Part:BBa_K117010:Experience

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Preparation of AI-2 for characterization:

Pure AI-2 is very difficult to harvest or synthesize. However, as a quorum sensing messenger, AI-2 is present in intercellular medium in considerable amount. In this project, AI-2 was indirectly harvested by obtaining cell supernatants at different time points of cell growth. Glucose was added to the growth medium as it promotes AI-2 production significantly (Liang Wang, 2005).

A colony of E. coli W3110 was grown overnight in LB for 16 hours at 37°C . The overnight culture was then diluted in LB plus 0.8% glucose to an optical density at 600 nm (OD600) of 0.02. The cells were incubated at 37°C with shaking at 225 rpm in 500ml Erlenmeyer flask. Cell aliquots were removed at each time point for measurement of the OD600. The measured values were shown in the following table:

Time (hrs)	OD of W3110
0	0.002
1	0.047
2	0.112
3	0.341
4	0.628
5	0.921
6	1.032
7	1.172
8	1.252

Based on the data, a graph of OD600 versus time was constructed as below:

OD of W3110 vs time

Maximal AI-2 production activity is typically observed during mid- to late exponential phase (Surette, 1998). Therefore, 4ml of samples corresponding to 2, 3, 3.333, 3.667, 4, 6 time points were collected and centrifuged at 13,200 rpm for 10 min in a microcentrifuge to separate cell pellets from supernatants. Cleared supernatants were filtered using 0.2 µm pore size filters to remove unwanted big proteins. Supernatants were stored at -20°C.

Bacteria strains and media

Bacteria used in all the experiments were LuxS mutants derived from the wild type E coli strain W3110. By knocking out LuxS gene, the cells were unable to synthesize AI-2. As a result, lsrA promoter could only be induced in the presence of external AI-2 produced by other bacteria rather than the host cells. LuxS mutants were first made chemically competent in order to be able to take in biobrick plasmids during transformation. The media used for overnight growth of cells were Luria-Bertani broth (LB). Cells were diluted in M9 media to an OD600 of 1 before ready to be read under Fluorescence Microplate Reader or Absorbance reader.

pLsrA-lysis plasmid (part BBa_K117010) in normal W3310 E.coli.(left) and LuxS mutants (right): In the first case, the bacteria produce their own AI-2 to cause themselves to lyze

Characterization:

Graphs showing cell density change when bacteria carrying plsrA-lysis plasmid were induced with AI-2:It can be observed that initially, the curves of cell samples with LB and AI-2 have slightly higher slopes than that of sample with cells only. This can be easily understood because the supernatants also contain considerable amounts of LB, which promotes cell growth. However, after about 3 hours, the cell induced with AI-2 reached steady state while the control samples were still growing. This can be well explained by the effect of lysis, which counters cell growth at the same time.

References:

• Liang Wang, Yoshifumi Hashimoto, Chen-Yu Tsao, James J.Valdes, and William E.Bentley,Cyclic AMP (cAMP) and cAMP receptor protein influence both synthesis and uptake of extracellular Autoinducer 2 in Escherichia coli,Journal of Bacteriology,2005,2066-2076.
• Surette, M. G., and B. L. Bassler,Quorum sensing in Escherichia coli and Salmonella typhimurium,Proc. Natl. Acad. Sci.,1998,7046-7050.