THE SHAPE OF BACTERIAL CELLS 17 



parasitic staphylococci and streptococci usually measure between 0-75 jj, and 1-25 fx 

 in diameter. Some forms of micrococci or sarcinae may show an average diameter of 

 1-5 to 2^. Among the rod-forms, a relatively large bacillus, such as B. anthracis, has 

 a transverse diameter varying between 1 and 1-25 ft and a length varying between 3 

 and 8fj,. A medium-sized bacillus, such as Bad. coli, has a transverse diameter vary- 

 ing between 0-5 and 1 fi, and a length of 2 to 3 /.i. A very small bacillus, such as the 

 influenza bacillus, has an average diameter of 0-2 to O-i/n, and a length of 0-7 to 1-5/^. 

 Some bacilli, such as CI. tetani, may combine a small transverse diameter, 0-3 to 

 04 ju, with considerable length, 3 to 5 /^. All such measurements must, of course, 

 be taken as representing a modal size, corresponding to the modal form, and wide 

 variations may occur. Some species of bacilli, for example, may show occasional 

 filamentous forms, measuring 100 ju or more in length. It remains true that the 

 modal size of a bacterium is one of its distinctive characteristics, and has the 

 advantage that it can be stated in numerical terms. In such forms as the Actino- 

 myces, which are normally filamentous, it is to the transverse diameter alone that 

 we can assign a modal value. 



The Shape of Bacterial Cells. — We can recognize three main types of bacterial 

 cell — the coccal or spheroidal, the bacillary or cylindrical, and the spirillar. 



The coccal form is distinguished by the fact that any one axis of the bacterial 

 cell is approximately equal to any other. Many forms approximate to true spheres, 

 although it is doubtful whether any living cell is truly spherical. In many cases 

 the spherical form is widely departed from, and the individual cells may be 

 ellipsoidal or show conical distortions, flattenings or indentations, which give the 

 cells, when observed in film preparations, shapes which may be likened to a bean, 

 or a kidney, or a lancet, as the case may be. 



The bacillary form is distinguished by the fact that one axis of the cell is 

 markedly longer than either of the others, which are themselves approximately 

 equal. Since it is customary to examine bacteria in film preparations, and to 

 describe them as though they were two-dimensional bodies, it is usual to refer 

 to the long axis and the transverse axis, ignoring any possible departure from 

 the circular form in the cross section of the cylinder. The ratio between the 

 length of these cylindrical cells and their transverse diameter may vary over an 

 enormous range ; so that, while some may be almost coccal in appearance, others 

 may be filamentous. Certain modifications of the general cylindrical shape are 

 characteristic of particular bacterial groups. The average ratio of the long axis 

 to the transverse diameter, that is, the thinness or thickness of the cell, is one 

 such character. The ends of the bacillus often show modifications of form which 

 are of differential value. They may be square-cut, rounded, or acutely pointed, 

 or may form definite clubs. Other irregularities in contour, due to the presence 

 within the cell of structures which cause distortion of the cylindrical form, will be 

 considered later. 



The spirillar form is characterized by a bending or twisting of the cells, so that 

 they assume curved or spiral shapes. It has become customary to speak of a 

 bacterium which shows a single curve, thus assuming the so-called comma 

 shape, as a vibrio, and of a bacterium which shows a series of curves or twists, 

 thus assuming a corkscrew form, as a spirillum. Vibrios and spirilla are of 

 necessity relatively elongated cells, and they always have rounded or pointed 

 ends. 



The study of the finer details of bacterial structure may be carried out in four 



