One possible explanation for the sudden \"threshold effect\" for toxin secretion
ID: 79290 • Letter: O
Question
One possible explanation for the sudden "threshold effect" for toxin secretion seen in S. aureus cultures is that this bacteria regulates toxin secretion by quorum sensing. Quorum sensing is a way that bacteria can sense when their population size reaches a particular density, and then do something different in response. In other words, quorum sensing allows an entire population of bacteria to carry out coordinated activity. Individual bacteria secrete signaling molecules that other individuals can detect. As the population grows, the concentration of the signaling molecule in the environment increases. Once the concentration of the signaling molecule reaches a threshold level, as detected by receptors on the bacteria's surface, signal transduction pathways in the bacteria are triggered that induce expression of a target gene. The target gene may code for a protein involved in bioluminescence, the formation of a biofilm, or toxin production. Quorum sensing ensures that individual bacteria don't express a gene until every individual in the population simultaneously expresses that same gene. To test the hypothesis that toxin secretion in S. aureus is dependent on quorum sensing, you examine mutants that are known to block two separate quorum-sensing pathways: the agr pathway and the TRAP pathway. You use completely normal ("wild-type") cells as a control. You also decide to examine the ztr- mutant as an additional control, since this mutant is known not to be involved in quorum sensing, and you wish to test for an effect specific to quorum-sensing mutations. For each mutant, you grow cultures to a standardized high density, measure the amount of toxin they produce, and compare them to the wild-type control cells. The following steps are listed from a proposed model of how S. aureus changes from a harmless skin flora to a serious wound pathogen. Order the steps by filling in a number in the blank next to each step, with 1 representing the first step and 5 representing the last. a. S. aureus grows to high densities in tissues (a) _____ b. S. aureus secretes toxins (b) _____ c. S. aureus enters the tissue through a wound (c) _____ d. S. aureus grows at low density on the skin surface (d) _____Explanation / Answer
(a) 3
(b) 5
(c) 2
(d) 1
(e) 4
S. aureus grows at low density on skin surface. It enters the tissues through wounds and grow their to high densities. This is because, they can easily take nutrition from tissues, which is not possible on outer skin surface due to barrier of keratin. Then quorum sensing results in release of toxins when bacterial population reaches a threshold size to control the growth. But harm host too.
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