72 MICRO-ORGANISMS AND FERMENTATION. 



0, 0, 6- 6, 3, and 9 growths ; whereas, in beer, none of these 

 water samples gave any growth. In another series, Koch's 

 gelatine gave for 1 c.c. of water 222 growths, wort gelatine 30 : 

 but none of the flasks containing wort or beer, after infection 

 with the water, showed any development of organisms. Thus, 

 only very few of the great number of germs living in the water 

 developed in wort or beer. 



Hansen has further shown that in zymotechnical analyses 

 of water and air, it is a mistake to employ gelatine at the 

 outset, and then to transfer the colonies that have been formed 

 into wort. Thus, he demonstrated by experiment that several 

 of the bacterial germs occurring in atmospheric dust and in 

 water are capable of developing in nutrient gelatine, but not 

 in wort ; whilst several of these species become invigorated to 

 such a degree, after having formed a new growth in the gelatine, 

 that they are then enabled to develop in the less favourable 

 medium, wort. Another, and a still greater, objection to the 

 gelatine method is that several organisms, and just those of 

 importance, do not develop at all when transferred directly to 

 the gelatine in the enfeebled condition in which they generally 

 occur in atmospheric dust and in water. 



Temperature naturally plays a much more important part 

 in the development of the germs on gelatine plates than in 

 nutritive liquid ; at a less favourable temperature they will 

 develop with greater difficulty in gelatine, owing to its defi- 

 ciency in nutriment. 



Reference may also be made in this connection to the 

 difficulties that are encountered in determining the number 

 of germs which can develop on gelatine plates. Species of 

 frequent occurrence in water, that tend to liquefy gelatine, 

 generally develop very rapidly, and may encroach so exten- 

 sively on the space that other species do not succeed in forming 

 colonies at all ; others require so long an incubation before 

 visible colonies are formed that the examination of the plates 

 is often concluded before these growths appear.* 



* In comparative investigations, as, for instance, the examination of air 

 and water before and after filtration, gelatine plates are usually employed ; the 

 possible sources of error accompanying their use must, therefore, be borne in 

 mind. 



