Chapter 16
Microbial Communication: Bacteria-Bacteria and Bacteria-Host
Leland S. Pierson III, Raina M. Maier, and Ian L. Pepper
1. Compare the benefits and limitations of AHL-mediated QS to peptide-mediated QS.
This is a discussion question posed to force students to begin to think more mechanistically about signaling. Points to incorporate in a discussion could include:
A. Differences in diffusion of AHL signals and small peptides across cell walls, including different requirements for transport systems.
B. Differences in the chemical nature of the signals. For example, all AHL signals contain a core homoserine lactone ring with a fatty acid side chain. The major differences are in the length of the side chain and the substituted groups. In comparison, each small peptide could be composed of completely different amino acids. How would this affect recognition of specific signals across a range of bacteria.
C. Differences in the predicted stability of each type of signal outside the cell. Are they easily broken down, do they adsorb to surfaces thereby reducing their ability to re-enter the cell.
2. Design an experiment to determine the effect of bypassing QS control (i.e., making gene expression constitutive) on the ability of a human pathogenic bacterium to infect its host.
One hypothesis is that pathogens utilize QS to regulate the expression of virulence genes to enable it to reach sufficient population density to increase the probability of a successful infection. If the bacterium in essence ‘tips its hand’ too soon, the host has a greater chance of mounting a successful defense response against a lower population density of the pathogen.
One could take a virulence gene normally subject to QS control in a pathogen and make its expression constitutive (by using a strong unregulated promoter) and follow bacterial persistence and disease levels in a model host system.
3. Since higher organisms evolved in the presence of bacteria, it makes inherent sense that these different kingdoms of organisms have the ability to communicate with each other. What processes can you think of that might require bacterial-host cooperation?
This is a discussion question with no single answer. Possible processes could include:
A. Activation of gene expression in a commensal bacterium by host signals that serve to induce expression of genes that inhibit the ability of another bacterium, such as a pathogen, to colonize.
B. Production of negatively acting signals by the host to directly affect the ability of a bacterium to colonize a tissue or organ.
C. Uptake of host or bacterial signals by the other member of the interaction could influence available signal levels and modulate gene expression patterns.
D. Coordination of a joint metabolic pathway to break down compounds or synthesize other compounds.

4. Is cross-communication community-wide, or could selected subpopulations be differentially affected? Can you devise an experiment to test this idea?
This discussion question tries to make students formulate hypotheses regarding spatial or biological influences on signaling within a mixed microbial community. These influences could include:
A. Recognition of different signals by different subgroups of bacteria within the community.
B. Difference in activation thresholds by the same signal in different members of the community.
C. The existence of signal cascades where one organism, in response to a specific signal produced by another, then produces its own signal that is recognized by yet another subgroup of the community.