MICROBIOLOGY LAB 5 – 7 PRACTICAL EXAM
QUESTIONS WITH CORRECT ANSWERS
pour plate method, the aliquots are placed in an - ✔✔ empty sterile Petri dish and the appropriate
melted and tempered (cooled to approximately 45°C) medium is added and mixed with the
sample
The pour plate method works best for enumeration of microorganisms that are - ✔✔ facultative
anaerobes - ones that are capable of growth in either the presence or absence of oxygen.
In viable plate counts, several aliquots (subsamples) of each dilution are plated. This allows the
investigator to obtain an average estimate for each dilution. Usually either - ✔✔ 3 or 5 plates per
dilution are used to obtain the most reliable estimates
A countable plate is one that yields between - ✔✔ 30 and 300 colonies on a standard Petri plate
. Plates with fewer than 30 colonies are not counted because th - ✔✔ variability between plates
renders the estimates of cell density statistically unreliable
. Plates with more than 300 colonies are not considered countable for several reasons. The high
number of colonies may have depleted the available nutrients in the agar so that some cells may
not have been able to grow to form visible colonies - ✔✔ depleted the available nutrients in the
agar so that some cells may not have been able to grow to form visible colonies
,The estimate of cell density in the original sample is obtained by counting the number of
colonies on a plate and then dividing by the aliquot size and the dilution factor of the sample, as
shown below: - ✔✔ CFU/ml= Colonies/(Aliquote, ml X Dilution)
For instance, if 1.0 ml from a 10-5 dilution yielded a count of 87 colonies on a plate, the number
of CFU/ml or CFU/gm is - ✔✔ (87 colonies) / [(1.0ml)X(10^-5)] = 8.7 X 10^6 CFU/ml
If a 0.1 ml aliquot of a 10-6 dilution yielded 119 colonies, the estimate would be - ✔✔ (119) /
[(0.1ml) X (10^-6)] = 1.19 X 10^9 CFU/ml.
Serial dilution allows for - ✔✔ accurate and quantifiable dilution of a sample
The dilution factor is calculated as the ratio of sample added to total volume of the dilution. For
instance, if 1 ml of a sample is added to 9 ml of a diluent, the ratio of sample to total volume is -
✔✔ 1 ml sample to 10 ml total, or a 1:10 = 10^-1 dilution.
If 1 ml of a sample is added to 99 ml of diluent, the ratio of sample to total volume is - ✔✔ 1 ml
sample to 100 ml total, or a 1:100 = 10^-2 dilution
If 1 ml of a 10-4 dilution is added to 99 ml of diluent, the resulting solution has a total dilution of
- ✔✔ (10-4)(10-2) = 10^-6.
Where would you expect an obligate anaerobe to grow in a tube of fluid thyioglycollate
medium? - ✔✔ Lower part of the tube
,Where would you expect to see growth for an obligate aerobe? - ✔✔ On the plate in the
anaerobe jar and also an oxygen-rich environment.
Rank the following, from the highest growth temperature range to the lowest growth temperature
range. - ✔✔ 1. extreme thermophile
2. thermophile
3. mesophile
4. psychrotroph
5. psychrophile
If comparing growth of a microbe in broth tubes incubated at different temperatures, the tube
incubated at the organism's optimum temperature would _________. - ✔✔ be the most turbid
Fill in your results for your FTM tube experiment.
C. sporogenes is a?
E. coli is a?
M. luteus is a?
, A.
facultative anaerobe
B.
obligate aerobe
C.
obligate anaerobe - ✔✔
What items and/or surfaces in the kiosk were probably sampled? - ✔✔ Kiosk countertop
Bottle containing antiseptic used to clean ears
Manual ear-piercing tool
Pump handle of alcohol-based sanitizer bottle
The kiosk workers claimed they couldn't have transferred the pathogen from their hands to the
customers because they wore gloves. Choose the most logical statement related to this claim.
Select the single best answer. - ✔✔ The gloves could have been involved in the pathogen's
transmission, because gloved hands may have contacted contaminated objects during the
piercing procedure.
Staphylococcus aureus - ✔✔ Streak onto mannitol salt agar may be incubated under aerobic
conditions.
QUESTIONS WITH CORRECT ANSWERS
pour plate method, the aliquots are placed in an - ✔✔ empty sterile Petri dish and the appropriate
melted and tempered (cooled to approximately 45°C) medium is added and mixed with the
sample
The pour plate method works best for enumeration of microorganisms that are - ✔✔ facultative
anaerobes - ones that are capable of growth in either the presence or absence of oxygen.
In viable plate counts, several aliquots (subsamples) of each dilution are plated. This allows the
investigator to obtain an average estimate for each dilution. Usually either - ✔✔ 3 or 5 plates per
dilution are used to obtain the most reliable estimates
A countable plate is one that yields between - ✔✔ 30 and 300 colonies on a standard Petri plate
. Plates with fewer than 30 colonies are not counted because th - ✔✔ variability between plates
renders the estimates of cell density statistically unreliable
. Plates with more than 300 colonies are not considered countable for several reasons. The high
number of colonies may have depleted the available nutrients in the agar so that some cells may
not have been able to grow to form visible colonies - ✔✔ depleted the available nutrients in the
agar so that some cells may not have been able to grow to form visible colonies
,The estimate of cell density in the original sample is obtained by counting the number of
colonies on a plate and then dividing by the aliquot size and the dilution factor of the sample, as
shown below: - ✔✔ CFU/ml= Colonies/(Aliquote, ml X Dilution)
For instance, if 1.0 ml from a 10-5 dilution yielded a count of 87 colonies on a plate, the number
of CFU/ml or CFU/gm is - ✔✔ (87 colonies) / [(1.0ml)X(10^-5)] = 8.7 X 10^6 CFU/ml
If a 0.1 ml aliquot of a 10-6 dilution yielded 119 colonies, the estimate would be - ✔✔ (119) /
[(0.1ml) X (10^-6)] = 1.19 X 10^9 CFU/ml.
Serial dilution allows for - ✔✔ accurate and quantifiable dilution of a sample
The dilution factor is calculated as the ratio of sample added to total volume of the dilution. For
instance, if 1 ml of a sample is added to 9 ml of a diluent, the ratio of sample to total volume is -
✔✔ 1 ml sample to 10 ml total, or a 1:10 = 10^-1 dilution.
If 1 ml of a sample is added to 99 ml of diluent, the ratio of sample to total volume is - ✔✔ 1 ml
sample to 100 ml total, or a 1:100 = 10^-2 dilution
If 1 ml of a 10-4 dilution is added to 99 ml of diluent, the resulting solution has a total dilution of
- ✔✔ (10-4)(10-2) = 10^-6.
Where would you expect an obligate anaerobe to grow in a tube of fluid thyioglycollate
medium? - ✔✔ Lower part of the tube
,Where would you expect to see growth for an obligate aerobe? - ✔✔ On the plate in the
anaerobe jar and also an oxygen-rich environment.
Rank the following, from the highest growth temperature range to the lowest growth temperature
range. - ✔✔ 1. extreme thermophile
2. thermophile
3. mesophile
4. psychrotroph
5. psychrophile
If comparing growth of a microbe in broth tubes incubated at different temperatures, the tube
incubated at the organism's optimum temperature would _________. - ✔✔ be the most turbid
Fill in your results for your FTM tube experiment.
C. sporogenes is a?
E. coli is a?
M. luteus is a?
, A.
facultative anaerobe
B.
obligate aerobe
C.
obligate anaerobe - ✔✔
What items and/or surfaces in the kiosk were probably sampled? - ✔✔ Kiosk countertop
Bottle containing antiseptic used to clean ears
Manual ear-piercing tool
Pump handle of alcohol-based sanitizer bottle
The kiosk workers claimed they couldn't have transferred the pathogen from their hands to the
customers because they wore gloves. Choose the most logical statement related to this claim.
Select the single best answer. - ✔✔ The gloves could have been involved in the pathogen's
transmission, because gloved hands may have contacted contaminated objects during the
piercing procedure.
Staphylococcus aureus - ✔✔ Streak onto mannitol salt agar may be incubated under aerobic
conditions.
