Il8 BACTERIOLOGICAL METHODS 



colon bacillus has already been explained. It is suggested that 

 i cc., o.io cc. and o.oi cc. quantities of the water be run into 

 fermentation tubes with lactose-bile medium, making five sets of 

 these tube cultures, and incubate at 37 C. for 48 hi., noting pos- 

 sible gas formation. Gas formation indicates sewage contamina- 

 tion. If the gas is formed quickly, in 6 to 1 2 hr., the contamination 

 is probably recent, if more slowly, 24 to 36 hr., the contamination 

 is probably older. Gas in the o.oi cc. quantities or less, indicates 

 very high sewage contamination, gas in the o.oi to o.io cc. quan- 

 tities indicates serious contamination, and condemnation of the 

 water supply for drinking purposes may be based on the presence 

 of gas formation in two out of three tubes containing o.io cc. 

 quantities, or three out of five of the i cc. quantities, also tak- 

 ing into consideration the rate of gas formation and the numerical 

 plate count as well as the findings based on the direct microscop- 

 ical examination. In brief, condemnation of water supplies in- 

 tended for drinking purposes must be based upon the judgment of 

 a competent sanitarian, one who comprehends the significance of the 

 findings in relation to the source of the water supply and the sources 

 of the contaminations. It is not practicable to lay down hard and 

 fast rules. Each case must be 'considered by itself. In one in- 

 stance the gas formation may develop in 0.3 cc. quantities (three 

 out of five tubes containing o.io cc. quantities) or even in o.io cc. 

 quantities and yet the water may be considered potable, as might 

 be the case in deep well water into which street and road dust is 

 carried, or which might contain surface drainage from field or 

 garden. Again the water may be quite unfit for drinking pur- 

 poses with colon bacilli in 10 cc. or in 100 cc. quantities, as might 

 be the case in wells or springs highly contaminated with old or 

 much weathered sewage contamination. 



14. Bacteriological Examination of Mineral Waters 



The bacteriological analysis of bottled waters is very important 

 because it is an efficient means of ascertaining the conditions at the 



