'28 



PRINCIPLES OF BACTERIOLOGY 



there are many exceptions to this typical form. Thus frequently the 

 motile bacteria have rounded ends (Fig. 3) ; many of the more slender 

 forms have the long axis bent; some few species, such as the diphtheria 

 bacilli (Fig. 4), invariably produce many cells whose thickness is very 

 unequal at different portions. Spore formation also causes an irregu- 

 larity of the cell outline (Figs. 7, 17 and 18). 



FIG. 7 





Large bacilli in chains. 



Spores in centre of bacilli. 

 (From Kolle and Wasserman.) 



The bacilli except when they develop from spoies or granules divide 

 only in the plane perpendicular to their long axis. A classification, 

 therefore, of bacilli according to their manner of grouping is much 

 simpler than in the case of the cocci. We may thus have bacilli as 

 isolated cells, as pairs, or as longer or shorter chains. 



3. SPIRAL FORM, OR SPIRILLUM. The members of the third mor- 

 phological group are spiral in shape, or rather segments of a spiral. 

 Here, too, we have large and small, slender and thick spirals. The 



FIG. 8 



FIG. 



Medium-sized spirilla. 



twisting of the long axis, which here lies in two planes, is the chief 

 characteristic of this group of bacteria. Under normal conditions the 

 twisting is equal throughout the entire length of the cell. The spirilla, 

 like the bacilli, divide only in one direction. A single cell, a pair, or the 

 union of two or more elements may thus present the appearance of a 

 short segment of a spiral or a comma-shaped form, an S-shaped form, 

 or a complete spiral or corkscrew-like form (Figs. 8, 9, 10 and 11). 



