1. Introduction

The InterLab studies have been contributing in developing a robust measurement procedure for green fluorescent protein (GFP) over several years now. The goal of this year’s InterLab to identify and correct the sources of systematic variability in synthetic biology measurements of GFP. In order to answer the following research question formulated by the iGEM Measurement Committee: Can we reduce lab-to-lab variability in fluorescence measurements by normalizing to absolute cell count or colony-forming units (CFU) instead of OD? The following two approaches were used to obtain results for this study:

   1. Converting between absorbance of cells to absorbance of a known
   concentration of beads.

   2. Counting colony-forming units (CFUs) from the sample.

The results of these two approaches were compared to determine how much the results were in accordance with each other to see whether using one (or both) of the methods can help to reduce the lab-to-lab variability in measurements. To get the best comparable results between all the teams performing the InterLab studies the InterLab Study Protocol was followed explicitly. The first step performed was the transformation of 8 strains with plasmids from the iGEM 2018 Distribution Kit. The different strains contained a Negative control, Positive control and 6 test strains which expressed different levels of GFP. The strains and their corresponding plasmids (parts) used from the Distribution Kit for transformation are shown in table 1. For transformation the iGEM protocol was used.

Strain	Part Number	Plate	Location
Negative control	BBa_R0040	Kit plate 7	Well 2D
Positive control	BBa_I20270	Kit plate 7	Well 2B
Test Device 1	BBa_J364000	Kit plate 7	Well 2F
Test Device 2	BBa_J364001	Kit plate 7	Well 2H
Test Device 3	BBa_J364002	Kit plate 7	Well 2J
Test Device 4	BBa_J364007	Kit plate 7	Well 2L
Test Device 5	BBa_J364008	Kit plate 7	Well 2N
Test Device 6	BBa_J364009	Kit plate 7	Well 2P

Table 1: Test Devices. The 8 Test Devices created by transformation of Escherichia coli DH5⍺ with the 8 plasmids from the Distribution Kit.

2. Experimental Setup

When the transformation of the test devices was confirmed these strains could be used for for the two experiments of the InterLab. But first, the three calibration protocols should be completed. The three calibration protocols consisted of measuring an OD600 reference point using the provided LUDOX CL-X (45% colloidal silica suspension), creating a Particle Standard Curve using the provided Microspheres and creating a Fluorescence Standard Curve using the provided Fluorescein. The Particle Standard and the Fluorescence Standard Curve are shown in figure 1 and 2.

3. Results

After calibration the next step was performing the cell measurement protocol to relate the absorbance and the known concentration of beads (model cells) and to perform the Colony forming units experiment to relate the absorbance and fluorescence to the amount of colonies formed. The results obtained are shown in figure 3 and 4 and table 2.

3.1 Cell measurement

3.2 Colony forming units

4. Conclusion

With these results and with that the completion of our InterLab studies we hope to have contributed well to the InterLab study of 2018.