Experiment 1 Characterization of lytic activity with time
To characterize the lytic activity of λ lysis cassette in more detail, we focused on when cell lysis occurs after induction by IPTG. We did the experiment as this protocol, and we got the following data. Protocol
- Pour 3mL of supplemented M9 medium to a Falcon tube.
- Pick out a colony on the plate of E.coli and put into the Falcon tube.
- Grow it at 30 degrees for 16h.
- Dilute it 50 folds with the same media and incubate until OD550 is about 0.15.
- Ditribute 3ml of the culture to falcon tubes and add certain amount of IPTG.
- Incubate the cultures at 30 degrees and measure OD550 of them every 30 min
As Fig.1 shows, when E.coli starts to lyse depends on the strength of induction of IPTG. When the concentration of IPTG is 1mM, E.coli starts to lyse after 1.5h, in contrast, when it is 0.03mM, E.coli starts to lyse after 6h. This result suggests the following two facts.
- To evaluate the lytic activity quantitatively, we must evaluate it after cell lysis becomes a steady state.
- We can regulate freely when E.coli are lysed, by changing the strength of induction of λ Lysis cassette.
Experiment 2-1 Quantitative characterization of lytic activity of λ lysis cassette
A steady state of cell lysis caused by λ lysis cassette exists. So we evaluated the lytic activity by measuring OD550 of the cultures that were steady state of cell lysis.
As table1 shows, all of the cultures in 18h seem to be a steady state of cell lysis.
RPU is relative activity of promoter, so Fig. 2 shows the relationship between the lytic activity of λ lysis cassette and its expression level. The lysis cassette effectively induced cell lysis even when the expression was very repressed (0.2 RPU). Moreover, the activity of the cell lysis has a distinct threshold around 0.1 RPU. In case of the higher induction than 0.2 RPU, the cell populations reached a plateau but not zero.
At 18h, we did CFU assay on samples: 0, 0.02, 0.05, 0.5, 1.0mM IPTG concentration. We only succeeded on two samples: 0.05 and 1.0mM. On the sample of 0.05mM, the value was 3.4x105c.f.u./ml, and 1.0mM, the value was 2.3x104c.f.u./ml.
Experiment 2-2 Quantitative characterization of lytic activity of λ lysis cassette
We also did the another experiment to observe the steady state.
As Fig.3 shows, the cell population had become to the steady state.