2 THE MORPHOLOGY OF BACTERIA 



culiar type of transverse division was early noted and later worked out in great detail 

 by Gross who described it as "multiplication by incurvation." It is of interest in this 

 connection to note that some years ago Gotschlich, in his studies on plasmolysis and 

 plasmoptysis, came to the conclusion that bacterial cells may break up into small bits 

 of protoplasm, resembling in no way the original cell in size or shape but serving as 

 starting-points from which new cells germinate. Gotschlich admitted that no positive 

 evidence exists to show such a kind of reproduction, suggesting it merely as a possibil- 

 ity which cannot be excluded. 



During the past few years a number of observations have been made which indi- 

 cate that many of our earlier ideas in regard to the morphology of the bacterial cells 

 must be subjected to rigid scrutiny, and the new conceptions recently advanced will, 

 if proved, modify our entire point of view with respect to these microscropical organ- 

 isms. 



PLEOMORPHISM 



It has long been recognized that bacterial cells exhibit what we regard as their 

 characteristic morphology under certain favorable conditions which we find in the 

 laboratory, usually in young cultures and on media peculiarly favorable to their 

 growth. Our effort, indeed, is to subject organisms to constant conditions of light, 

 temperature, and nutrition and so maintain constancy of form. Certain species are 

 peculiarly susceptible to environmental influences and prone to exhibit departures 

 from their standard morphology. Thus the cholera vibrio and Vibrio proteus assume 

 in old cultures the most bizarre shapes, swollen spheres, crescents, half-moons, and 

 distorted cells with knoblike protuberances. Similar changes are seen when these 

 species are cultivated on acid media. Transfer of these distorted forms to media of the 

 proper alkaline reaction yields normal vibrios as evidence of their potential viability. 

 Large spherical forms are common in old cultures of the micrococci and are sometimes 

 seen in the streptococci. The plague bacillus is especially likely to produce enlarged 

 distorted forms on certain kinds of media (salt agar), and their production is regarded 

 as practically diagnostic. In the animal body also departures of bacteria from their 

 characteristic morphology have been frequently noted. One can find in streptococcus 

 infections of the throat large cocci (megacocci) bearing little resemblance to ordinary 

 streptococci, yet cultures from such throats may yield only streptococci. With the 

 plague bacillus the appearance of enlarged, distorted forms is not uncommon in both 

 natural and artificial infections. 



Organisms exhibiting an atypical morphology are usually called "degeneration" 

 or "involution" forms. A number of investigations bearing upon their origin have ap- 

 peared within fairly recent years. Thus Wilson' has found that when Bacillus coli, 

 Bacillus typhosus, Bacillus enteritidis of Gartner, the Friedliinder bacillus, and the 

 plague bacillus are cultivated on media containing urine, a great diversity of forms re- 

 sults. The organisms are large and distorted and filamentous elements are common. 

 Hata^ has shown that the addition of magnesium chloride to agar upon which dysen- 

 tery and plague bacilU are grown leads to marked variation in the morphology of 



' Wilson, W. J.: /. Path, b" Bad., ii, 394. 1906. 

 'Hata, S.: Centralbl.f. Bakteriol., 46, 289. 1908. 



