22 THE MORPHOLOGY OF BACTERIA 



shaped diplococci, exemplified in the meningococcus and gonococcus, 

 or to an elongation of the axes perpendicular to the plane of apposition, 

 in which event the organisms are "lance-shaped" diplococci, as for 

 example the pneumococcus. 



Bacilli. Bacilli are rod-shaped, cylindrical organisms in which a 

 longer and a shorter dimension may be recognized. They are typi- 

 cally circular in cross-section. When division is taking place the shorter 

 bacilli may be temporarily oval or even circular in outline. The dimen- 

 sions of bacilli vary considerably: some are habitually long, some are 

 short, some are thick, some are thin. The ends may be convex, less 

 commonly flat or even concave. A few bacilli are not typically isodia- 

 metric, but appear in outline as club-shaped, spindle-shaped, or even 

 more or less conical (cuneate) rods. Less commonly, slightly curved 

 rods are met with; the curvature takes place along the longer dimension. 



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FIG. 1. The normal types of bacteria. 1-6, cocci; 7-13, bacilli; 14-16, spirilla; 

 1, micrococcus; 2 and 3, diplococci; 4, tetracoccus; 5, sarcina; 6, streptococcus (the 

 lower chain includes an arthrospore) ; 7 and 8, bacilli; 9, 10, 12, and 13, bacilli with 

 various granules; 11, streptobacillus ; 14, vibrio; 15, spirillum; 16, Spirocheta trepo- 

 nema. 



Spirilla. Spiral bacteria, like the bacilli, exhibit a longer and 

 a shorter dimension; unlike the bacilli, the longer axis is curved in 

 three planes of space. The curvature may be slight, less than a com- 

 plete turn, in which event the organism is "comma-shaped" when 

 viewed under the microscope; it may be a series of open curves, giving 

 the organism a sinuous outline; or it may be very much curved, so 

 that the organism resembles a somewhat closely coiled spring in out- 

 line. As a rule, the curvature is symmetrical and uniform in each 

 instance. 



The cocci, through almost imperceptible morphological gradations, 

 merge into the bacilli, and the bacilli, through the slightly curved 

 forms, merge into the spirilla. Even in the spirilla slight differences 

 in curvature are usually discernible. Thus, a culture of the cholera 

 vibrio may contain many straight, uncurved organisms in addition 



