BACTERIA IN MILK 117 



A very common form is a short non-motile rod (1-2 ju by 0-5 jx), facultatively- 

 anaerobic (spores unknown), and not liquefying gelatine (Fig. 24, d). It 

 is known by various names, Bacillus aerogenes, B, acidi lactici, &c, and 

 comprises probably several races of one species. They may be called 

 the typical bacteria of lactic fermentation. Mingled with them are found, 

 sometimes in great numbers, the spherical or ovoidal cells which cause 

 the coagulation of milk. The lactic fermentation of mash in distilleries 

 is caused by yet another form, B. acidificans longissimus (87). This 

 bacterium is about 1 fx broad and something more than 2-5 p long. The 

 number of lactic acid bacteria is great and their determination difficult. 

 The manifold importance of the lactic ferments can be best illustrated by 

 some concrete examples. 



Milk and other Dairy Products (86), etc. 



Cow's milk, being neutral or slightly alkaline, and containing as it does 

 from four to five per cent, of milk sugar, 4 per cent, casein, and 0-7 per cent, 

 of the necessary mineral salts, is an excellent nutritive medium for bacteria. 

 They multiply in it very rapidly, and within a few hours after it is drawn 

 are present in enormous numbers (from 100 to 6,000,000 per c.c). As 

 might be expected, the numbers vary greatly according to the cleanliness 

 of handling during milking and afterwards. In consequence of the 

 favourable conditions thus offered for the multiplication of bacteria, the 

 sterilization of milk, particularly for children, has become a matter of great 

 importance *, and numerous inventions have been put forward to attain 

 a thorough and efficient sterilization. But milk, although sterile when 

 drawn from the udder, becomes infected with micro-organisms at once 

 on contact with the outside world, and invariably contains spore- 

 bearing bacteria. Some of the spores are so resistant that even boiling 

 for an hour, or an hour and a half in a Soxleth boiler, does not destroy 

 them, so that at present it seems impossible to render milk absolutely 

 sterile without exposing it to heat so great that its chemical characters are 

 altered. Fortunately, the majority of the bacteria in milk are sporeless 

 cells which are killed by boiling for five or ten minutes, and for this reason 

 expensive sterilizing apparatus is being largely discarded in favour of the 

 old domestic method of boiling the milk, and then keeping it as cool as 

 possible, so that the uninjured spores may not germinate (88). 



Besides the predominating lactic bacteria, there are always present 

 species that secrete an enzyme resembling rennet, and frequently chromo- 

 genic forms in small number. 



Since the presence of pathogenic germs in milk may become a source 



* In Germany a matter of State and municipal control. 



