HIE BACTERIA 



449 





The spores can resist even the temperature of boiling water, and pass 

 still living into the decoction. There they germinate into active 



Bacilli, motile in the liquid 

 by cilia (Fig. 346, a, d). 

 But those which rise to the 

 surface lose their motility 

 (b), though continuing to 

 divide ; they form thick 

 gelatinous walls, and so 

 they remain associated to- 

 gether as the scum of the 

 zoogloea (e). If the supply 

 of organic material is ex- 

 hausted they pass again 

 into the resistant spore- 

 stage. It thus appears that 

 a single boiling of the 

 medium containing spores 

 of B. subtilis is not enough 

 to sterilise it, for the spores 

 can resist 100° C, at least 

 for a time. For complete 

 sterilisation it is necessary after boiling to incubate the culture at a 

 favourable temperature of 37 C. for 48 hours, during which time the 

 spores will all pass into the active but vulnerable state. Then a 

 second boiling will completely sterilise the liquid. This method is 

 commonly used in the preparation of media for the culture of Bacteria. 

 It is important to realise the great rapidity of multiplication of 

 Bacteria. Under favourable conditions B. subtilis is found to divide 

 once in about 20 minutes. If this pace be continued by all the 

 progeny for 8 hours the result from a single Bacillus would be over 

 16 millions. It is not, however, the rapid multiplication and easy 

 transfer of these minute bodies alone which gives the Bacteria their 

 importance. A still more interesting feature is the variety of their 

 physiological powers. Being as a rule parasitic or saprophytic plants, 

 they depend for their supply of food and energy upon breaking down 

 more complex organic compounds into simpler ones : and they do this 

 in the most various ways. The end products of such changes are 

 chiefly carbon-dioxide and water. In fact, Bacteria are the great 

 scavengers of the world, restoring organic material to the sources from 

 which it came. But in the course of the process many steps may 



Fig. 346. 



Bacillus subtilis. a, d, motile cells and chains of cells ; 

 b, non-motile cells and chains of cells ; c, spores from the 

 zoogloea ; e, the zoogloea. (After A. Fischer, a-d x 

 1500 ; e x 500.) (After Strasburger.) 



