236 PATHOGENIC BACTERIA. 



them before it was frozen. In Hndson-River ice Prud- 

 den found an average of 398 colonies in a cubic centi- 

 meter. 



A sample of water when collected for examination 

 should be placed in a clean sterile bottle or in a her- 

 metically-sealed pre-sterilized glass bulb, and must be 

 examined as soon as possible, as the bacteria multiply 

 rapidly in water which is allowed to stand for a short 

 time. In determining the species of bacteria found in 

 the water reference must be made to the numerous mono- 

 graphs upon the subject, and to tables such as those 

 compiled by Eisenberg. 



The discovery of certain important pathogenic bacteria, 

 as those of cholera and typhoid, will be considered under 

 the specific headings. 



Unfortunately, the bacteriologic examination of waters 

 does not throw satisfactory light upon their exact hygi- 

 enic usefulness. Of course, if cholera or typhoid-fever 

 bacteria are present, the water is harmful, but the quality 

 of the water cannot be gauged by the number of bacteria 

 it contains. 



The drinking-water furnished large cities is not infre- 

 quently contaminated with sewage, and contains intes- 

 tinal bacteria — Bacillus coli communis. For the ready 

 determination of this organism, which is an important one 

 as an indicator that the water is polluted, Smith 1 has 

 made use of the fermentation-tube in addition to the 

 plate. His method is to add to each of the fermentation- 

 tubes containing 1 per cent, dextrose-bouillon a certain 

 quantity of water. The evolution of 50-60 per cent, of 

 gas by the third day is a strong indication that the colon 

 bacillus is present. Plates may be used to confirm the 

 presence of the bacillus, but are hardly necessary, as 

 there is scarcely another bacterium met with in water 

 that is capable of producing so much gas. 



Filtration with sand, etc. diminishes the number of 

 bacteria for a time, but, as the organisms multiply in 



1 American "Journal of the Aledical Sciences, 1895, IIO, p. 301. 



