MICROBES, MOULDS, AND YEAST. 20I 



of producing spores, but in the majority of cases spores 

 do not occur. These bacterium " spores " are quite 

 different also from the spores of other plants. They 

 are really resting states, and are formed in a peculiar 

 and unusual way. The protoplasm of a cell changes in 

 appearance and becomes of a finely granular character. 

 A small denser part appears in the centre, and gradually 

 enlarges till it absorbs the protoplasm and becomes the 

 spore. This lies inside the membrane or wall of the 

 original cell, but possesses a distinct wall of its own. 

 It is not difficult to get the spores of Bacillus subtilis. 

 It is only necessary to keep the yeast (see below, p. 215) 

 long enough upon the plaster of Paris, and they are 

 pretty sure to be attacked by Bacillus subtilis, which 

 will form spores. 



The cell wall is very seldom visible, and it is only 

 distinct in spore formation. The wall is probably of 

 quite a different kind to that found in other plants. The 

 resistance of bacteria to extremes of temperature is very 

 remarkable, and the spores are even more resistant than 

 the ordinary cells. Certain bacteria (Typhus, Colicom- 

 munis, and Diphtheria) withstood a cold of — 182° to 



— 1 92° C. for twenty hours. No less than 44 organisms 

 in one experiment survived exposure to a cold of 



— 2 1 0° C. Certain spores of bacteria can stand a heat 

 of 150° C. without destruction. Even sudden and 

 violent changes do not always kill bacteria. If milk, 

 for instance, is quickly heated to 70° C, and then 

 rapidly cooled in ice water, only some 90 per cent, 

 of the bacteria die ; it is necessary to wait and 

 allow the spores to germinate, and then repeat the 

 experiment. 



On the other hand, germs are very susceptible to 

 sunshine. Only four hours of sunshine are sufficient 



