Microbiology Lab Exam # 1 with Verified
Solutions
Considering the cultures used to inoculate each medium in this exercise, how many different
microbial types should you expect to see in/on each medium? (1.4) Correct Ans-You should
only have 1 if using good sterile technique. A successful aseptic transfer will have only
micrococcus lutes growth- this would be a pure culture
You were asked to describe differences in appearance of growth in each culture, if present. In
which medium was this the most difficult to determine? What made it difficult? (1.4) Correct
Ans-More difficult to see in broth because you can only see if growth occurred on the top or
bottom etc. Better in slant because you can observe different growth patterns.
Which medium was most difficult for you to transfer from? Which medium was most difficult
for you to inoculate? Why? (1.4) Correct Ans-Most people don't like working with slants
(solid)
If you got growth on sterile NA and NB slant rubes, why? (1.4) Correct Ans-Possible
contamination include not heating the loop to orange-hot, not holding the open tubes on an angle,
and/or placing the cap on the table surface during the process.
Did you notice a difference in density (turbidity) of growth in NB tubes inoculated from NB and
NA slants? Possible reasons?( 1.4) Correct Ans-Generally, more cells are transferred from
growth on a solid medium than from a broth culture. Therefore, broth cultures made from growth
on a solid medium will show greater turbidity than those inoculated from a broth culture.
,Did you notice a difference in density of growth on NA slants inoculated from NA slants and
NB? (1.4) Correct Ans-Generally there will be denser growth on the slant inoculated from the
NA slant, because more cells are transferred from solid medium than broth.
Pure culture (1.4) Correct Ans-When a culture ( a medium that contains living microbes)
contains a single species.
Broths (1.4) Correct Ans-A medium used to grow microbes when fresh cultures or large
numbers of cells are required. Used for ID.
Agar Slant (1.4) Correct Ans-A type of medium used to grow stock cultures that can be
refrigerated after incubation and maintained for several weeks.
Plated Media (1.4) Correct Ans-A type of medium used for obtaining isolation of species,
differential testing, and quantifying bacterial densities.
Inoculating loops (1.4) Correct Ans-An instrument used to inoculate a medium.
Inoculation (1.4) Correct Ans-To introduce (cells or organisms) into a culture medium.
Using a pencil, draw a quadrant streak (1.5) Correct Ans-Should look like this. You should
also rotate a little less than 90 Degrees each streak, and heat the loop so you can get good results.
Also, let the loop cool!!
,Mixed Culture (1.5) Correct Ans-A microbial culture consisting of two or more species.
What is generally the first step in identifying a microbial organism? (1.5) Correct Ans-
Obtaining isolations of Individual colonies. The technique we used in class was the isolation
technique- the streak plate. Cells that have been sufficiently isolated will grow into colonies,
consisting only of the original cell type.
Colonies (1.5) Correct Ans-Individual microbial cell type. They can also form from a pair,
chain, or cluster of cells.
Colony Forming Unit (CFU) (1.5) Correct Ans-A more correct description of the colony
origin
Zigzag Inoculation (1.5) Correct Ans-A type of inoculation pattern used to when the sample
does not have a high enough cell density.
Quadrant streak technique: (1.5)
- how much space should you try to use?
- describe what each of the quadrants should look like
- describe order Correct Ans-- maximize the space used
, - Q1: confluent growth (colonies overlapping), max growth
- Q2: less growth, some separation
- Q3: even less growth
- Q4: isolated colonies
- flame, only go to culture once at the beginning, Q1, flame, Q2, flame, Q3, flame, Q4, flame
Quadrant plate questions: (1.5)
- What if no overlap?
- too much overlap?
- plate with white and yellow colonies is what type of plate? Correct Ans-- no overlap --> no
isolation
- too much overlap --> too much confluent growth
- white colonies & yellow colonies --> mixed plate
Confluent Growth (1.5) Correct Ans-When you have confluent growth, the parental bacteria
are too close together in space, and so as they reproduce and their progeny reproduce, you see a
mass of cells that cover the surface of the agar - and in this case, it isn't possible to separate out
which bacteria are the descendants from a single original cell.
Most colonies on streak plates grow from isolated colony-forming units. On rare occasions,
however, a colony can be a mixture of two different organisms. If a culture is started from this
colony (thinking it's pure), correct identification will be next to impossible because the extra
Solutions
Considering the cultures used to inoculate each medium in this exercise, how many different
microbial types should you expect to see in/on each medium? (1.4) Correct Ans-You should
only have 1 if using good sterile technique. A successful aseptic transfer will have only
micrococcus lutes growth- this would be a pure culture
You were asked to describe differences in appearance of growth in each culture, if present. In
which medium was this the most difficult to determine? What made it difficult? (1.4) Correct
Ans-More difficult to see in broth because you can only see if growth occurred on the top or
bottom etc. Better in slant because you can observe different growth patterns.
Which medium was most difficult for you to transfer from? Which medium was most difficult
for you to inoculate? Why? (1.4) Correct Ans-Most people don't like working with slants
(solid)
If you got growth on sterile NA and NB slant rubes, why? (1.4) Correct Ans-Possible
contamination include not heating the loop to orange-hot, not holding the open tubes on an angle,
and/or placing the cap on the table surface during the process.
Did you notice a difference in density (turbidity) of growth in NB tubes inoculated from NB and
NA slants? Possible reasons?( 1.4) Correct Ans-Generally, more cells are transferred from
growth on a solid medium than from a broth culture. Therefore, broth cultures made from growth
on a solid medium will show greater turbidity than those inoculated from a broth culture.
,Did you notice a difference in density of growth on NA slants inoculated from NA slants and
NB? (1.4) Correct Ans-Generally there will be denser growth on the slant inoculated from the
NA slant, because more cells are transferred from solid medium than broth.
Pure culture (1.4) Correct Ans-When a culture ( a medium that contains living microbes)
contains a single species.
Broths (1.4) Correct Ans-A medium used to grow microbes when fresh cultures or large
numbers of cells are required. Used for ID.
Agar Slant (1.4) Correct Ans-A type of medium used to grow stock cultures that can be
refrigerated after incubation and maintained for several weeks.
Plated Media (1.4) Correct Ans-A type of medium used for obtaining isolation of species,
differential testing, and quantifying bacterial densities.
Inoculating loops (1.4) Correct Ans-An instrument used to inoculate a medium.
Inoculation (1.4) Correct Ans-To introduce (cells or organisms) into a culture medium.
Using a pencil, draw a quadrant streak (1.5) Correct Ans-Should look like this. You should
also rotate a little less than 90 Degrees each streak, and heat the loop so you can get good results.
Also, let the loop cool!!
,Mixed Culture (1.5) Correct Ans-A microbial culture consisting of two or more species.
What is generally the first step in identifying a microbial organism? (1.5) Correct Ans-
Obtaining isolations of Individual colonies. The technique we used in class was the isolation
technique- the streak plate. Cells that have been sufficiently isolated will grow into colonies,
consisting only of the original cell type.
Colonies (1.5) Correct Ans-Individual microbial cell type. They can also form from a pair,
chain, or cluster of cells.
Colony Forming Unit (CFU) (1.5) Correct Ans-A more correct description of the colony
origin
Zigzag Inoculation (1.5) Correct Ans-A type of inoculation pattern used to when the sample
does not have a high enough cell density.
Quadrant streak technique: (1.5)
- how much space should you try to use?
- describe what each of the quadrants should look like
- describe order Correct Ans-- maximize the space used
, - Q1: confluent growth (colonies overlapping), max growth
- Q2: less growth, some separation
- Q3: even less growth
- Q4: isolated colonies
- flame, only go to culture once at the beginning, Q1, flame, Q2, flame, Q3, flame, Q4, flame
Quadrant plate questions: (1.5)
- What if no overlap?
- too much overlap?
- plate with white and yellow colonies is what type of plate? Correct Ans-- no overlap --> no
isolation
- too much overlap --> too much confluent growth
- white colonies & yellow colonies --> mixed plate
Confluent Growth (1.5) Correct Ans-When you have confluent growth, the parental bacteria
are too close together in space, and so as they reproduce and their progeny reproduce, you see a
mass of cells that cover the surface of the agar - and in this case, it isn't possible to separate out
which bacteria are the descendants from a single original cell.
Most colonies on streak plates grow from isolated colony-forming units. On rare occasions,
however, a colony can be a mixture of two different organisms. If a culture is started from this
colony (thinking it's pure), correct identification will be next to impossible because the extra
