STERILISATION 



87 



condition. Material containing bacteria was placed in boiling water. 

 The figures indicate the time required to destroy the bacteria : 



BACTERIA IN -SPORE CONDITION. 



BACTERIA IN 

 VEGETATIVE CONDITION. 



All killed in 

 2 minutes. 



Bac. tumescens, - 



cohaerens, - 



,, simplex, 



,, mycoides, - 



pumilis, 



, , fusiformis, - 



carotarum, - 



,, Ellenbachensis, 



,, graveolens, - 



,, subtilis, 



,, ruminatus, - 



Although in the case of Bac. subtilis continuous heating for 150-180 

 minutes at 100 C. was necessary in order to incapacitate it for germina- 

 tion, it must not be thought that the protoplasm of this organism is 

 more resistant than that of other bacteria ; the greater resistance is due 

 to the greater efficiency of the spore-coats. The method of intermittent 

 sterilisation is based on a knowledge of the difference in resisting power 

 of bacteria in the spore- and in the vegetative-condition. A nutrient 

 medium is placed in a steam-steriliser for about 20 minutes on three 

 successive days. On the first application of heat all the vegetative 

 cells are killed, but probably not all the spores. During the next 

 24 hours the surviving spores, being in a good nutrient medium, 

 germinate : when the nutrient medium is now again placed in the 

 steam-steriliser, the germinated cells are quickly killed. On the third 

 day, after the third application, the medium may be regarded as 

 sterile. 



We must now describe the steam-steriliser. This is shown in Fig. 65, 

 which represents the first kind in use, known as Koch's Steam-steriliser, 

 It consists essentially of a chamber with a receptacle at the bottom for 

 holding water (A). This latter is separated from the chamber by a 

 detachable, perforated, tin disc (B). The tap at the side is for pouring 

 out the water when required, and the arrangement at C enables the 

 operator to find out how much water is contained in the receptacle. 

 The object to be sterilised is placed on B, and heat is applied by 

 placing a burner underneath. The chamber is soon filled with steam, 

 and the object attains the same temperature as the steam. The great 

 defect of this kind of steriliser is that the steam escapes into the 



