TITLE: Pooled, Growth-Based Assays
AUTHORS: David Ross

[DESCRIPTION]
This protocol outlines a pooled, growth-based assay for measuring the fitness of a library of variants in E.Coli. This protocol defines paths for either:
only measuring a pool of control variants
or an entire library of variants pooled in addition to the controls.

The inputs for this protocol are: 1) barcoded normalization variants, 2) barcoded calibration variants, and depending on the branch taken 3) a pooled library of barcoded variants containing 100,000-500,000 members as well. The protocol begins with several growths which convert the separate glycerol stocks into pooled cultures that have reach stationary phase in a 96-well plate. The glycerol stocks are first grown overnight in separate tubes and flasks. The next morning, the optical density (OD) of each culture is measured, after which all cultures are pooled into a single flask and grown for 4-5 hours to achieve one doubling. The OD of this pooled culture is then measured again before distributing it into a 96-well growth plate. This plate is placed in a plate reader/incubator to grow to stationary phase (~12 hours) without antibiotics or additives (except those required for plasmid maintenance).  These cultures are then used as an input for the next growth cycle (i.e., timepoint 1), explained in further detail in the next section.

The 4 subsequent growth cycles (i.e., timepoints 1-4) will produce culture samples that will eventually be sequenced and used in the fitness calculation. These growth cycles (i.e., timepoints 1-4) are all ~3 hours long, so that cells stay in mid-log phase. At the end of each growth cycle, a small amount of each culture acts as input for the subsequent growth cycle, while the remaining culture is processed for downstream measurement (see the Notes section below for details on subsequent steps). The media for the first of these 4 growth cycles (i.e., timepoint 1) contains only additives to initiate gene expression (i.e., inducers), but no selection antibiotic. The media for the following 3 growth cycles contain both the additives and the selection antibiotic.

Notes: 
Throughout all growths involving 96-well plates, OD and fluorescent measurements are recommended to be taken every 5 minutes and at the end of each growth plate's incubation. The OD measured at the end of each ~3 hour growth should be constant or just slightly decreasing across timepoints 1-4.
Immediately after each growth plate is done incubating and a sample from each well has been transferred to the next growth plate, perform a DNA extraction protocol on the remaining culture in each well.
After DNA extraction is complete for each timepoint, you can proceed to the Automated Bar-Seq Library Preparation and Pooling protocol

[STEPS]
SECTION: Culture Preparation &amp; Overnight Growth
3. Start a culture of the mixed calibration variants in a 15 mL snap-cap culture tube.
Take a scraping from the glycerol stock of the mixed calibration variants and put it in 5 mL of M9 Media to start culture.
SECTION: Culture Preparation &amp; Overnight Growth
5. Incubate all cultures overnight
37 C, with shaking at 300 rpm
Approximately 18 hours 
SECTION: Pool cultures &amp; grow for one doubling
7. In a new 250 mL baffled flask, combine the following:
50 mL M9 Media
0.5 mL of each normalization variant culture's overnight tube
1 mL of mixed calibration variant culture's overnight tube
48 mL of the variant library culture 
SECTION: Pool cultures &amp; grow for one doubling
8. Incubate for 4-5 hours to allow for approximately one doubling.
37 C, w/ shaking at 300 rpm
SECTION: First QC check: Overnight OD600 measurement
6. Measure the OD600 of all four cultures.
SECTION: Prepare the automation system or liquid handler
9. Load the reagents and labware required for the 96-well growth plate prep into automation system or deck of automated liquid handler:
The combined cell culture
M9 Media
Selection Antibiotic stocks, typically at 10x higher concentration than the final concentration to be used in the wells of the growth plates
Additional additives needed for the experiment, e.g., inducers for control of selection circuit; ligand stocks for sensor protein assay; inhibitors (i.e., drugs) for protease assay, maintenance antibiotics
Growth plates: square-well polystyrene plate with 1.0 mL per well max volume, made by Agilent, part no. 204799-100 (only fill cultures up to 0.5 mL per well).
SECTION: Culture Preparation &amp; Overnight Growth
4. Start a culture of your variant library in a 250 mL baffled culture flask.
Use one aliquot of glycerol stock and put it in 100 mL of M9 media.
SECTION: Using this protocol
1. If you are running a pooled, growth based assay for the first time (i.e., only using control variants pooled together) then follow the 'Controls Only' branch below. 
If you are running a pooled, growth-based assay on an entire library of variants pooled together (including controls as well), then use the 'Variant Library' branch below.
SECTION: Culture Preparation &amp; Overnight Growth
2. Start separate cultures of the two  “best” normalization variants, each in their own 15 mL snap-cap culture tubes. 
Take a scraping from the glycerol stock for each of the normalization (always-on) variants and put it in 5 mL of M9 Media to start cultures
SECTION: Prepare the automation system or liquid handler
8. Load the reagents and labware required for the 96-well growth plate prep into automation system or deck of automated liquid handler:
The combined cell culture
M9 Media
Selection Antibiotic stocks, typically at 10x higher concentration than the final concentration to be used in the wells of the growth plates
Additional additives needed for the experiment, e.g., inducers for control of selection circuit; ligand stocks for sensor protein assay; inhibitors (i.e., drugs) for protease assay, maintenance antibiotics
Growth plates: square-well polystyrene plate with 1.0 mL per well max volume, made by Agilent, part no. 204799-100 (only fill cultures up to 0.5 mL per well).
SECTION: The first growth plate: cells reach stationary phase
9. Pipette 450 uL M9 Media into each well being used in the growth plate
Each condition should be represented by 4 replicate wells.
SECTION: The first growth plate: cells reach stationary phase
10. Pipette 50 uL of the combined cell culture into each well
SECTION: The first growth plate: cells reach stationary phase
11. Apply gas-permeable seal to plate
SECTION: The first growth plate: cells reach stationary phase
12. Incubate first growth plate for 12 hours
37 C, with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The first growth plate: cells reach stationary phase
12.2. During incubation, prepare the second growth plate with 490 uL mixed media per well, where the mixed media now includes:
M9 Media
additives (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)

SECTION: The first growth plate: cells reach stationary phase
12.3. Approximately ten minutes before the end of the 12-hour incubation, pre-warm the second growth plate.
Adjust pre-warming temperature and timing so that the media temperature in the plate is 37C at the end of the pre-warming, and so that the pre-warming step ends at the same time as the 12-hour incubation.
SECTION: The second growth plate (timepoint 1)
13. After 12-hour incubation, remove gas-permeable seal from the first growth plate.
SECTION: The second growth plate (timepoint 1)
14. Transfer 10 uL from each well in the first growth plate to the corresponding well in the second growth plate
If using 96 channel head, there are some subtleties required to get a reproducible transfer. Details are described in the supplemental information of this paper.
SECTION: The second growth plate (timepoint 1)
15. Apply gas-permeable seal to second growth plate.
SECTION: The second growth plate (timepoint 1)
16. Incubate the second growth plate for approximately 3 hours and prepare the third growth plate, with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
The exact incubation time for this step needs to be worked out during the testing phase. The time needs to be adjusted so that in the wells with the fastest growing cultures (e.g., with zero selection antibiotic) fulfill the following conditions:
 The cells are always in mid-log phase or lower.
The cell density at the end point of each subsequent incubation step is constant or slightly decreasing. 
SECTION: The second growth plate (timepoint 1)
16.2. During incubation, prepare the third growth plate with 450 uL mixed media per well, where the mixed media now includes:
M9 Media
The same additives as the second growth plate (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (can be at varying concentrations, depending on the pooled assay design)
SECTION: The second growth plate (timepoint 1)
16.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the third growth plate.
SECTION: The third growth plate (timepoint 2)
17. After 3-hour incubation, remove gas-permeable seal from the second growth plate.
SECTION: The third growth plate (timepoint 2)
18. Transfer 50 uL from each well in the second growth plate to the corresponding well in the third growth plate.
SECTION: The third growth plate (timepoint 2)
19. Apply gas-permeable seal to third growth plate and place into incubator.
SECTION: The third growth plate (timepoint 2)
20. Incubate the third growth plate for approximately 3 hours (or the same time as second growth plate) with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The third growth plate (timepoint 2)
20.2. During incubation, prepare the fourth growth plate with 450 uL of the same mixed media as the third growth plate:
M9 Media
The same additives  (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (at the same concentrations as the third growth plate)
SECTION: The third growth plate (timepoint 2)
20.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the fourth growth plate.
SECTION: The fourth growth plate (timepoint 3)
21. After 3-hour incubation, remove gas-permeable seal from the third growth plate.
SECTION: The fourth growth plate (timepoint 3)
22. Transfer 50 uL from each well in the third growth plate to the corresponding well in the fourth growth plate.
SECTION: The fourth growth plate (timepoint 3)
23. Apply gas-permeable seal to fourth growth plate and place in incubator.
SECTION: The fourth growth plate (timepoint 3)
24. Incubate the fourth growth plate for approximately 3 hours (or the same time as second growth plate), with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The fourth growth plate (timepoint 3)
24.2. During incubation, prepare the fifth growth plate with 450 uL of the same mixed media as the fourth growth plate:
M9 Media
The same additives  (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (at the concentrations as the third growth plate)
SECTION: The fourth growth plate (timepoint 3)
24.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the fifth growth plate.
SECTION: The fifth growth plate (timepoint 4)
25. After 3-hour incubation, remove gas-permeable seal from the fourth growth plate.
SECTION: The fifth growth plate (timepoint 4)
30. Combine each set of 4 replicate wells from each condition of the fifth growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: Culture Preparation &amp; Overnight Growth
2. Start separate cultures of the two  “best” two normalization variants, each in their own 15 mL snap-cap culture tubes. 
Take a scraping from the glycerol stock for each of the normalization (always-on) variants and put it in 5 mL of M9 Media to start cultures
SECTION: Culture Preparation &amp; Overnight Growth
3. Start a culture of the mixed calibration variants in a 250 mL baffled flask.
Take a generous scraping from the glycerol stock of the mixed calibration variants and put it in 100 mL M9 Media.
SECTION: Culture Preparation &amp; Overnight Growth
4. Incubate all cultures overnight
37 C, with shaking at 300 rpm
18 hours 
SECTION: Pool cultures &amp; grow for one doubling
6. In a 250 mL baffled flask, combine the following:
50 mL M9 Media
0.5 mL of each normalization variant culture's overnight tube
49 mL of mixed calibration variant culture's overnight flask.
SECTION: Pool cultures &amp; grow for one doubling
7. Incubate for 4-5 hours to allow for approximately one doubling.
37 C, w/ shaking at 300 rpm
SECTION: First QC check: Overnight OD600 measurement
5. Measure the OD600 of all three cultures.
SECTION: The first growth plate: cells reach stationary phase
12.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The second growth plate (timepoint 1)
16.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The third growth plate (timepoint 2)
19.1. Immediately after the third growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the second growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA elution (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The third growth plate (timepoint 2)
20.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The fourth growth plate (timepoint 3)
23.1. Immediately after the fourth growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the third growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The fourth growth plate (timepoint 3)
24.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The fifth growth plate (timepoint 4)
26. Transfer 50 uL from each well in the fourth growth plate to the corresponding well in the fifth growth plate.
SECTION: The fifth growth plate (timepoint 4)
27. Apply gas-permeable seal to fifth growth plate and place in incubator.
SECTION: The fifth growth plate (timepoint 4)
27.1. Immediately after the fifth growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the fourth growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The fifth growth plate (timepoint 4)
28. Incubate the fifth growth plate for approximately 3 hours (or the same time as second growth plate), with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The fifth growth plate (timepoint 4)
28.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The fifth growth plate (timepoint 4)
29. After 3-hour incubation, remove gas-permeable seal from the fourth growth plate.
SECTION: The first growth plate: cells reach stationary phase
10. Pipette 450 uL M9 Media into each well being used in the growth plate
Each condition should be represented by 4 replicate wells.
SECTION: The first growth plate: cells reach stationary phase
11. Pipette 50 uL of the combined cell culture into each well
SECTION: The first growth plate: cells reach stationary phase
12. Apply gas-permeable seal to plate
SECTION: The first growth plate: cells reach stationary phase
13. Incubate first growth plate for 12 hours
37 C, with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The first growth plate: cells reach stationary phase
13.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The first growth plate: cells reach stationary phase
13.2. During incubation, prepare the second growth plate with 490 uL mixed media per well, where the mixed media now includes:
M9 Media
additives (e.g., inducers, ligands, maintenance antibiotics, or inhibitors if they are used in the assay)

SECTION: The first growth plate: cells reach stationary phase
13.3. Approximately ten minutes before the end of the 12-hour incubation, pre-warm the second growth plate.
Adjust pre-warming temperature and timing so that the media temperature in the plate is 37C at the end of the pre-warming, and so that the pre-warming step ends at the same time as the 12-hour incubation.
SECTION: The second growth plate (timepoint 1)
14. After 12-hour incubation, remove gas-permeable seal from the first growth plate.
SECTION: The second growth plate (timepoint 1)
15. Transfer 10 uL from each well in the first growth plate to the corresponding well in the second growth plate
If using 96 channel head, there are some subtleties required to get a reproducible transfer. Details are described in the supplemental information of this paper.
SECTION: The second growth plate (timepoint 1)
16. Apply gas-permeable seal to second growth plate.
SECTION: The second growth plate (timepoint 1)
17. Incubate the second growth plate for approximately 3 hours and prepare the third growth plate, with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
The exact incubation time for this step needs to be worked out during the testing phase. The time needs to be adjusted so that in the wells with the fastest growing cultures (e.g., with zero selection antibiotic) fulfill the following conditions:
 The cells are always in mid-log phase or lower.
The cell density at the end point of each subsequent incubation step is constant or slightly decreasing. 
SECTION: The second growth plate (timepoint 1)
17.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The second growth plate (timepoint 1)
17.2. During incubation, prepare the third growth plate with 450 uL mixed media per well, where the mixed media now includes:
M9 Media
The same additives as the second growth plate (e.g., inducers, ligands, maintenance antibiotics, or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (can be at varying concentrations, depending on the pooled assay design)
SECTION: The second growth plate (timepoint 1)
17.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the third growth plate.
SECTION: The third growth plate (timepoint 2)
18. After 3-hour incubation, remove gas-permeable seal from the second growth plate.
SECTION: The third growth plate (timepoint 2)
19. Transfer 50 uL from each well in the second growth plate to the corresponding well in the third growth plate.
SECTION: The third growth plate (timepoint 2)
20. Apply gas-permeable seal to third growth plate and place into incubator.
SECTION: The third growth plate (timepoint 2)
20.1. Immediately after the third growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the second growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA elution (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The third growth plate (timepoint 2)
21. Incubate the third growth plate for approximately 3 hours (or the same time as second growth plate) with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The third growth plate (timepoint 2)
21.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The third growth plate (timepoint 2)
21.2. During incubation, prepare the fourth growth plate with 450 uL of the same mixed media as the third growth plate:
M9 Media
The same additives  (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (at the same concentrations as the third growth plate)
SECTION: The fourth growth plate (timepoint 3)
22. After 3-hour incubation, remove gas-permeable seal from the second growth plate.
SECTION: The fourth growth plate (timepoint 3)
23. Transfer 50 uL from each well in the second growth plate to the corresponding well in the third growth plate.
SECTION: The fourth growth plate (timepoint 3)
24. Apply gas-permeable seal to third growth plate and place into incubator.
SECTION: The fourth growth plate (timepoint 3)
24.1. Immediately after the third growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the second growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA elution (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The fourth growth plate (timepoint 3)
25. Incubate the third growth plate for approximately 3 hours (or the same time as second growth plate) with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The fourth growth plate (timepoint 3)
25.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The fourth growth plate (timepoint 3)
25.2. During incubation, prepare the fourth growth plate with 450 uL of the same mixed media as the third growth plate:
M9 Media
The same additives  (e.g., inducers, ligands, maintenance antibiotics or inhibitors if they are used in the assay)
Selection antibiotic in some of the wells (at the same concentrations as the third growth plate)
SECTION: The fourth growth plate (timepoint 3)
25.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the fourth growth plate.
SECTION: The fifth growth plate (timepoint 4)
26. After 3-hour incubation, remove gas-permeable seal from the fourth growth plate.
SECTION: The fifth growth plate (timepoint 4)
27. Transfer 50 uL from each well in the fourth growth plate to the corresponding well in the fifth growth plate.
SECTION: The fifth growth plate (timepoint 4)
28. Apply gas-permeable seal to fifth growth plate and place in incubator.
SECTION: The fifth growth plate (timepoint 4)
28.1. Immediately after the fifth growth plate goes into the incubator, combine each set of 4 replicate wells from each condition of the fourth growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The fifth growth plate (timepoint 4)
29. Incubate the fifth growth plate for approximately 3 hours (or the same time as second growth plate), with fastest shaking possible in the plate reader (e.g., in Biotek Neo2SM reader: double orbital shaking at 807 cpm and 1 mm shaking diameter)
SECTION: The fifth growth plate (timepoint 4)
29.1. Measure OD600 and fluorescence every 5 minutes
Start next steps during the last hour of the 12-hour incubation
SECTION: The fifth growth plate (timepoint 4)
30. After 3-hour incubation, remove gas-permeable seal from the fourth growth plate.
SECTION: The fifth growth plate (timepoint 4)
31. Combine each set of 4 replicate wells from each condition of the fifth growth plate into a single well of a new polypropylene deep well plate (suitable for centrifugation). Run the plasmid DNA extraction protocol with the combined cultures in the deep well plate.
Store the resulting extracted DNA until ready to run the BarSeq library prep protocol. If barcode sequencing prep is to be performed immediately after DNA extraction, nuclease-free water can be used for the final plasmid DNA (as in the last two steps of the protocol on protocols.io). Otherwise, substitute “EB buffer” or “TE-4” for the final elution step to avoid possible DNA degradation from uncontrolled (low) pH.
SECTION: The third growth plate (timepoint 2)
21.3. Approximately ten minutes before the end of the 3-hour incubation, pre-warm the fourth growth plate.