The question
Name: ______________________ Date: ____________ Quorum sensing: light output and a released signal (4 points)
Data table
Spent medium and light from sparse bacteria
Researchers grew the bacterium Vibrio fischeri to a high density (8 × 10⁸ cells/mL), then filtered out the cells to obtain cell-free "spent" medium. They placed fresh cells at a low density (1 × 10⁷ cells/mL) in mixtures of fresh medium and spent medium and measured light per cell after 1 hour; the cells did not multiply noticeably in that time. In a last treatment they used spent medium from a strain that cannot make the autoinducer. Values are means ± standard error of 3 cultures, in relative units.
| Spent medium in the mixture (%) | Source of spent medium | Light per cell (relative units) |
|---|---|---|
| 0 | — | 1.0 ± 0.2 |
| 5 | Normal strain | 1.2 ± 0.2 |
| 10 | Normal strain | 4.5 ± 0.6 |
| 20 | Normal strain | 140 ± 12 |
| 40 | Normal strain | 410 ± 30 |
| 80 | Normal strain | 450 ± 35 |
| 80 | Strain that cannot make autoinducer | 1.1 ± 0.2 |
(a) Describe how light per cell changes as the percentage of spent medium from the normal strain increases. [1 point]
(b) Calculate how many times greater the light per cell is with 20% spent medium than with 10% spent medium. [1 point]
(c) Identify what the treatment with spent medium from the strain that cannot make autoinducer rules out. [1 point]
(d) A student claims that the bacteria switch on their light in response to the concentration of a signal molecule, not to the number of cells around them. Support the claim with evidence from the data. [1 point]