{"id":3813,"date":"2019-06-19T20:03:19","date_gmt":"2019-06-19T11:03:19","guid":{"rendered":"http:\/\/163.180.4.222\/lab\/?p=3813"},"modified":"2019-06-19T20:03:19","modified_gmt":"2019-06-19T11:03:19","slug":"selective-killing-of-antibiotic-resistant-bacteria-from-within","status":"publish","type":"post","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=3813","title":{"rendered":"Selective killing of antibiotic-resistant bacteria from within"},"content":{"rendered":"

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Some bacteria naturally transfer pieces of their DNA within and between species. Such a piece of DNA has been engineered to act as a molecular \u2018Trojan horse\u2019 that unleashes a toxin to selectively kill antibiotic-resistant\u00a0Vibrio cholerae<\/i>\u00a0bacteria.<\/h5>\n

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Antibiotic resistance among infectious bacteria is an increasing problem worldwide, resulting in large part from the overuse of antibiotics.\u00a0Writing in\u00a0Nature Biotechnology<\/i><\/a>, L\u00f3pez-Igual\u00a0et al.<\/i>1<\/a><\/sup>\u00a0demonstrate a nifty way to selectively poison antibiotic-resistant\u00a0Vibrio cholerae<\/i>\u00a0bacteria \u2014 the species that causes cholera \u2014 from the inside. The authors\u2019 aim is to offer a highly targeted alternative to standard broad-brush antibiotics.<\/p>\n

Our present scattergun overuse of antibiotics has caused several problems, one being the emergence of antibiotic-resistant bacteria. Another is that typical broad-spectrum antibiotics affect not only the target disease-causing (pathogenic) bacteria but also our normal beneficial bacteria, which protect us against infection and might influence many other aspects in humans, including weight, mood and allergies (see\u00a0go.nature.com\/31x0csa<\/a>). The specificity of the approach proposed by L\u00f3pez-Igual and colleagues could avoid both of these issues.<\/p>\n

The authors\u2019 method builds on the ability of bacteria to transfer certain pieces of genetic material (\u2018mobilizable\u2019 DNA) on cell-to-cell contact, in a process known as conjugation. L\u00f3pez-Igual\u00a0et al.<\/i>\u00a0take advantage of this phenomenon to transfer a set of genes that encode a toxin (a protein called CcdB) and its antidote (a protein known as CcdA) from donor bacteria into their neighbours. The system is designed so that the toxin will be made only in\u00a0V. cholerae\u00a0<\/i>and the antidote will be made only in the\u00a0V. cholerae<\/i>\u00a0bacteria that are antibiotic-sensitive, so that just antibiotic-resistant\u00a0V. cholerae<\/i>\u00a0will be killed (Fig. 1).<\/p>\n

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Figure 1 | Time-delayed destruction of\u00a0<\/b>Vibrio cholerae<\/i><\/b>.<\/b>\u00a0L\u00f3pez-Igual\u00a0et al.<\/i>1<\/a><\/sup>\u00a0have designed a system with which to selectively kill antibiotic-resistant\u00a0V. cholerae<\/i>, the bacterial species that causes cholera. They engineered a circular piece of genetic material that encodes both a toxin (red) that is interrupted by a component known as an intein (yellow) and an antidote to the toxin (blue). These genes are inserted into a donor bacterium, and can then be transferred into other bacteria in a population through a transfer process called conjugation (dotted arrows).\u00a0a<\/b>, If the bacterium that receives the genetic material is not\u00a0V. cholerae<\/i>, then the toxin and antidote are not expressed because the bacterium lacks the appropriate transcription-factor protein that drives their expression. Such cells survive.\u00a0b<\/b>, If the recipient is antibiotic-sensitive\u00a0V. cholerae<\/i>, the bacterium makes the antidote and a toxin that is non-functional because it contains an intein. Over time, the intein removes itself from the toxin. However, the resulting functional toxin is inactivated by the antidote, and the bacterium lives.\u00a0c<\/b>, If the recipient is an antibiotic-resistant\u00a0V. cholerae<\/i>, expression of the antidote is blocked by a repressor protein called SetR, which is encoded by a gene that contributes to antibiotic resistance. When the intein removes itself from the toxin, this generates active toxin and the bacterium dies.<\/span><\/p>\n<\/figcaption><\/figure>\n

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L\u00f3pez-Igual\u00a0et al.<\/i>\u00a0used several clever tricks to ensure that toxicity occurred only in the target cells. First, they engineered the toxin-encoding genes to be under the control of a\u00a0Vibrio<\/i>-specific protein, the transcription factor ToxR (which is essential for\u00a0V. cholerae<\/i>\u00a0to cause disease). This means that, if the toxin-encoding genes were ever transferred by conjugation to other bacterial species, no toxin could be made. Second, to ensure that any antibiotic-sensitive\u00a0V. cholerae<\/i>recipients do not die, the authors included the gene that encodes the antidote on the mobilizable gene set. This antidote is made in all antibiotic-sensitive\u00a0V. cholerae<\/i>\u00a0but not in antibiotic-resistant\u00a0V. cholerae,<\/i>because in the latter bacteria, antidote production is turned off by a repressor protein, SetR, which is encoded by, and also regulates, the antibiotic-resistance genetic element\u00a0SXT<\/i>\u00a0in these bacteria.<\/p>\n

However, simply having the antidote present might not be enough to prevent the killing of bacteria that are not intended to be targeted. The CcdB toxin is highly potent and acts rapidly, killing bacteria by causing extensive damage to their genetic material \u2014 it acts by inhibiting an enzyme called gyrase, locking it to DNA, which results in DNA breaks. The authors therefore built in a delay mechanism \u2014 generating a ticking time-bomb that becomes deadly only after some time.<\/p>\n

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