Lawn Sale said:
I am curious as to how they do this. I have heard this and another statement, that waterborne organisms can build a resistance to chlorine, for years, but am not sure I'm buying it.
I am not trying to start a fight, I am just interested in the mechanics behind it, as it makes no sense to me. I equate the statement to saying a person can build a resistance to bullets or a Mack truck if they are exposed to them often enough. Or more accurately, that someone else can build a resistance to bullets if someone else gets shot.
Dead is still dead is my understanding, but I’m no biologist and don’t play one on TV. When you kill something it doesn't come back to life, unless it was never dead in the first place.
Pete's comment, "evolution in action" is right on the money.
In more detail:
Three cases: (This logic applies to a wide range of stressors, eg chemicals, radiation, natural or human selection (as in pulling weeds).) I also will refer to death, but anything which reduces, slows, or stops reproduction can have similar effects.
* 1. A stressor has no effect. Simple: no effect is no effect.
* 2. The slightest trace of a stressor is absolutely lethal. Dead is dead. Not much chance to adapt...
* 3. A stressor is partially lethal (perhaps due to a low concentration or inadequate contact time). There will be some genetic variation across a population of an organism, some will be more resistant than others. The resistant ones are more likely to live, the sensitive ones are more likely to die. Over time the population will contain a higher percentage of resistant organisms. And since variation is continually introduced into the gene pool by genetic copying errors, radiation damage, mutations, etc, the population as a whole can become more and more resistant. (Most such errors, radiation damages, and mutatations are harmful or lethal to the organism, but it only takes a few "good" ones...)
Examples:
1. A human scans bins of seeds for weed seeds and removes them. But he misses some of the weed seeds that look like the desired seeds. Thus over time, the weed seeds end up looking more like the desireable seeds. (Same for manual weeding of the growing plants, exept that the weed plants end up looking like the desired plants.)
2. Selective breeding. A human selects which individual organisms get to reproduce and kills those which are considered undesirable. Radiation or chemical insults may be used to increase the rate of genetic variation. Cross-breeding may also be used in introduce desireable genetic variation.
3. Antibiotic resistance. A human takes an antibiotic for some bacterial infection but stops when he "feels better". (A classic scenario...) At this point, most but not all of the bacteria are likely to have been killed. The most susceptable bacteria have died, those that remain are the resistant ones. Now free from the antibiotic, they multiply and you now have a population of resistant bacteria when you started with a population which only contained a few resistant bacteria. (This is why you should always take the full course of an antibiotic--you want to kill all of the bacteria before the population can become resistant.) Constant use of low levels of an antibiotic (such as is frequently used in the livestock industry) can have the same effect because the low dose may only kill some of the bacteria.
It is easier to adapt to some stressors than others. If a stressor attacks a specific vital molecule, perhaps a minor change in the molecule still does the biological job, but is not attacked by the stressor. (This appears to apply to the development of resistance to a number of drugs.) Stressors like chlorine and iodine (chemical ozidizers) are probably much broader in their effect and probably harder to adapt to. But these adaptation mechanisms are
very powerful and will likely achieve some degree of resistance--higher concentrations may be required to kill the organisms.
Another effect of a stressor would be to alter the species makeup of an ecosystem--the population of more susceptable organisms decreases and the population of less susceptable organisms increases.
Frequent sub-lethal doses of any stressor could have either or both effects.
What affects one group of organisms cannot affect the other unless some of the originals survive, and then they would have to mutate. It's not like the organisms can transmit data back to their comrade’s telling them this or that, they have no organizational link and thus cannot develop “resistance”.
Not quite true: Some simpler (single celled etc) organisms can engage in conjugation. Two individuals (which, IIRC, can be of different species) connect and exchange gentic material. Thus a trait can cross species rather than having to be developed twice independently.
What is happening is that we are the ones that are reducing our immunity to the organisms by using such destructive products. Our bodies stop producing the right kind and amount of antigens needed to combat the foreign invaders by letting the products, rather than our bodies, do the work. That is how we become more susceptible to disease, not that the organisms have developed a resistance to biocides.
This is a separate effect. Our immune systems appear to adapt in both general and specific ways to the stressors in our environments. So growing up in clean city houses (rather than playing in the dirt and being exposed to farm animals) is suspected to be a factor in the increase in allergies (generally over-sensitive immune systems). Prior exposure to some pathogens can result in specfic immunity to that pathogen or a specific strain of that pathogen (eg childhood diseases, flu). The same probably applies to a local's tolerance of the pathogens in the local drinking water which may not be shared by an unhappy traveler.
Disclaimer: I am not a biochemist. This is just my (hopefully accurate) understanding of the topic.
Perhaps a little long-winded, but I hope this helps to answer your questions.
Doug
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