Quorum sensing (introduced in Ch. 11, Section 11.4) allows bacteria to detect the number of neighboring cells and to trigger a response only when this number reaches a critical level. Quorum sensing is used by V. fischeri in light production and by many pathogenic bacteria, including Vibrio cholerae, to turn on genes for toxin production only when a critical cell density is reached. Why might quorum sensing be beneficial to pathogenic bacteria?

Video transcript

Hi everyone. Here's our next question. It says how does quorum sensing help the bacterium pseudomonas aeruginosa in fact its host. Well quorum sensing we can recall from our content video is a means of cell to cell communication that allows bacteria to alter their gene expression based on its population density. Um We can think of the word quorum if you've ever heard of it in a like a civic sense, a quorum is a minimum number of people. You need to start a meeting or take a vote. So quorum sensing has to do with sensing a minimum level of population to do something. So when population density is low the bacteria express certain genes certain genes are repressed with quorum sensing. Um as the bacteria grow they're releasing extra cellular signal signaling molecules called a eyes. When these molecules reach a high enough level because the bacteria have multiplied enough, gotten dense enough then. So when population reaches the crucial level the quorum new jeans are expressed and why would this be a benefit to bacteria. It's because there are certain processes that are very energetically expensive. And the bacteria can hold off on carrying those out until they reach a significant population so that the impact of those processes will have a maximum impact on the host or the environment. So genes that get expressed this way our genes that cause it to form a biofilm And they can be genes that increase the virulence of the bacteria. So let's look at our answer choices and see which makes sense with this process of quorum sensing. So choice A says it makes them undetectable if they have an insufficient population size. And we talked here more about things that get expressed when they reach a great enough density of population. But the flip side is before they reach that they kind of sneak along at low levels until they reach a sufficient size to just overwhelm the host with this biofilm formation and increase in virulence. So Choice A. Oops. Choice A. Is correct. And that allows them to have this sort of sneaky mode until they've built up a sufficient reserve to go on the attack. Choice B says it makes them rapidly increase their population size. Well that's not our answer. Quorum sensing allows the bacteria to respond to an increase in population size but it doesn't actually cause that increase choice C. Says it weakens the immune system of the host. Well again that's not the mechanism here. Um It does mean that when there are low levels they evade the immune system of the host. Uh Since they're not carrying out these adverse activities, Choice D. Says it destroys the enzymes that will attempt to eliminate them. Again that's not what quorum sensing does. It uses this um This mode of switching to higher virulence biofilm formation to overwhelm the defenses of the host only when it has sufficient resources sufficient numbers to do so. So again how does quorum sensing help the bacterium pseudomonas aeruginosa infect its host choice A. It makes them undetectable if they have an insufficient population size, see you in the next video.