Published online by Cambridge University Press: 15 May 2009
The investigation deals with the bacteriological examination of 386 samples of water from Manchester swimming baths where the method of purification could be studied.
There were 339 samples from chlorinated pools of which 160 gave acid and gas in MacConkey broth at 37° C. and 2130 strains were isolated. The remaining forty-seven samples were from untreated open-air pools. Thirty of these samples were positive and 270 strains were isolated. The strains were classified according to Wilson's method into the coli, intermediate, aerogenes and irregular groups, and only 1·78% of the total were found to be irregular. Both series of samples included more than 72% which contained coli, and among the chlorinated samples no less than 70% yielded aerogenes and nearly 49% intermediate type, while among the non-chlorinated baths there were 43% with aerogenes and 40% with intermediate type. There was a similar difference in the proportions of coli, aerogenes and intermediate type in chlorinated and in non-chlorinated baths when the results were considered on the basis of strains (Table II).
Since these differences in proportion were considered significant, and since the chief variation in conditions between outdoor and indoor baths consists of chlorination, a series of experiments was devised to test the effect of chlorine on pure cultures of coli, aerogenes, and intermediate type. There was very little change after adding 0·1 part of chlorine per million, but the addition of 0·3 and 0·5 part was followed by very rapid reduction in numbers with diminution in free chlorine. This reduction was on the whole least with aerogenes. The numbers continued to fall for about 48 hr. until all trace of chlorine had disappeared and then regrowth occurred. This regrowth was most vigorous and most rapid with aerogenes (see figures) but was very slow in all cases.