Do constant sanitizer levels promote bacteria growth?

[pkn2]

I did not see a specific bromine forum. I apologize if this is the wrong place, but I would think that this same question would apply to chlorine only at proper levels.

Do constant 3 to 4 ppm (generally accepted recommended levels) of bromine promote bacteria growth? Or does varying the ppm levels promote better efficacy of sanitation?

Is there a plus to letting the levels drop to 1 ppm and then raising the levels? Does the constant level allow the bacteria to "get used to it"?

[chem geek]

While constant exposure to antibiotics and other chemicals that kill using a single mode-of-action can allow for mutations to have bacteria develop resistance, this doesn't happen with chlorine or bromine because the levels are much higher than for inhibition (i.e. they are fast kill) and they operate via multiple modes of action where mutations cannot easily work around it. Of course, given enough time, evolution can certainly do its magic, but it took millions of years for protozoan oocysts to develop their very thick exterior shells that are chlorine resistant (especially for Cryptosporidium parvum) and such resistance developed long before chlorine was every used.

Now if bacteria are not killed quickly (because the chlorine level is very near zero), then they can attach to surfaces and form biofilms where they excrete chemicals that make them more resistant to chemical attack in general (including chlorine and bromine), though again such resistance developed long before the use of such products and was more for general environmental tolerance. As described in this paper, 1-3 ppm Free Chlorine (FC) was able to essentially prevent biofilm formation on plaster coupons where no chlorine resulted in 1.26x10^6 cfu/cm^2 while 1-3 ppm FC chlorine resulted in only 3.7 cfu/cm^2. Though it is true that superchlorination weekly lowered this to 1.1 cfu/cm^2, having only 4 bacteria (colony forming units) per square centimeter given the extraordinarily high bather loads equivalent to 1 bather per 350 gallons for 24 hours per day is essentially negligible. Now that same experiment found that in sand filters, the control with no chlorine had 1.10x10^9 cfu/g while 1-3 ppm FC had 220 cfu/g and 16 hour superchlorination to 10 ppm FC got this down to 1.3 cfu/g. So for sand filters in commercial/public high bather-load pools, superchlorination may make some sense. However, for residential pools that are much lower bather-load, it isn't necessary as is proven by the tens of thousands of pool owners reporting in on multiple forums maintaining chlorine levels without shocking and not having biofilm or other bacteria issues (or algae problems either, if the FC is maintained high enough relative to the CYA level -- algae are much harder to kill than bacteria).