42 American Scientist, V olume 102
S c i e n c e S o u r c e
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44 American Scientist, V olume 102
经济分析
golden age of discovery当断不断
horizontal transmission bacterial transformation bacterial transduction relea of DNA relea of phage
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is a substantial interest in broadly characterizing the gene rervoirs to improve our overall understand-ing of how swapping genes from one rervoir to another contributes to the evolution of antibiotic resistance in bacterial pathogens. How Pathogens Fight Back Just as there are a number of different ways for bacteria to acquire an antibi-otic resistance gene, the genes them-lves reprent a number of different strategies to encode resistance. The genes that confer antibiotic resistance can be looly parated into four groups, each with their own unique mechanism for combating antibiotic exposure (e figure on page 45). Bacteria in the impermeable-barrier group are naturally resistant to certain antibiotics, either becau they lack the target of the antibiotic or becau their membranes are impermeable to the drug. In the cond group, target mod-ification, bacteria acquire mutations in genes that modify the target of the antibiotic, dampening its effectiveness. Each antibiotic is designed to target a well-defined esntial bacterial pro-cess. For example, fluoroquinolones are a widely ud class of antibiotics, for the treatment of skin, lung, or uri-nary tract infections. The antibiot-ics target DNA, disrupting the proper functioning of proteins involved in un-winding DNA’s helix during replica-tion. Mutations that confer resistance toward fluoroquinolone antibiotics of-ten change the conformation of the proteins, reducing the binding of the drug to its target and thus increasing the concentration necessary to block the process. In antibiotic modification, a resis-tance gene can encode an enzyme that helps break down or modify th
e antibi-otic before it can kill the bacteria. This tactic is often ud against beta-lactams, the most widely prescribed and diver chemical class of antibiotics, which include the well-known drug penicil-lin. Penicillin inhibits enzymes that re-model the bacterial cell wall and are esntial for the cell during growth. Re-sistance toward penicillin is frequently conferred by beta-lactamas, enzymes that cleave the penicillin molecule to render it ineffective in inhibiting the cell wall modification enzymes. Finally, efflux occurs when a resis-tance gene codes for proteins that ac-tively pump the antibiotic out of the cell, keeping its internal concentration low enough to prevent inhibition. This resistance mechanism is deployed for all antibiotics that have targets with-in the cell; in many cas such efflux pumps are able to pump out veral different antibiotics, resulting in mul-
tidrug resistance. An example of this has been obrved for tetracycline, an agent ud to treat a wid
e variety of infections. Resistance to the drug can stem from tetracycline efflux genes, which code for proteins that sit in the bacterial membrane and export the an-tibiotic out of the cell. Further complicating matters, resis-tance toward any one drug typically results from more than one mecha-nism. For instance, tetracycline re-sistance has been obrved to occur through target modification, antibiotic modification, and efflux mechanisms.Though the term “antibiotic re-sistome” didn’t emerge until five years ago, the concept encapsulates decades of rearch on the transmission and I n the first veral decades after the discovery of antibiotics, rearch-ers studied the emerging problem of antibiotic resistance by growing target microorganisms in the lab. The reliance on such methods traces back to the founder of modern bacteriolo-gy , Robert Koch, who work in cul-tured bacteria made “pure culture” the gold standard in clinical micro-biology laboratories. We know now that studying individual organisms grown in pure culture ignores the in-creasing number of dias caud not by one pathogenic bacterium but by veral acting in concert. In addi-tion, most environmental microbes cannot be readily cultivated in the lab. Recent technical breakthroughs have led to three established culture-independent strategies for explor-ing antibiotic resistance much more fully, in both pathogenic and non-pathogenic bacteria.The polymera chain reaction (PCR), lectively amplifies specific antibiotic resistance genes from com-plex microbial communities so they can be easily identified. PCR is well suited for studying the prevalence of already known genes
but cannot be ud to discover new ones. Metagenomic quencing identi-fies all of the DNA from a particu-lar environment, irrespective of its origin. The quences are then as-mbled and scanned for new genes that are similar to already known re-sistance genes
拍卖底价
互联网时代观后感Metagenomic functional lec-tions combine the cultivation-bad methods of old with new, culture-in-dependent techniques. A host organ-ism that is normally susceptible to antibiotics is genetically engineered to posss various chunks of DNA taken from a microbial community of interest. The modified hosts are then expod to antibiotics. The only survivors will be tho that acquired a resistance gene. The microbes can then be characterized to reveal the quence that confers resistance. In addition, we have developed a novel approach called PARFuMS (for Parallel Annotation and Reas-mbly of Functional Metagenomic Selections) that integrates culture-in-dependent functional metagenomic lections, next-generation DNA -quencing, and optimized computa-tional quence asmbly and annota-tion algorithms to profile resistomes.
Exploring Resistance Outside the Petri Dish 46 American Scientist, V olume 102Scientists once relied heavily on methods
that required culturing bacteria in the lab-
英帕尔
二年级古诗二首>金寨
香芹炒牛肉oratory (usually in petri dishes, as shown
at left) to study antibiotic resistance in var-
ious microbial populations. Today the
culture-dependent methods are being
eclipd by other approaches that enable
rearchers to characterize the large per-
centage—as much as 99 percent—of bac-
teria that cannot be grown in pure culture.
Doncaster and Bastlaw Hospitals/Science Source
