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Wikipedia mentions this as a biocide. It also mentions that low concentrations of certain bacteriocides is bacteriostatic. (I am assuming that biocide and bacteriocide play the same role, here.)

Hence, the question. (The only information I can find about MI is with regards to allergies.)

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Yes, MI/MCI can behave as a bacteriostatic at low concentrations.

Check out table 1 in this paper: The Mechanism of Action of Isothiazolone Biocides. For a bunch of different isothiazolones (including the two you're interested in, though they call them MIT/CMIT instead of MI/MCI) it gives the Minimum Inhibitory Concentration (the concentration at which the isothiazolone acts as a microbiostatic) and the Minimum Biocidal Concentration (the concentration at which the isothiazolone acts as a biocide)*.

It's my understanding that basically everything that can act as a bacteriocide can also act as a bacteriostatic at a lower concentration. At low concentrations bacteriocides will kill some cells while leaving others alive in a stochastic (ie random) fashion. At some concentration this stochastic killing will balance out with the natural growth rate of the bacteria, and so thus act as a bacteriostatic.

*It should be noted that the paper lists these values for isothiazolone activity against yeasts, but the results would likely follow the same trends for isothiazolone activity against bacteria, since "CMIT/MIT has broad spectrum efficacy versus bacteria, algae, and fungi" (from the above cited paper).

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  • $\begingroup$ Is my understanding right that "this sort of bacteriostaticity is different from the one caused due to bacteriostatic drugs such as tetracarbon/tetracycline"? And should we be worried about MI/MCI, present in Dettol Liquid handwash and some cosmetics, going down in water drains, where bacteria may develop resistance for it, due to its low concentration present there? Also, can we discard the risk that MI/MCI can lead to bacterial-resistance against other bactericides? (This risk was present with tetracycline & other bacteriostats, which FDA banned.) [Sorry for the late reply; not-my-field.] $\endgroup$ – digikar Mar 27 '17 at 6:33
  • $\begingroup$ There are plenty of antibiotics that can only be administered to human patients at bacteriostatic concentrations, but any bacteriostatic will become a bacteriocide at some concentration. $\endgroup$ – tel Mar 27 '17 at 6:50
  • $\begingroup$ @Shubham1999 The answer to your question about resistance is that yes, of course it can develop, and low concentrations are usually where it occurs. There's been a lot of good research work recently about how populations of bacteria will "pioneer" niches with successively higher concentrations of some antibiotic. $\endgroup$ – tel Mar 27 '17 at 6:54
  • $\begingroup$ @Shubham1999 While multidrug resistance is a real thing (which probably hasn't received enough study to date), it's not necessarily something you have to worry about. In fact, there's good evidence that in many cases a bacteria that becomes resistant to one drug will become more vulnerable to one or more others. Essentially, a bacterium will always pay a price (in terms of its overall fitness) for any given resistance. $\endgroup$ – tel Mar 27 '17 at 6:58
  • $\begingroup$ This paper is something - tetracycline as a bactericide! So, all-in-all, would it be right to avoid using bactericide/bacteriostat (antibiotic) in places where soap-water does the job to the required quality, and get done away with antibiotic containing products if their efficacy is to be maintained as long as possible? (Is this question more suitable for Sustainable-Living-Stackexchange?) $\endgroup$ – digikar Mar 31 '17 at 8:10

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