Example Answers to PEA, MSA, MAC, and Thioglycolate Questions PDF

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PEA Questions 1.

What is a selective medium? What is a differential medium? A selective medium is a microbiological growth medium that contains ingredients that inhibit the growth of one type of bacterium, while allowing another type to grow. A differential medium is a microbiological growth medium that contains ingredients that react with, change in response to, or appear differently after growth, due to some characteristic of microbial growth or metabolism.

2.

What is the function of PEA? PEA is a selective medium used to determine if a bacterium is Gram +ve or Gram -ve.

3.

What does PEA ‘select for’? PEA selects for Gram +ve bacteria.

4.

What is the selective ingredient in PEA? How does this ingredient inhibit bacterial growth? The selective ingredient in PEA is phenylethyl alcohol (2.5%). This ingredient interferes with the structure of Gram -ve outer membranes, inhibiting their growth. Gram +ve bacteria, with no outer membrane, are capable of growth on this concentration of alcohol.

5.

What is observed to interpret PEA? Growth is observed to interpret PEA. Inhibited growth indicates that an organism is Gram -ve; healthy growth indicates an organism is Gram +ve.

6.

Why must a control plate (TSA) be used when interpreting selective media? A positive control for growth is necessary for the interpretation of selective media. Without a positive control, it cannot be determined if lack of growth on a selective plate is due to inhibition by the medium or due to the culture being inviable.

PEA Results Questions 1.

What are your observations and conclusions for the organisms (A, B, C) on this PEA plate? All organisms were viable and grew on TSA. Observations A – growth of organism B – growth of organism C – inhibited growth of organism

Conclusions A – bacterium is Gram +ve B – bacterium is Gram +ve C – bacterium is Gram -ve

MSA Questions 1.

What is the function of MSA? MSA is a selective and differential medium that is used to select for halotolerance and differentiates for the ability to ferment mannitol in halotolerant bacteria. The medium is commonly used to identify Staphylococcus aureus.

2.

What is the selective ingredient in MSA? What does MSA ‘select for’? MSA contains NaCl (7.5%) as the selective ingredient. MSA selects for halotolerant bacteria.

3.

What are the differential ingredients in MSA? MSA contains mannitol (a sugar) and phenol red (a pH indicator) as differential ingredients.

4.

Which should be interpreted first, the MSA selective assay or the MSA differential assay? The MSA selective assay (growth) must be interpreted before the differential assay.

5.

What is observed to interpret the MSA selective assay? Observe for growth to interpret the MSA selective assay.

6.

What is observed to interpret the MSA differential assay? Observe for color of medium to interpret the MSA differential assay.

7.

An organism has failed to grow on MSA, but grows on TSA. What conclusion can you draw regarding the ability of this organism to ferment mannitol? No conclusion can be drawn for mannitol fermentation when no growth has occurred.

MSA Results Questions 1.

What are your observations and conclusions for the organisms (A, B, C) on this MSA plate? All organisms were viable and grew on TSA. Observations A – no growth B – growth, medium is yellow C – growth, medium is not yellow Conclusions

A – organism is not halotolerant B – organism is halotolerant, can ferment mannitol C – organism is halotolerant, cannot ferment mannitol

MAC Questions 1.

What is the function of MacConkey Agar? MAC is a selective and differential medium that selects for Gram -ve bacteria and differentiates for the ability of Gram -ve bacteria to ferment lactose.

2.

What are the selective ingredients in MAC? What does MAC ‘select for’? MAC contains crystal violet and bile salts as selective ingredients. MAC selects for Gram -ve bacteria.

3.

What are the differential ingredients in MAC? MAC contains lactose (a sugar) and neutral red (a pH indicator) as differential ingredients.

4.

What should be interpreted first, the MAC selective assay or the MAC differential assay? The MAC selective assay (growth) must be interpreted before making observations for the differential assay.

5.

What is observed to interpret the MAC selective assay? Growth is observed to interpret the MAC selective assay.

6.

What is observed to interpret the MAC differential assay? Color of growth (pink v. not pink) is observed to interpret the MAC differential assay.

7.

An organism has failed to grow on MAC but grows on TSA. What conclusion can you draw regarding the ability of this organism to ferment mannitol? No conclusion can be made about the ability of the organism to ferment lactose.

MAC Results Questions 1.

What are your observations and conclusions for the organisms (A, B, C, and D) on this MAC plate? All organisms were viable and grew on TSA. Observations A – growth, culture is pink B – no growth, (NA) C – growth, culture is pink, medium is pink D – growth, culture is not pink (colorless) Conclusions

A – organism is Gram -ve, organism can ferment lactose B – organism is Gram +ve, (NA) C – organism is Gram -ve, organism can ferment lactose D – organism is Gram -ve, organism cannot ferment lactose

Thioglycolate Questions 1.

What is the function of Thioglycolate broth? Thioglycolate broth is used to determine an organism’s oxygen requirements. The thioglycolate broth is used to determine if the unknown organism is an obligate aerobe or a facultative anaerobe.

2.

What is the function of 0.075% agar? The agar prevents mixing of the aerobic and anaerobic portions of the medium.

3.

What is the function of sodium thioglycolate? Sodium thioglycolate acts as an oxygen scrubber, maintaining an anaerobic area within the medium.

4.

What is the function of resazurin? Resazurin is an oxygen indicator. The lavender color of resazurin indicates that oxygen is present, resazurin is colorless in the absence of oxygen.

5.

What is observed to interpret thioglycolate broth? Position and distribution of growth (anaerobic and aerobic growth) is used to interpret thioglycolate broth.

Thioglycolate Results Questions 1.

What are your observations and conclusions for the organisms (A, B, C, D, and E) in these thioglycolate broth tubes? Observations A – growth in both aerobic and anaerobic zones B – growth in both aerobic and anaerobic zones C – growth in anaerobic zone only, no growth in aerobic zone D – growth in aerobic zone only, no growth in anaerobic zone E – growth in aerobic zone only, no growth in anaerobic zone Conclusions A – organism is a facultative anaerobe B – organism is a facultative anaerobe C – organism is an obligate anaerobe D – organism is an obligate aerobe E – organism is an obligate aerobe

2.

What are your observations and conclusions for the organisms (A, B, C, D, and E) in these thioglycolate broth tubes? Observations A – growth in both aerobic and anaerobic zones B – growth in anaerobic zone only, no growth in aerobic zone C – growth in both aerobic and anaerobic zones D – growth in anaerobic zone only, no growth in aerobic zone E – growth in aerobic zone only, no growth in anaerobic zone Conclusions A – organism is a facultative anaerobe B – organism is an obligate anaerobe C – organism is a facultative anaerobe D – organism is an obligate anaerobe E – organism is an obligate aerobe...