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Chlorination has been used for over a century to disinfect water supplies. Why haven't microorganisms evolved immunity to this chlorine by now?

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    $\begingroup$ It's aspecific, like ethanol. A bit like humans can't evolve to sustain bullets. $\endgroup$ – AliceD Nov 9 '15 at 4:35
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    $\begingroup$ Antibiotic resistance works because most antibiotics target specific sites on specific proteins. If these sites can be mutated to avoid antibiotic binding, while maintaining the proper protein function, they become resistant. Some resistance is mediated by enzymes that destroy the antibiotic, or by pumps that keep it out of the bacterial cell. Chlorination is a very harsh treatment that causes general damage to many biomolecules, it would be very difficult for any organism to develop a defense against it. TLDR, antibiotics is like picking a lock, chlorination is like breaking the door down. $\endgroup$ – user137 Nov 9 '15 at 4:35
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    $\begingroup$ @Dexter posted a nice answer on this: biology.stackexchange.com/questions/39931/… $\endgroup$ – AliceD Nov 9 '15 at 4:36
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    $\begingroup$ It is possible for microbes to acquire resistance to non-specific agents. Radiation resistant bacteria exist. Some bacteria can also tolerate high amount of oxidative stress (they can do that by upregulating enzymes like SOD). You may find this post useful. $\endgroup$ – WYSIWYG Nov 9 '15 at 5:25
  • $\begingroup$ @WYSIWYG You are right that there are organisms that can resist the effects of chlorine. It takes a little less than 11 days for Cryptosporidium to be killed in a chlorinated swimming pool, and given that water supplies likely use much lower concentrations of chlorine, there survival time in drinking water likely increases. cdc.gov/healthywater/swimming/pools/… $\endgroup$ – AMR Nov 10 '15 at 2:16
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The reason chlorination still works is that it is aspecific, just as @AliceD mentioned.

The way chlorination works is mainly by oxidizing biological molecules. Chlorine, along with other halogens, is a strong oxidizing agent. It breaks down the phospholipid bilayer membrane, proteins and enzymes. There is no way organisms can overcome this, unless it uses none of the chemical molecules that react with chlorine, which is virtually none.

On the other hand, antibiotics inhibit particular enzymes and proteins in a particular way and by making one or two amino acids different in the protein, the microorganisms can easily overcome this.

Reference - http://www.sswm.info/sites/default/files/reference_attachments/TUE%202011%20The%20Chlorine%20Dilemma.pdf

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  • $\begingroup$ @WYSIWYG But enzymes like SOD are mainly for tolerating oxygen radicals and reactive-oxygen species, not chlorine oxidation. $\endgroup$ – TanMath Nov 9 '15 at 5:29
  • $\begingroup$ Chlorination refers to the addition of chlorate (ClO3-), not Cl2. Hence, it is not halogens being added to water - that would be bad :) $\endgroup$ – AliceD Nov 9 '15 at 10:51
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    $\begingroup$ @AliceD Wikipedia doesn't back up your statement en.wikipedia.org/wiki/Water_chlorination $\endgroup$ – March Ho Nov 9 '15 at 13:29
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    $\begingroup$ @AliceD technically you sre correct as the chlorine gas, when dissolved in water, forms thar ion... But I didn't include all the details. I just wanted to make it simplified. $\endgroup$ – TanMath Nov 9 '15 at 21:25
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    $\begingroup$ Well, there are microorganisms that have adapted or were resistant in the first place... that is why cryptosporidium is such a problem in the potable water supply. While it is not much of an issue for healthy adults, it can be very bad for immunocompromised people. cdc.gov/healthywater/swimming/pools/… $\endgroup$ – AMR Nov 10 '15 at 2:19

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