Posted on 02/13/2011 2:39:33 PM PST by decimon
First evidence of gene transfer from human host to bacterial pathogen offers new view of evolution, disease
CHICAGO --- If a human cell and a bacterial cell met at a speed-dating event, they would never be expected to exchange phone numbers, much less genetic material. In more scientific terms, a direct transfer of DNA has never been recorded from humans to bacteria.
Until now. Northwestern Medicine researchers have discovered the first evidence of a human DNA fragment in a bacterial genome – in this case, Neisseria gonorrhoeae, the bacterium that causes gonorrhea. Further research showed the gene transfer appears to be a recent evolutionary event.
The discovery offers insight into evolution as well as gonorrhea's nimble ability to continually adapt and survive in its human hosts. Gonorrhea, which is transmitted through sexual contact, is one of the oldest recorded diseases and one of a few exclusive to humans.
"This has evolutionary significance because it shows you can take broad evolutionary steps when you're able to acquire these pieces of DNA," said study senior author Hank Seifert, professor of microbiology and immunology at Northwestern University Feinberg School of Medicine. "The bacterium is getting a genetic sequence from the very host it's infecting. That could have far reaching implications as far as how the bacteria can adapt to the host."
It's known that gene transfer occurs between different bacteria and even between bacteria and yeast cells. "But human DNA to a bacterium is a very large jump," said lead author Mark Anderson, a postdoctoral fellow in microbiology. "This bacterium had to overcome several obstacles in order to acquire this DNA sequence."
The paper will be published Feb. 14 in the online journal mBio.
The finding suggests gonorrhea's ability to acquire DNA from its human host may enable it to develop new and different strains of itself. "But whether this particular event has provided an advantage for the gonorrhea bacterium, we don't know yet, " Seifert said.
Every year an estimated 700,000 people in the United States and 50 million worldwide acquire gonorrhea. While the disease is curable with antibiotics, only one drug is now recommended for treatment because the disease developed resistance to previously used antibiotic options over the past four decades.
Gonorrhea is a particularly serious disease for women. If left untreated, gonorrhea can lead to pelvic inflammatory disease, a painful condition that can cause sterility and ectopic pregnancy. In rare cases, men and women can develop a form of the disease that leaves the genital tract and enters the bloodstream, causing arthritis and endocarditis, an infection of the inner lining of the heart.
An ancient disease that sounds like gonorrhea is described in the Bible, noted Seifert, who has studied the disease for 28 years. Most of his research focuses on how the bacterium evades the human immune system by altering its appearance and modulating the action of white blood cells.
The gene transfer was discovered when the genomic sequences of several gonorrhea clinical isolates were determined at the Broad Institute in Cambridge, Mass. Three of the 14 isolates had a piece of DNA where the sequence of DNA bases (A's, T's, C's and G's) was identical to an L1 DNA element found in humans.
In Seifert's Feinberg lab, Anderson sequenced the fragment to reconfirm it was indeed identical to the human one. He also showed that this human sequence is present in about 11 percent of the screened gonorrhea isolates.
Anderson also screened the bacterium that causes meningitis, Neisseria meningitidis, and is very closely related to gonorrhea bacteria at the genetic level. There was no sign of the human fragment, suggesting the gene transfer is a recent evolutionary event.
"The next step is to figure out what this piece of DNA is doing," Seifert said.
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The research was sponsored by the National Institutes of Health.
NORTHWESTERN NEWS: www.northwestern.edu/newscenter/
I can't help thinking about what this means for those who fool around. The odds for those who are young and not sexually active, for those in a monogamous relationship, and for those who abstain from sex are all excellent. As for those who engage in casual sex or who patronize prostitutes, the odds are not pretty.
Yes there is. Bacteria are not only adept at gaining foreign DNA, they are ‘experts’ at getting rid of any DNA that isn’t ‘worth its salt’.
If any at all present in your body didn’t get the DNA your immune system (by some unknown mechanism) was introducing, then those that didn’t get the DNA would dominate subsequent generations. Those that gained it would experience selective pressure to rid themselves of it, and any mutation that rendered it inoperable would be favored.
It would be, first of all an amazing unknown mechanism, and secondly - an attempt to beat a bacteria at its own game.
You don’t win that way, it is an outlandish strategy presupposing unknown mechanisms that, even if true, wouldn’t be effective.
Reality is, by most accounts, weird enough. Our immune system introducing DNA into bacteria is weird, but not weird enough - because it wouldn’t work.
Excellent and astute series of replies. Thanks for sharing.
They refer to the human sequence the bacteria has acquired as an “element”. This usually denotes a sequence of DNA that is bound by a specific 3-D protein, usually a transcription activator or repressor.
For example, the difference between lactose tolerance as an adult and lactose intolerance is a mutation in a transcription repressor ‘element’ near the lactose gene. In almost all mammals this repressor turns off the gene in adulthood, in Northern European populations and some African cattle herding populations this DNA ‘element’ is mutated, so that the lactose gene is expressed as an adult.
It will be interesting to see what, if anything, this DNA is doing, and what advantage (if any) it confers upon those bacteria with it.
I never said a bacteria was “too smart”. That is your own misconception.
Evolution would work against your proposed mechanism, there is no “smart” required.
A change in the dangerous characteristics of an invading pathogen through introduction of new DNA into its genome would not work for the three reasons I already outlined, and there was no “smart” required from the bacteria in either....
1) unknown mechanism completely removed from anything observed in how the immune system works. This is obviously the weakest objection because absence of evidence is not evidence of absence. But it presupposes a rather complex and bizarre mechanism that has never been observed, Occum’s razor precludes taking this assumption seriously without any evidence. This MIGHT be evidence of what you suggest, but first we will have to see what the human DNA element is doing in the bacteria and if having it is advantageous or disadvantageous for the bacteria.
2) Even if the mechanism existed and was highly effective at introducing human DNA into the bacteria, natural selection would favor any that were not affected until they dominated each subsequent generation.
3) Selective pressure would work towards elimination or mutation of the foreign DNA sequence if it conferred a disadvantage.
It might be easier to deal with the arguments I didn't make, but it isn't exactly logical discourse at that point, is it? Can you deal with the arguments I actually DID make?
They are “experts” in that bacteria routinely gain and rid themselves of foreign DNA. It is a trait, a characteristic, an intrinsic property.
Yes, natural selection.
If 10% of the bacterial population has the human DNA and it is advantageous there will eventually be 11% of the bacterial population that will have that trait.
It is a mathematical inevitability.
If it is disadvantageous then eventually only 9% of the population will have that trait.
That is why a fantastical mechanism whereby the immune system would add DNA to a bacteria would be idiotic, because the more detrimental the trait it passed on - the quicker it would be selected against and eliminated from the population.
The immune system is in the business of identifying foreign 3D structures and KILLING THEM, not messing with their DNA in the hopes that it will make them act nicer.
I mean what sense does that make? But I sense where you are coming from, and it doesn't seem so strange now that you prefer fantastical illogical magical mechanisms to actual science.
I said the mechanism you proposed was idiotic because it wouldn't work. I didn't call you an idiot. If you actually understood the subject and still insisted it would work, I might, but you obviously don't.
Intelligent design isn't a buzz word that makes natural selection go away. The formulator of the philosophy accepts natural selection, micro-evolution, even the common descent of species. He just argues it had to have a guiding hand. Do you accept natural selection but think it needs a guiding hand, or do you reject it?
Nothing in evolution requires “progression” of species from simple to complex. That is another misconception you have. Nice that you rejected something that you never really understood.
So if the DNA supposedly added to the bacteria was a detriment to its continued survival and pathogenicity in the human environment and 10% of the population had it, it is a mathematical inevitability that eventually LESS THAN 10% will have it.
Can you understand that point? If so we can move on. If not I can perhaps explain it to you in greater detail.
"Protect your daughters," eh? Probably every perv there is bought that video.
“Guilty by association”. Ever hear of that?
Look, you can’t just call an idea “idiotic” while divorcing the idea from the source. Thus, logically speaking, if you deem an idea to be idiotic, the source of the idea is, by association, also being called “idiotic”. Nice try in trying to spin your way around that. Ideas don’t materialize from nowhere.
Also, natural selection might be part of the evolutionary process, which might all be part of an “intelligent” design. But, from observations in nature, ever since humans can recall, there hasn’t been any observed “evolution” and most of what has actually been observed is “adaptation” by a species in order to survive or thrive. But, those characteristic for adaptation were already part of the “design”. Can you point to any “adaptation”, or trigger, which essentially caused a jump from one kind of species to another, even if the change is very subtle? I mean, genetically speaking, has there ever been a “mutation” that has been observed? They might have happened, but, they haven’t been proven.
Now, going back to the bacteria...
So, why is it so preposterous that DNA from a species could be “introduced” as a defense mechanism? You’re the type that, if you had been around 100 years ago, and heard about how human blood goes around with its “tiny soldiers” destroying invading “enemies”, you would’ve scoffed at the idea. Too fantastic, you would say, and too much science fiction. So, why not a defense mechanism that “renders” an invader “neutral” while not necessarily killing that invader?
And, like I said before, the best time to destroy an enemy or to neutralize it, is in the initial stages, when the invaders are low in number, and not as hard to fight or destroy as it would be after they’ve had a chance to multiply. At that point, when they’re still low in numbers, the neutralization or destruction could be at 100% rather than your 10% (where you then hypothesize that, the rest would still be available to carry out their damage).
Even a genius can come up with an idiotic idea or two, sometimes three or four! ;)
Adaptation is evolution. Semantics. Darwin's theory becomes “Adaptation through natural selection of genetic variation”, but the mechanism is exactly the same.
Yes, mutations happen every generation. DNA cannot be perfectly replicated, as such every time it is copied errors are introduced by DNA polymerase.
Bacteria in fact have a gene for an error prone DNA polymerase that is expressed instead of the high fidelity DNA polymerase when the bacteria experiences stress.
In other words part of the bacterial stress response is to induce mutations.
I repeat, the immune system is interested in identifying a foreign 3-D structure and KILLING it, not identifying a foreign 3-D structure and introducing new DNA into it in the hopes that now it will behave itself.
“He also showed that this human sequence is present in about 11 percent of the screened gonorrhea isolates.”
I said 10%, please excuse the rounding error.
If the human sequence is present in 11% of the bacteria and it is advantageous in the human environment - then eventually 12% of the bacteria will have the trait. If it is disadvantageous in the human environment - then eventually less than 10% will have it.
It is a mathematical inevitability.
Now why don't you try to tell me how the immune system would hope to eliminated a pathogen by introducing detrimental DNA to some 11% of the population - when that addition is just going to “wash out” in subsequent generations through less reproductive viability, loss of plasmid DNA, or mutation - because it is detrimental.
How is it going to go from 11% of the population to 100% when any fraction not affected is going to compete better than those affected, and any that ARE affected are going to be under selective pressure towards losing the DNA through mutation or plasmid removal?
Adaptation is through a CHANGE in DNA. Change in DNA is mutation. “adaptation is a characteristic, already built into the DNA of a species” is only true if by “built into the DNA of a species” you mean that inevitable CHANGE is a feature “built into the DNA”. You come far closer to the truth when you say “What you want to call mutations, others would call “adaptations””. Adaptations are though changes in DNA. Changes in DNA are, by definition, mutations.
“If it's the same exact DNA” that is my point, it is not a change in how the DNA is expressed, it is a change in the DNA itself. Expression of error prone DNA polymerase during stress ensures that it is not “the same exact DNA”.
Evolution is neither “progression” and neither does it imply or necessitate a change into a different species.
Evolution is not progression, I have told you that before.
Evolution is not speciation, any more than gravity is planet formation. Planet formation is the RESULT of gravity, but gravity itself is not dependent upon a planet being formed.
An immune system “Catch and release” program would never be as effective as a “Catch and KILL” program. That is what makes your argument so facile.
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