86 LABORATORY GUIDE IN BACTERIOLOGY 



tions 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 ex- 

 amined 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. — If Bacillus coli and strep- 

 tococci are present in relatively large number, sewage 

 pollution is indicated. 



Method of examination for B. coli and strepto- 

 cocci — 



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

 diluted sample, is added to each of a series of ten 

 fermentation tubes, containing sterile i per cent 

 dextrose 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 number of B. coli 

 per cubic centimeter may be estimated. 



Formula for determining the number of B. coli 

 present in water: 



— I — = Number of B. coh m i c.c. water. 



N=Number of tubes with gas; D = Dilution; I=Total 

 number of tubes inoculated. 



