78 OUTLINES OF BACTERIOLOGY 



atmosphere : the greater the number of bacteria in the soil the greater 

 the number that are found in the atmosphere. It therefore follows 

 that the air above soils containing much organic matter, e.g. a tract of 

 cultivated ground, contains more bacteria than the atmosphere above, 

 for example, a sandy waste which has little or no organic matter in its 

 constitution. 



2. BACTERIA IN WATER. 



The number of bacteria in any liquid depends entirely on the 

 capacity of the latter to sustain bacterial life. Bacteria are entirely 

 aquatic in their habits, for even when growing on an apparently solid 

 medium, e.g. nutrient agar, each individual is covered with a film of 

 water which completely surrounds it. If, therefore, a liquid contains 

 all the elements necessary for bacterial life, and other conditions are 

 favourable, the number of bacteria becomes enormous. In milk and 

 sewage we have examples of such liquids. The following table shows 

 how numerous bacteria are in sewage water. 



CRUDE SEWAGE. No. OF BACTERIA PER c.c. 



1. Chiefly domestic sewage, - 14,240,000 



2. Mixed sewage, - 7,800,000 



3. Chiefly domestic sewage, 4,800,000 



4. Mixed sewage and trade eflluent, 36,000,000 



5. Hospital sewage, 2,800,000 



6. Domestic sewage and trade effluent, 4,100,000 



7. Domestic sewage, 28,100,000 



8. Mixed sewage, - 21,100,000 



Analyses of samples of milk, which have been allowed to stand for 

 some hours, also show enormous numbers. In the case of one sample 

 of a milk of bad quality, as many as 124 million bacteria were present 

 in every cubic centimetre. In ordinary water, however, the amount of 

 food material is not sufficient to support such large numbers. In 

 ascertaining whether water has been polluted or not, the usual method 

 of examination is to find out how many bacteria are present in the 

 water. The following table indicates the standard accepted by many 

 bacteriologists : 



BACTERIA PER c.c. 



Very pure water, 0-50 



Good water, 50-500 



Passable water, 500-3,000 



Bad water, 3,000-10,000 



Very bad water, - * 10,000-100,000 and over. 



