Q: We have had chlorinated public water systems since the early 20th century—enough time for Cryptosporidium or Clostridium perfringens to develop metabolic or behavioral changes to survive chlorination. Why don't microorganisims become resistant to chlorine? A: Water purification is the critical link in controlling disease transmission by water. Water purification usually involves a series of steps: (1) a sedimentation basin, (2) a settling basin, (3) rapid sand filter, and (4) treatment with a disinfectant such as chlorine.
The preceding purification process effectively removes or inactivates disease-causing bacteria and indicator microorganisms (coliforms). However, this process does not consistently and reliably remove
Giardia lamblia cysts, Cryptosporidium oocysts, Cyclorspora, and viruses. Giardia, a cause of human diarrhea, is now recognized as the most commonly identified waterborne pathogen in the United States. In the last few years, Cryptosporidium has become an even greater problem than Giardia. Major sources of Giardia and Cryptosporidium are contaminated soil, plant material, and water. Clostridium perfringens is not a waterborne microorganism and is not found in water supplies. Transmission of this bacterium usually occurs via contact with bowel flora or exposure of injured tissue to soil contaminated with C. perfringens spores. (The protozoan (not a bacterium), Cryptosporidium forms a very resistant cyst that is the only known life cycle stage among microorganisms to resist chlorination in commercial water supplies.)
Many different chemicals are available for use as disinfectants. Examples include phenolics, alcohols, halogens (chlorine, bromine, iodine, fluorine), heavy metals (mercury, silver, arsenic, zinc, copper), quaternary ammonium compounds, aldehydes, and sterilizing gases. As noted above, chlorine is the usual disinfectant for municipal water supplies. It may be applied as a gas, sodium hypochlorite, or calcium hypochlorite, all of which yield hypochlorous acid. The result is oxidation of all cellular materials and destruction of vegetative bacteria and fungi, but not spores, cysts (
Giardia), or oocysts ( Cryptosporidium). Death of almost all microorganisms usually occurs within 30 minutes. As a result, the microorganisms do not have a chance to mutate and genetically develop resistance to this form of disinfection.
However, microbiologists now know that overuse of the disinfectant triclosan can leads to resistance. Triclosan seems to be everywhere; it is found in products such as deodorants, mouthwashes, and soaps, on cutting boards, and on baby toys. Unfortunately, we are already seeing the emergence of triclosan-resistant bacteria that can actively pump the antiseptic out of the cell. Bacteria seem to be responding to antiseptic overuse in the same way they reacted to overuse of antibiotics; fortunately they have not developed resistance to the halogen disinfectants such as chlorine.
John P. Harley