Monday, January 15, 2007
How 'DNA parasites' can increase spread of antibiotic resistance
Stealth technology maintains fitness after sex: Pathogens can become superbugs without their even knowing it, research published today in the journal Science shows. 'Stealth' plasmids* - circular DNA 'parasites' of bacteria that can carry antibiotic-resistance genes - produce a protein that increases the chances of survival and spread of the antibiotic-resistant strain.
Low-cost plasmids, described for the first time in the study are a threat to the use of antibiotics.
Plasmids are naturally occurring 'DNA parasites' of many bacterial species and have been known about for over 30 years. Some are able to transfer themselves from one bacterial cell to another through a sex-like process called conjugation, contributing to bacterial evolution. Worryingly, as well as copying themselves plasmids can pick up and transfer bacterial genes, such as those that make pathogens resistant to antibiotics.
However, the plasmid comes at a cost to the host bacterium: gaining a plasmid can reduce the host's ability to grow and reduce its fitness. When antibiotic treatment is stopped, the new microbe-plasmid combination will be eliminated quickly through fierce competition from more 'fit', plasmid-free bacteria.
The research teams, led by Professor Charles J. Dorman (research) at Trinity College Dublin, Ireland, and Dr John Wain (research) at the Wellcome Trust Sanger Institute** in Cambridge, UK, have discovered that an important class of plasmids use a stealth gene (called sfh) to allow entry into a new bacterium with minimal reduction in fitness.
With the low-cost version of the resistance plasmid they have described in Salmonella, resistant bacteria are likely to survive and the resistance genes to persist even if antibiotic therapy is stopped.
Their research shows that sfh codes for a protein that is very similar to another bacterial protein: the role of which is to organise the genetic material within each bacterium and control activity of many genes, including those involved in causing disease. The sfh protein binds to the new plasmid DNA, preventing its detection by the bacterium.
"The bacterial protein, called H-NS, is a very important molecule and affects the way a bacterial pathogen operates. By bringing in its own supply of the H-NS-like stealth protein (called Sfh), the plasmid avoids interfering with the natural balance of H-NS and DNA in the cell," explained Professor Dorman.
"Our work suggests that bacterial fitness can be manipulated by altering the proportions of H-NS and DNA in the cell, perhaps through the use of drugs, an insight that may be exploited in the future to prevent or to fight infection."
Bringing its own supply of the host-like protein is clearly an advantage for the plasmid, suggesting that the normal supply of H-NS in the bacterium may become limited when new DNA is imported. If a modified plasmid, lacking the sfh gene, is transferred to Salmonella, the effects of the plasmid are very rapidly detected.
Bacteria can acquire and transfer resistance genes through a variety of methods, but this new study shows how a single gene has the potential to increase dramatically the chance of successful - and health-threatening - transfer and survival of a battery of antibiotic-resistance genes.
The consequences for managing disease - especially in developing countries - are significant, explained Dr John Wain: "These plasmids are found in many pathogenic bacteria including those that cause typhoid and paratyphoid fever. Both of these diseases are increasing in the developing world and in the UK we are seeing more and more imported cases."
"But understanding is not enough: we now need to exploit this information to try to prevent the plasmid spreading any further."
Source: Wellcome Trust Sanger Institute (11th January 2007)
Based on the paper:
An H-NS-like Stealth Protein Aids Horizontal DNA Transmission in Bacteria
Doyle M, Fookes M, Ivens A, Mangan MW, Wain J, Dorman CJ
Science. 2007;315;251-2. PMID: 17218529 DOI: 10.1126/science.1137550
The Sfh protein is encoded by self-transmissible plasmids involved in human typhoid and is closely related to the global regulator H-NS. We have found that Sfh provides a stealth function that allows the plasmids to be transmitted to new bacterial hosts with minimal effects on their fitness. Introducing the plasmid without the sfh gene imposes a mild H-NS- phenotype and a severe loss of fitness due to titration of the cellular pool of H-NS by the A+T-rich plasmid. This stealth strategy seems to be used widely to aid horizontal DNA transmission and has important implications for bacterial evolution.
*A 1998 paper by Kado CI:
Origin and evolution of plasmids
Studies on the origin and evolution of plasmids may provide valuable insights on the promiscuous nature of DNA. The first examples of the selfish nature of nucleic acids are exemplified by primordial oligoribonucleotides which evolved into primitive replicons. The propagation of these molecules were likely patterned after the current viral RNA ribozymes, which have been recently shown to possess RNA synthesizing and template mediated polymerizing capabilities in the absence of proteins.
The parasitic nature of nucleic acids is depicted by satellite nucleic acid molecules associated with viruses. The satellite of adenovirus and tobacco ringspot virus serve as established examples: they contain no open reading frames. Comparative analysis of the replication origins of virions and plasmids show them to be conserved, originating from the simplest autocatalytic replicon to highly complex and evolved plasmids, replicating by a rolling circle mechanism.
The eventual association of proteins with nucleic acids provided added efficiency and protective advantages for molecular perpetuation. The promiscuous and selfish nature of plasmids is demonstrated by their ability to genetically engineer their host so that the host cell is best able to cope and survive in hostile environments. Survival of the host ensures survival of the plasmid. Sequestering of genes by plasmids occurs when the environmental conditions negatively affect the host. The sequestering mechanism is fundamental and forms the outreach mechanisms to generate and propagate macromolecules of increasing size when necessary for survival.
The level of sophistication of plasmids increases with the addition of new genes such as those that allow the host to occupy a specific environment normally inhospitable to the host cell. The vast range of plasmid types which have obtained genes interchangeably reflect the levels of sophistication achieved by these macromolecules. The Ti plasmid in Agrobacterium tumefaciens and the pSym and accessory plasmids in Rhizobium illustrate the level of complexity attained by replicons.
Antonie van Leeuwenhoek, Volume 73, Number 1, January 1998, pp. 117-126(10)
"The Wellcome Trust Sanger Institute, which receives the majority of its funding from the Wellcome Trust, was founded in 1992 as the focus for UK sequencing efforts. The Institute is responsible for the completion of the sequence of approximately one-third of the human genome as well as genomes of model organisms such as mouse and zebrafish, and more than 90 pathogen genomes. In October 2005, new funding was awarded by the Wellcome Trust to enable the Institute to build on its world-class scientific achievements and exploit the wealth of genome data now available to answer important questions about health and disease. These programmes are built around a Faculty of more than 30 senior researchers. The Wellcome Trust Sanger Institute is based in Hinxton, Cambridge, UK."
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