CHAPTER IV 



MORPHOLOGICAL CHANGES DURING THE 

 GROWTH OF BACTERIA 



PAUL F. CLARK 



University of Wisconsin 



In spite of the efforts of investigators for more than half a century, three general 

 conceptions of the morphology of bacteria still exist. The first assumes that bacteria 

 are simple in form and structure. According to this theory, the size and shape of each 

 species are fixed, varying only within narrow limits, and the organisms multiply only 

 by transverse fission into two daughter-cells of similar size. A second conception at 

 the other extreme has been emphasized especially during the last decade. Several ob- 

 servers have described complicated life-cycles in bacteria including reproduction by 

 budding, and complex mitoses following conjugation; marked changes in form have 

 been reported in presumably pure cultures, even to the extent of the metamorphosis 

 of a diphtheroid into a streptococcus and of filterable viruses into spore-formers. The 

 third position, as one might expect, follows a middle path with the acceptance of a 

 considerable degree of pleomorphism, especially in certain genera such as Corynebac- 

 teria and Azotobacter, but with a skepticism concerning the demonstration of proved 

 life-cycles. This conception also includes an emphasis on the importance of observing 

 the changes in form and size during the growth of the organism. 



The first-mentioned idea of bacterial morphology exists now chiefly on the pages 

 of the briefer textbooks and in the minds of beginning students. Everyone who has 

 examined ordinary stained preparations of root-nodule bacteria, diphtheria bacilli, 

 streptococci, or members of the Spirillaceae grown on a variety of media will have 

 noted the occurrence of aberrant morphological types to such a degree that the fixed 

 morphology concept must be discarded. Furthermore, some of these forms appear in 

 young cultures (two to eight hours) as well as in those which have been incubated for 

 several days, so that they cannot be dismissed as involution or degeneration forms. 



Although this idea of the fixity of bacterial form has been largely discarded in its 

 more precise interpretation, systematic bacteriology is, nevertheless, largely based on 

 this conception; and, in routine, we find it possible, though at times difficult, to work on 

 this foundation. The usual procedures tend to emphasize uniformity; the standard- 

 ized artificial media select those organisms best fitted to grow under these saprophytic 

 conditions; the crude drying and flaming methods of fixation and the gross overstain- 

 ing effectively conceal any fine differences in structure; the diurnal rotation of the 

 earth gives us a standard time for the examination of cultures based on our convenience 

 rather than on any physiological cycles in the development of bacteria. Furthermore, 

 bacteria are so fascinatingly varied in their functions and apparently so alisurdly simple 

 in morphology — what they do is so vastly more important biologically than what 

 their form and structure may be — that bacteriologists quite generally have become 



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