rich medium - answers - made up of natural products, like meat, plant, or
yeast extract
-the exact amount of each element is not known
What is a third type of microbiological media? - answers - living medium
Living medium - answers - used for cultivating viruses and bacteria that only grow inside
a eukaryotic cell
-tissue culture is covered with a nutrient broth and incubated at the appropriate
temperature
Why do we sterilize culture media and practice aseptic techniques? - answers - to
prevent undesired microorganisms from contaminating a pure culture and also prevent
pure cultures from contaminating the surrounding environment
Autoclaves - answers - - how wet sterilization is done
-designed to achieve an operating pressure of 15 psi above the normal atmospheric
pressure at sea level causing water to boil at 121c (instead of 100c)
-kills all forms of life (cells and endospores) when maintained for 20 min
Define aseptic technique - answers - - to prevent contamination of cultures and media
from microbes in the environment.
- to transfer cultures from one medium by inoculating another medium
Standard plate count method - answers - counting only living cells
What does high bacterial numbers in milk suggest? - answers - it can suggest a
diseased cow, poor sanitation of milking devices and milk cans, poor sanitation of milk
handlers, contamination of milk during transport to the pasteurization plant,
contaminated pasteurization machinery, and even insufficient pasteurization
Laws state that pasteurized milk must have what kind of bacterial count? Raw milk? -
answers - - less than 20,000 bacteria per ml and 10 coliforms per ml
-no more than 30,000 bacteria per ml
What can happen if milk is not refrigerated properly during transportation? - answers -
natural flora in the milk can grow to potentially dangerous levels
, What is the purpose of pasteurization? - answers - to eliminate any potentially
pathogenic bacteria in raw milk
***how to perform serial dilutions - answers - 1:10, 1:100, 1:1000 and so on
-inoculate one-tenth ml of the desired dilution into the tube of melted agar and pour the
agar into the plate. Doing this with each dilution will allow you to eventually reach a
point where the original inoculum is sufficiently diluted in order to obtain isolated
colonies
Pour plates - answers - a way in which you can obtain isolated colonies by mechanically
diluting a culture
How does the pour plate differ from the streak plate method? - answers - in the pour
plate method the agar medium is inoculated while it is still liquid therefor colonies will
develop on the surface as well as throughout the medium
-on the surface of the agar, the colonies are circular and flat
-the colonies embedded or trapped in the medium are smaller and spindle shaped
Water blanks (dilution blanks) - answers - sterile tubes or bottles containing a measured
amount of sterile water
Plating - answers - the act of pipetting from the dilutions to the empty, sterile petri
dishes
Serial dilutions - answers - dilutions of the original specimen in increments of 10 and/or
100 with sterile water
Plating medium - answers - agar medium to be added to the plated specimens and
dilutions of the specimens
Plated specimens - answers - well-mixed plating medium and specimens or dilutions
that are allowed to solidify
Cfu - answers - colony forming units
To calculate:
Colonies x positive dilution factor = cfu/ml
Calculate cfu if you counted 40 colonies on an 10^-6 plate - answers - 40 cfu * 10^6/ml
= 40,000,000 cfu/ml
3 types of growth patterns that are more commonly used to describe a bacterium: -
answers - 1. An isolated colony on the surface of a nutrient agar plate
2. A nutrient broth culture
3. An agar slant
