274 BACTERIOLOGICAL ANALYSIS OF MILK. 



milk may not, however, be out of place. Normal milk should 

 contain a moderate number of Bact. lactis acidi, the number 

 and percentage varying, as we have already indicated, with 

 the age of the milk. Fresh milk should contain them in small 

 numbers, while milk that is older should have a larger pro- 

 portion. Normal milk will also contain large numbers of 

 neutral forms, that is, bacteria which produce colonies on 

 gelatin that are neither acid nor alkaline, nor have any other 

 distinctive characteristic. Normal milk will commonly also 

 contain some liquefiers. The number of liquefiers, however, 

 will be quite variable. If the number is very high it renders 

 the milk suspicious and suggests that it has been subjected to 

 great contamination at the dairy, or at all events that the 

 protecting lactic bacteria have not developed as they nor- 

 mally should in the milk. While, then, the presence of 

 large numbers of liquefying bacteria will not positively con- 

 demn such milk, it does render it somewhat suspicious, and 

 suggests that something unusual has produced a high con- 

 tamination of the milk with the liquefying and putrefactive 

 forms. Milk which is not strictly fresh should be found to 

 contain the lactic bacteria in considerable proportion, a per- 

 centage which will be likely to vary from 40% to 60% or 

 even higher. If the number of bacteria is very high in total 

 numbers, ranging in the millions, and the percentage of lactic 

 bacteria is low, the milk is an abnormal form of milk and 

 is at once suspicious, for normal milk produced under proper 

 conditions will, by the time it has bacteria in great quantities, 

 have a very large percentage of lactic forms. 



It may be convenient to give here a brief description of the 

 kind of colonies which the chief dairy bacteria produce in the 

 litmus gelatin. The normal forms of milk bacteria are as 

 follows : 



Bact. lactis acidi. This produces in the litmus gelatin a 

 small, opaque colony, which is red after the growth of a few 

 days, and soon becomes surrounded with a red halo. It never 



