WATER 59 



Determinations of the number of streptococci have been made 

 much less frequently than in the case of B. coli. As a provisional 

 guide, and without attaching an equal significance to the findings, 

 a standard similar to that for B. coli may be employed i.e. y 

 their presence in 100 c.c. or less of deep- well or spring water, or 

 in 10 c.c. or less of surface and shallow-well waters, would justify 

 an adverse opinion as to the purity of the water in question. 



On its negative side the streptococcus test is not of great 

 value, and the absence of streptococci, even in a considerable 

 bulk of water, cannot be taken as showing purity or freedom 

 from danger. 



Opinion is not united as to the value of B. enteritidis 

 sporogenes as an indicator of pollution. It is fairly abundant 

 in sewage and excreta, but it is a spore-bearing organism with 

 prolonged powers of resistance, and therefore, even if it be 

 admitted that its presence indicates pollution, such pollution 

 may have taken place at some long antecedent period, a con- 

 tamination so old as to be of no significance. Its absence in a 

 large quantity of water is some evidence of purity. 



The essential limitation to the value of bacteriological 

 examinations is that they only supply information as to the 

 existing conditions in the water at the time of sampling. They 

 cannot indicate liability to contamination which is not actually 

 taking place. This important limitation must never be lost 

 sight of, and it sets a decidedly restricted value upon the 

 bacteriological examination results of chance samples. 



When the bacteriological data discloses distinct evidence of 

 undesirable and potentially harmful contamination it is safe to 

 report adversely upon such a supply, since unless the sources 

 of contamination can be and are removed once contaminated 

 always liable to contamination. Even in such cases a con- 

 demnatory report comes with much greater weight if more than 

 one sample has been found to be polluted. 



It is on its negative side that caution is required. As stated 

 above contamination is often intermittent and chance samples 

 may not disclose it. Opinions on individual samples in such 

 cases should therefore be always statements of fact and not of 

 inference as to the purity of the particular supply. 



