ii6 LABORATORY GUIDE IN BACTERIOLOGY 



more accurate, as no loss takes place by adhesion to the 

 tube. 



Estimation of colonies. — The colonies are then counted 

 after 48 hours, by means of a colony-counter (Fig. 35). 

 Plates should be counted which contain no more than 

 200-300 colonies. If it is necessary to count plates with 

 a large number of colonies, an estimate must be made 

 by counting different sections of the plate-counter and 

 averaging. the result for the whole plate. 



Species determination. — If the different species of 

 bacteria are to be studied, the colonies must be examined 

 by the naked eye and the low power. Then those which 

 appear to be different are transferred to slant-agar tubes, 

 and from these to the ordinary media. 



Sewage contamination. — The presence of Bacillus 

 coli and streptococci is sufficient indication of sewage 

 contamination in water. 



Method oj examination for B. coli and streptococci. — 



1. I c.c. of the sample, or, if necessary, of the diluted 

 sample, is added to a series of ten fermentation-tubes, 

 containing sterile 2 per cent, glucose-broth. 



2. Place in thermostat. 



3. Examine after 12-18 hours. 



4. Examine a loopful of the sediment in a stained 

 preparation. 



Example. — If i c.c. of the sample is added to each 

 fermentation-tube, and six show gas-formation, there 

 would be six colon bacilli in each 10 c.c. if undiluted 

 water is employed. By this method fairly accurate 

 results may be obtained. 



Isolation is accomplished by plating in an agar 

 medium containing 2 per cent, lactose and 10 per cent, 

 litmus solution. 



