48 BACTERIA 



As to to the quantitative examination, that is fulfilled by 

 counting the number of colonies which appear, say by the 

 third and fourth day, upon the gelatine plates. Each colony 

 has arisen, it is assumed, from one individual, so that if we 

 count the colonies, though we do not thereby know how 

 many organisms we have upon our plate, we do know 

 approximately how many organisms there were when the 

 plate was first poured out, which are the figures we require, 

 and which can at once be multiplied and returned as so 

 many organisms per cubic centimetre. There is, unfort- 

 unately, at present no exact standard to which all bacterio- 

 logists may refer. 



Miquel and Crookshank have suggested standards which 

 allow " very pure water " to contain up to 100 micro-organ- 

 isms per cc. Pure water must not contain more than 1000, 

 and water containing up to 100,000 bacteria per cc. is con- 

 taminated with surface water or sewage. Mace gives the 

 following table : 



Very pure water o- 10 bacteria per cc. 



Very good water 20- 100 " " 



Good water 100- 200 " " 



Passable (mediocre) water 200- 500 " " 



Bad water 500- 1,000 " " 



Very bad water 1,000-10,000 and over " 



Koch first laid emphasis on the quantity of bacteria present 

 as an index of pollution, and whilst different authorities 

 have all agreed that there is a necessary quantitative limit, 

 it has been so far impossible to arrive at one settled standard 

 of permissible impurity. 



Besson adopts the standard suggested by Miquel, and, on 

 the whole, French bacteriologists follow suit. They also 

 agree with him, generally speaking, in not placing much 

 emphasis upon the numerical estimation of bacteria in water. 

 In Germany and England it is the custom to adopt a stricter 

 limit. Koch in 1893 fixed 100 bacteria per cc. as the maxi- 

 mum number of bacteria which should be present in a 



