26 



TAXUNOM Y 



the bacteria are living in agar cultures, in all our culture media in fact. 

 In such circumstances it is no wonder if some of the cells grow into mis- 

 shapen, feeble involution forms, with their physiological functions, such as 

 virulence or fermentative power, weakened or suppressed (16). 



Bacteria, like all other living things, produce crippled and deformed 

 individuals when forced to live under unsuitable conditions (Fig. 14). These 

 abnormal bacterial cells are known as ' involution forms.' The causes of 

 involution are of various kinds. Acetic bacteria, for instance (Fig. 14, c-d), 

 produce monstrosities if their own fermentation product (acetic acid) accu- 

 mulates beyond a certain point, and also if the temperature exceeds the 



optimum. In B. snbtilis involution 

 forms arise if the relative propor- 

 tions of nitrogen and carbon in the 

 culture medium are not suitable a 

 solution of o-i per cent, asparagin 

 and 10 per cent, sugar, for instance. 

 In other cases a high percentage of 

 neutral salts induces them. A curious 

 case of involution is that shown by 

 the bacteria in the root tubercles of 

 the leguminosae (see Chap. X). 



The shapes that the cells assume 

 arc very varied. Irregular, swollen, 

 or spindle-shaped rods and twisted 

 chains are common, and the protru- 

 sion of short lateral processes from 

 the cells often gives the growths the 

 appearance of a branched system 

 (Fig. 14). At the same time the 

 cell-contents are reduced and stain 

 badly, a few granules being appa- 

 rently all that is left in the cell. Cells which show a high degree of 

 involution are dead, and cannot, even in the most favourable conditions, 

 be revived. They are common in old cultures, and particularly so in those 

 of strictly parasitic forms, such as the diphtheria and tubercle bacilli, that 

 even in the most nourishing media do not find quite the conditions they 

 require. 



Such branched involution forms have been thought by some to show 

 that the bacteria of diphtheria and tubercle (Fig. 14, /i, g] are fungi of 

 a higher order of growth than their ordinary rod form would indicate. It 

 has been suggested that the rods are merely a stage in the development of 

 a true filamentous bacterium or even a hyphomycete, and new names have 

 accordingly been given to the organisms, the tubercle bacteria being placed 



... 



'"" ..." 

 .,..<*^- -:'" - 



FIG. 13. Bacillus subtilis in hay infusion. The 

 figure shows the complete cycle of forms, a, Peritri- 

 chous motile short rod ; b, non-motile rods and chains ; 

 d, motile chains ; c, spores in non-motile rods and 

 chains that unite on the surface of the infusion to form 

 a thick whitish pellicle e. Magn. a-d 1500, e (from 

 Brefeld) 250. 



