Using FlopR

Alex J H Fedorec 02/06/2021

• Prerequisite Knowledge
• Installation
• A Quick Note on .csv Files
• Plate Reader Calibration
• Processing Plate Reader Data
  • Autofluorescence normalisation details
• Flow Cytometry Processing
  • Process a single .fcs file
  • Process a folder of .fcs files

Prerequisite Knowledge

We have attempted to make our software usable with minimal prior knowledge of the R programming language or programming in general. You will need to be familiar with the idea of running commands from a console or writing basic scripts. For R beginners, this is a great starting point, there are some good resources here and we suggest using the RStudio application. It provides an environment for writing and running R code.

Installation

FlopR relies on several other R packages. Most of them are available through the “Comprehensive R Archive Network (CRAN)” which just means that they can be automatically installed. There are, however, a couple that need to be manually installed if you are using the flow cytometry processing functions. To do this use the following commands:

install.packages("devtools", repos = "https://cloud.r-project.org/")

if (!requireNamespace("BiocManager", quietly = TRUE))
    install.packages("BiocManager")

BiocManager::install(c("flowCore", "flowClust", "flowStats"))

Once those packages have been installed we are ready to install FlopR.

devtools::install_github("ucl-cssb/flopr")

This grabs the latest version of FlopR from GitHub and builds it on your computer. If it installed successfully we should now be able to load the FlopR package in R.

library(flopr)

The examples in this document can be run using the data in the “example” folder. Download the whole folder and make sure that you have set your current working directory (you can use the setwd() command in R) to the location where you saved it on your computer

A Quick Note on .csv Files

This package makes extensive use of .csv (comma separated variables) files for data and meta-data. Unfortunately, not all .csv are created equal.

• When using a Windows OS computer we recommend that you save your .csv files explicitly with a UTF-8 file encoding (as shown below). This means that your files can be used by someone running a different operating system without error.
• Some parts of the world use decimal commas instead of decimal points. In these places, .csv files use “;” as a separator instead of “,”. Unfortunately, we are unable to automatically detect which format your .csv files are in. As such, you will have to ensure that your files are saved using “,” as the separator and decimal points for the decimal mark. This article has some help for doing so.

Plate Reader Calibration

The experimental protocols for producing absorbance and fluorescence calibration data can be found here and here.

The data produced by plate readers from different manufacturers comes in different formats. The first step we need to do is to “parse” the data from the plate reader into a standard format. We have written an example parsers for use with data from Tecan plate readers and the Biotek Neo 2. If you need help writing a parser for your plate reader, please contact us and we’ll see what we can do.

For a Tecan plate reader, the data is saved as an Excel .xls file. As of flopR version 0.4.02, the spark_parse function accepts Excel files (.xls and .xlsx). If you are using an earlier version, the data first needs to be saved as a .csv file (open in Excel and “Save As” .csv) that can be read by R. We also need a .csv file telling us what is in each well of our microtitre plate. An example can be found in the “examples/plate_reader/tecan_spark” folder, but the first few rows looks like this:

calibrant	fluorophore	media	concentration	replicate	well
fluorescein	GFP	PBS	7.53000e+14	1	A1
fluorescein	GFP	PBS	3.76500e+14	1	A2
fluorescein	GFP	PBS	1.88250e+14	1	A3
fluorescein	GFP	PBS	9.41250e+13	1	A4
fluorescein	GFP	PBS	4.70625e+13	1	A5
fluorescein	GFP	PBS	2.35313e+13	1	A6

Once we have the calibration data and layout .csv files we can parse the data.

flopr::spark_parse(data_csv = "examples/plate_reader/tecan_spark/191219_calibration_membrane.csv",
                   layout_csv = "examples/plate_reader/tecan_spark/calibration_plate_layout.csv",
                   timeseries = FALSE)

The data_csv argument takes the path to the calibration data. layout_csv is the path to the plate layout .csv file. Finally, the Tecan plate readers save timeseries data differently from single timepoint data, so we have a Boolean flag, timeseries, that lets the parser know that this is not a timeseries.

The spark_parse() function saves the parsed calibration data in a new .csv file, in the same location as the calibration data, with "_parsed" appended to the filename. The first few rows look like this:

calibrant	fluorophore	media	concentration	replicate	well	OD600	OD700	GFP 40	GFP 50	GFP 60	GFP 70	GFP 80	GFP 90	GFP 100	GFP 110	GFP 120	row	column
fluorescein	GFP	PBS	10.0000	1	A1	0.0921	0.0831	1830	9811	36911	NA	NA	NA	NA	NA	NA	A	1
fluorescein	GFP	PBS	5.0000	1	A2	0.0942	0.0852	932	4993	19221	54510	NA	NA	NA	NA	NA	A	2
fluorescein	GFP	PBS	2.5000	1	A3	0.1015	0.0928	453	2434	9448	27864	NA	NA	NA	NA	NA	A	3
fluorescein	GFP	PBS	1.2500	1	A4	0.0985	0.0899	232	1253	4853	14513	36349	NA	NA	NA	NA	A	4
fluorescein	GFP	PBS	0.6250	1	A5	0.0957	0.0905	114	617	2404	7128	18282	41812	NA	NA	NA	A	5
fluorescein	GFP	PBS	0.3125	1	A6	0.1027	0.0965	54	294	1137	3388	8672	20095	42244	NA	NA	A	6

The parser has extracted the information we need from the calibration data and merged it with the plate layout information so that we now have columns containing each of the measurements for each well.

Now we can actually calculate our calibration coefficients. To do this we just need to use one function and give it our parsed data.

flopr::generate_cfs(calibration_csv = "examples/plate_reader/tecan_spark/191219_calibration_membrane_parsed.csv")

For details about how this process works, you can read our paper here. At the end, there should be two .pdf images showing the calibration curves for absorbance and fluorescence, along with a new .csv file, appended with "_cfs", containing the parameters for use in the future, the first few rows of which look like this:

cf	beta	calibrant	fluorophore	measure
184.7274	0.0057476	fluorescein	GFP	GFP 40
996.4472	-0.0035424	fluorescein	GFP	GFP 50
3821.0497	-0.0004737	fluorescein	GFP	GFP 60
11298.9531	0.0002015	fluorescein	GFP	GFP 70
29128.3783	-0.0007481	fluorescein	GFP	GFP 80
65834.9342	0.0012751	fluorescein	GFP	GFP 90

The “cf” column contains the calibration coefficients that will be used to calibrate our data in later experiments.

Before we get too excited we need to check the images to make sure that the calibration curves look sensible. The software attempts to remove data points which it deems are invalid, but this process isn’t perfect and occasionally may need you to remove data points from the "*_parsed.csv" file. Using the example data, you can see (here) that some of the fluorescein wells are considered valid absorbance measurements. In this case, it isn’t the end of the world since we would never use the parameters produced from those two curves.

n.b. Currently the software is setup to work with “microspheres” for calibrating cell number and “fluorescein” for calibrating GFP fluorescence. We hope to extend it in the near future to work with other calibrants.

Processing Plate Reader Data

As mentioned above, because plate readers from different manufacturers save the data in different formats, the first step we need to do is parse our raw data. The parser that we provide takes Tecan plate reader data in the form of a .csv file. We also need a .csv file telling us what is in each well of you microtitre plate. This can include any information that you wish; we include as much meta-data as possible as it makes our data analysis later much smoother. (Before version 0.4.01: The only requirement is that the last column must be named “well” and include an identifier (usually the well id i.e. B2) that can be matched to the same identifier in the plate reader data.) Here’s an example where we include information about the strains, plasmids, media, inducers, etc.:

strain	host	plasmid	plasmid_2	strain_2	media	sugar	amino_acids	inducer	concentration	init_ratio	init_dilution	well
									NA	NA	NA	A1
									NA	NA	NA	A2

Now we can use our parsing function.

flopr::spark_parse(data_csv = "examples/plate_reader/tecan_spark/200228_example_data.csv",
                   layout_csv = "examples/plate_reader/tecan_spark/200228_example_layout.csv",
                   timeseries = TRUE)

The data is extracted and the meta-data from the layout is attached. Note that this time the data a timeseries so we set the timeseries flag to TRUE. A new .csv file is produced containing the parsed data with "_parsed" appended to the filename.

Now we can start processing our data. There is one function that does all the work for: process_plate(). There are a few arguments that we need to give the function which will control what happens.

flopr::process_plate(data_csv = "examples/plate_reader/tecan_spark/200228_example_data_parsed.csv",
                     blank_well = c("C12", "D12"),
                     neg_well = c("C6", "D6", "E6"),
                     od_name = "OD700",
                     flu_names = c("GFP", "mCherry"),
                     af_model = "spline",
                     to_MEFL = TRUE,
                     flu_gains = 135,
                     conversion_factors_csv = "examples/plate_reader/tecan_spark/191219_calibration_membrane_parsed_cfs.csv")

Let’s walk through what each of the arguments do:

• data_csv is the path to our parsed data.
• blank_well are the well identifiers of wells containing media blanks. These are used for normalising absorbance. If you only have one blank well you can specify it using blank_well = "C12" for example.
• neg_well are the well identifiers of wells containing negative controls. These are used for normalising fluorescence. As above, if you only have one negative control well you can specify it using neg_well = "C6" for example.
• od_name is the name of the column containing our absorbance values in the parsed data .csv file. Currently we can only use one absorbance column, so if you record absorbance at multiple wavelengths (like I do), you will have to pick one.
• flu_names are the names of the columns containing our fluorescence values. You can include as many or as few of you fluorescence columns as you like. Whichever columns are named in here, we will attempt to normalise.
• af_model allows you to choose the type of model that we are going to use for fluorescence normalisation. We’ll discuss the available choices below.
• to_MEFL is a Boolean flag that lets you tell the function if you want to convert the fluorescence data into calibrated units. You can only do this if you have carried out the calibration as detailed above.
• flu_gains is where you specify the gain at which your fluorescence data was recorded for each fluorescence channel. Here we only have calibration parameters for “GFP” so we only specify one gain value.
• conversion_factors_csv is the path to the calibration parameters that you generated using the protocol detailed above.

When we run this function the absorbance is normalised, then the fluorescence and finally (if desired) the absorbance and fluorescence values are calibrated. Finally, the processed data is saved in a new .csv file with "_processed" appended to the filename and with additional columns for each of the processed values.

We also save some .pdf images comparing the raw and normalised data, and images showing the fluorescence normalisation curves. Using these plots, there are a few checks that we should make before celebrating.

• Check that none of the blank wells you used showed any contamination (it happens to the best of us). If there is growth in a blank well it will throw off the absorbance normalisation.
• Check that the fluorescence normalisation curves fit the data. Read below for more detail on possible issues.

Autofluorescence normalisation details

Autofluorescence is the fluorescence produced by anything other than the fluorophores that we are interested in measuring. A small amount usually comes from growth media and can be minimised by choosing certain medias. A large contribution comes from molecules produced and secreted by our cells. Some of these molecules show particularly strong emission at similar wavelengths to GFP. We observe that the level of autofluorescence is not simply proportional to the number of cells or optical density of our culture. As cells enter stationary phase, autofluorescence increases, perhaps due increased production of the autofluorescent molecules and changes in cell size.

In order to remove this autofluorescence from our sample data we fit a curve to our negative control data. We provide four different models that the user can choose to fit their data (and if desired more models can be added). There are two smoothing models: “loess” and “spline”. The primary difference to the user between these two models is that the “spline” is able to extrapolate beyond the negative control data provided. This means that if the range of absorbance values at which you have measurements of your negative control is smaller than the range of your samples, we can still make an attempt at normalisation. However, this extrapolation is very crude (linear from the last data point) and can produce poor normalisation in the extrapolated range. Fortunately, negative controls tend to grow better than fluorescent samples, so extrapolation is often not an issue. We also provide a second-order polynomial and an exponential model, specified by “polynomial” and “exponential” repsectively. These are inherently able to make predictions beyond the range of normalised data and therefore may be good starting points.

Prolonged periods in stationary phase can cause autofluorescence to increase while absorbance remains stable and in some cases absorbance can start decreasing while autofluorescence does not. In these circumstances, none of the models perform particularly well. The performance of each of them should be checked and if none of them perform satisfactorily it may be necessary to trim the data to remove confounding timepoints.

Flow Cytometry Processing

We have two functions for processing flow cytometry data:

• process_fcs takes a single .fcs file, removes debris and doublets and saves the trimmed data in a new .fcs file.
• process_fcs_dir takes a folder of .fcs files and performs the same trimming on each. It can also perform fluorescence normalisation if you have a negative control and fluorescence calibration if you have measured a calibrant.

Process a single .fcs file

To process a single .fcs file we can run the following command:

flopr::process_fcs(fcs_file = "examples/flow_cytometry/DATA/20191121/pWeak_None_0_1.fcs",
                   flu_channels = "BL1-H",
                   pre_cleaned = TRUE,
                   do_plot = TRUE)

We need to give the function four bits of information

• fcs_file is the path to the .fcs file that we want to process.
• flu_channels are the names of the fluorescence channels that we recorded. When processing a single .fcs file like this we don’t actually do anything with the fluorescence data. However, if you include the channel names you can see the data in a plot that is saved. If you have more than one fluorescence channel, the argument needs to be a vector, which will look something like this: flu_channels = c("BL1-H", "BL2-H", YL2-H")
• pre_cleaned lets the function know if you have gated out debris on the flow cytometer. Most people do this when running their experiments by setting a threshold on forward-scatter and side-scatter. But some people like to record everything and process the data later.
• do_plot lets the function know if you want a plot to be saved of the trimming process.

In the end we have a new .fcs file with "_processed" appended to the filename, and if you asked the function to save a plot we will have a .pdf that looks something like this.

Process a folder of .fcs files

It’s much more likely that we have more than one sample that we want to process. This is where the other function comes in handy.

flopr::process_fcs_dir(dir_path = "examples/flow_cytometry/DATA/20191121",
                       pattern = "*Med*.fcs",
                       flu_channels = "BL1-H",
                       pre_cleaned = TRUE,
                       do_plot = TRUE,
                       neg_fcs = "pNeg_None_0_1.fcs",
                       calibrate = TRUE,
                       mef_peaks = list(list(channel = "BL1-H",
                                             peaks = c(0, 822, 2114, 5911, 17013, 41837, 145365, 287558))))

There are a few more arguments to this function and some of them look a bit complicated so let’s go through them.

• dir_path is the path to the folder with your .fcs files in.
• pattern allows us to just process a subset of the .fcs files in the folder. Here we are just going to process files with “Med” in the filename. Without going into too much detail, this uses a simplified version of “regular expressions” called “globbing patterns”. We can use “wildcard” symbols to represent any character (? is a place holder for an single character and * is a place holder for 0 to any number of characters). In this example all we know is that “Med” appears somewhere in the filenames and they all end with “.fcs”. So we use the * character to show that there are some unknown characters before “Med” and between “Med” and “.fcs”. If you want to process all .fcs files in your folder, the pattern would be "*.fcs".
• flu_channels is as above.
• pre_cleaned is as above.
• do_plot is as above.
• neg_fcs is the filename of your negative control sample. It must be in the folder with the other .fcs files. This is used to normalise autofluorescence. If you don’t specify a filename here, the processing will be carried out without any normalisation steps.
• calibrate tells the function whether you want to calibrate the fluorescence measurements. To be able to calibrate you must have an .fcs file with data from calibration beads and it must have “beads” somewhere in the filename. For discussion about calibration beads, read our paper or check out TASBE and FlowCal.
• mef_peaks is where we tell the function what the true fluorophore values are for our beads. These will be available from the bead manufacturer. We need to specify peaks for each of the channels that we want to calibrate and the channel name needs to correspond to one given in flu_channels. Here we are just calibrating the “BL1-H” channel which corresponds to GFP on our flow cytometer. If you wanted to calibrate two channels it would look something like this:

mef_peaks = list(list(channel = "BL1-H", 
                      peaks = c(0, 822, 2114, 5911, 17013, 41837, 145365, 287558)),
                 list(channel = "YL2-H",
                      peaks = c(0, 218, 581, 1963, 6236, 15267, 68766, 181945)))

The function carries out the trimming as described above. Then, if a negative control file is given, it performs fluorescence normalisation by negating the geometric mean of the negative control’s fluorescence in each channel from each of the other samples. Finally, if the files are going to be calibrated, a calibration curve is fit to the bead peaks in each of the fluorescence channels in mef_peaks. We use a model developed for FlowCal for our calibration curve. We then calibrate both the raw and normalised data since normalisation can produce fluorescence values less than or equal to 0, which get removed during calibration due to working with logged data (the output will contain a both sets of calibration). The new data, any plots produced, and a data_summary.csv file (containing geometric statistics for each .fcs file) will be saved to a new folder with the same name as the original but with "_processed" appended.

There are a few checks that should be made to reassure yourself that everything has worked.

• To confirm correct trimming, we recommend setting do_plot = TRUE so that you can see which events have been removed during processing. Check that the debris, if there is any, and doublets have been correctly identified and removed.
• For each fluorescence calibration channel, a plot will be saved showing the bead peaks that have been identified and the calibration curve that we have fit to them. Sometime the bead peaks aren’t identified correctly. In this case, there is another argument that the function can take beads_dens_bw, which has a default value of 0.025. To identify the beads we use something called a Gaussian kernel to smooth the fluorescence data and pick the highest points. Sometimes this smoothing isn’t quite right; we might not smooth enough and one peak is identified as two, or we smooth too much and lose peaks. In the former case we need to increase beads_dens_bw and in the latter we decrease it. If tweaking this value doesn’t work, we also provide a way to manually specify identify the peaks using the manual_peaks argument. For this data, a manually specified set of peaks would look like this:

manual_peaks = list(list(channel = "BL1-H", 
                      peaks = c(1.9, 2.5, 2.9, 3.3, 3.7, 4.2, 4.6, 4.95)))