84 LABORATORY GUIDE IN BACTERIOLOGY 



SECTION 4 



BACTERIOLOGICAL EXAMINATION OF WATER, AIR 

 AND MILK 



EXERCISE I. BACTERIOLOGICAL ANALYSIS OF WATER 



References — 



Prescott and Winslow, Elements of Water Bacteriology, New 



York, igi4. 

 Savage, The Bacteriological Examination of Water Supplies, 

 London, 1906. 



A bacteriological examination of water is made for 

 the purpose of determining — 



1. Bacterial numbers. 



2. Bacterial species. 



3. Sewage 'pollution. 



Collection of samples. — Procure wide-mouthed, 

 glass-stoppered bottles, having a capacity of at least 

 100 c.c. After cleaning and drying, tie lead foil or 

 filter paper over the stopper, wrap the bottles indi- 

 vidually in paper and sterihze in the hot-air oven for i 

 hour at 160° C. ; then deposit them in a metal or wooden 

 case. The samples from surface waters should be taken 

 at least one foot below the surface, to avoid contamina- 

 tion with organisms from the air. If possible, samples 

 should be plated on the spot or in the laboratory within 

 an hour at the latest. But when a greater interval of 

 time must occur, the samples should be taken to the 

 laboratory packed in ice, despite the probability of 

 thus partially altering the bacterial flora. 



Method of examination. — A number o&»pipettes of 

 various sizes (i c.c, 2 c.c, 5 c.c, and 10 c.c.) are plugged 



