The author, in 1915, made a number of experiments to ascertain whether the B. coli found after chlorination were more resistant to chlorine than the original culture. The strains surviving treatment with comparatively large doses were fished into lactose broth and subjected to a second treatment, the process being repeated several times. The velocity of the germicidal reaction with the strains varied somewhat, but not always in the same direction, and the variations were not greater than were found in control experiments on the original culture. No evidence was obtained that the surviving strains were in any way more resistant to chlorine than the original strain; in considering the results it should be borne in mind that the surviving strains were cultivated twice on media free from chlorine before again being subjected to chlorination.
A number of the strains that survived several treatments were cultivated in lactose broth and the acidity determined quantitatively. All the cultures produced less acid than the original culture, and the average was materially less than the original. These results point to a diminution of the bio-chemical activity by action of the chlorine.
A point of perhaps more scientific interest than practical utility is the relative proportion of the various types of B. coli found before and after treatment with chlorine. The author, in 1914, commenced the differentiation of the types by means of dulcite and saccharose and obtained the results shown in [Table XVIII]. These figures are calculated from several hundreds of strains.
Although there is a slight difference in the relative proportions of the types found at Ottawa and Baltimore, both sets of results show definitely that there is no difference in the resistance of the various types to chlorination.
Aftergrowths. In [Tables XIII] ([p. 44]) and [XV] ([p. 51]), it will be noticed that, after the preliminary germicidal action has subsided, a second phase occurs in which there is a rapid growth of organisms. This is usually known as aftergrowth. When the contact period between chlorination and consumption is short, the reaction does not proceed beyond the first phase, but when the treated water is stored in service reservoirs the second phase may ensue. At one purification plant, where the service reservoirs are of large capacity, the aftergrowths amounted to 20,000 bacteria per c.cm. although the water left the purification plant with a bacterial count usually lower than 50 per c.cm.
TABLE XVIII.—TYPES OF B. COLI SURVIVING CHLORINATION
| Percentage of Organisms. | ||||||||
| B. coli communis | B. coli communior | B. lactis aerogenes | B. acidi lactici | |||||
| Raw. | Chlori- nated. | Raw. | Chlori- nated. | Raw. | Chlori- nated. | Raw. | Chlori- nated. | |
| Ottawa, 1914 | 5 | 4 | 40 | 48 | 44 | 36 | 11 | 12 |
| Ottawa, 1915 | 8 | 8 | 50 | 46 | 34 | 31 | 8 | 15 |
| Baltimore, 1913[C] | 11 | 14 | 33 | 25 | 35 | 31 | 21 | 30 |
| [C]Thomas and Sandman.[5] | ||||||||
Regarding the nature of this aftergrowth, there has been a considerable difference of opinion: some regard it as the result of the multiplication of a resistant minority of practically all the species of organisms present in the untreated water; others, that it is partially due to the organisms being merely “slugged” or “doped,” i.e. are in a state of suspended animation, and afterwards resume their anabolic functions; whilst others believe that with the correct dosage of chlorine, only spore-forming organisms escape destruction and that the aftergrowth is the result of these cells again becoming vegetative.
The aftergrowths obtained under the usual working conditions vary according to the dosage of chlorine employed, and none of the above hypotheses alone provides an adequate explanation. When the dosage is small, a small number of active organisms, in addition to the spore bearers, will escape destruction, and others will suffer a reduction of reproductive capacity. The flora of the aftergrowth in this case will only differ from the original flora by the elimination of a majority of the organisms that are most susceptible to the action of chlorine and the weaker members of other species of greater average resistance. As the dose is increased these factors become relatively less important until a stage is reached when only the most resistant cells, the spores, remain. The resultant aftergrowth must necessarily be almost entirely composed of spore-bearing organisms. A small number of the most resistant members of non-sporulating organisms may also be present but they will, in the majority of instances, form a very small minority. This is the condition that usually obtains in practice and it is necessary to consider whether the aftergrowth may have any sanitary significance.
Concerning the secondary development of B. coli, the usual index of pollution, there is but little information. H. E. Jordon[6] reported that, of 201 samples, 21 gave a positive B. coli reaction immediately after treatment, 39 after standing for twenty-four hours, and 42 after forty-eight hours. These increases were confined to the warm months, the cold months actually showing a decrease. The following figures, taken from the author’s routine tests for 1913 and 1914, show a similar tendency, but an analysis of the results by months did not show that this was confined to the warm season. The sequence of the results from left to right, in the [following Table], is in the same order as the contact period. Approximately 290 samples were taken at each sampling point.