Is it possible to block quorum-sensing pathways in vitro?

As the next step, you decide to test various agents to see if they can inhibit quorum sensing in vitro. You know from work done on a related bacterium that short proteins (peptides) can act as inhibitors of quorum sensing by blocking protein-protein binding events required for signaling. You test a large collection of synthetic peptides and find three candidates that appear to interfere with quorum sensing (peptides 1, 2, and 3).

In an experiment similar to the one in the previous part, you grow cultures of wild-type (non-mutant) S. aureus to the same standardized high density and measure the amount of toxin they secrete. For this experiment, however, you mix the candidate inhibitory peptides into the growth medium before starting the cultures. The control culture contains no peptide. 1. Of the peptides tested, ____ significantly inhibited toxin secretion in vitro.

2. Peptides 1 and 2 acted , suggesting that they inhibit _ quorum-sensing pathway(s).

3. Peptides 1 and 3 acted , suggesting that they inhibit _ quorum-sensing pathway(s).

4. Peptides 2 and 3 acted , suggesting that they inhibit _______ quorum-sensing pathway(s).

Choices:

peptides 1, 2, and 3

peptides 2 and 3

the same

peptides 1 and 3

peptides 1 and 2

additively

non-additively

different

This is a form of chemical communication between bacteria that regulates gene expression in response to cell population density. The chemical messengers are called autoinducers.

According to the graph, a significant change in toxin concentration in the growth medium is observed.
The control contains > 2μ moles/ml of toxins whereas the medium with Peptide 1, 2 and 3 contain 0.9, 0.7 and 1.25 μ moles/ml of toxin concentration.

Peptides 1, 2 and 2, 3 work additively as the culture medium containing both the combination has less than 0.5 μ moles/ml of toxin concentration.
In this experiment agr and TRAP signaling pathways are involved that regulate toxin gene expression. Since, both of them are involved, the inhibition of one will not yield significant inhibition of toxin production. Therefore, all peptides need to block both pathways at the same time to stop toxin production.
Hence, additive effect on the toxin concentration indicates that the peptides 1, 2 and 2,3 work on the same quorum sensing pathways.