36 MORPHOLOGY 



separate. The aggregates which are formed in this way are often highly char- 

 acteristic, and constitute an important factor in the identification of bacterial 

 species. In the spheroidal forms division may occur in any plane, while in the 

 cylindrical forms it always occurs in the transverse diameter. As a result the cocci 

 may, and do, show a greater variety of groupings than the bacillary or spiral 

 forms. 



When the successive divisions, in a coccal bacterium, occur in such a way that 

 the daughter cells remain united in pairs for a short period, but these pairs separate 

 before a further division occurs, the organism in question is described as a diplo- 

 coccus. When several successive divisions occur before separation of the resulting 

 aggregate, and these divisions follow no ordered sequence as regards the planes 

 in which they take place, irregular groups of cocci result, which have been com- 

 pared to bunches of grapes. Organisms which form this type of cell-aggregate 

 are known as staphylococci, and this term is used as a generic name. When such 

 a series of divisions without separation occurs in planes parallel to one another, 

 the aggregates so formed have the appearance of a chain or chaplet, and organisms 

 which behave in this way are known as streptococci, and are usually classed in 

 a single genus. In some species the cells remain attached while two divisions 

 occur, the second at right angles to the first. The resulting aggregate is a group 

 of four cocci ; such an organism is sometimes referred to as a tetracoccus. In 

 some species the typical cell grouping is produced by three divisions, the plane 

 of each being at right angles to the other two. In this way cubes, or packets, 

 each of eight cocci, are produced. The species which form groupings of this type 

 are classed in the genus Sarcina. 



Among the cylindrical forms, the only possible departure from the single-cell 

 formation is tlje adherence of two or more cells in pairs or chains. Such groupings 

 may be referred to as diplobacilli or streptobacilli ; but they are not sufficiently 

 constant in a particular bacterial species to be used for purposes of classification. 

 They occur, however, far more often in some species than in others, and their 

 relative frequency may be an important specific character. 



Apart from the formation of such united cell-aggregates, the way in which a 

 cell divides influences the grouping of the daughter cells. In certain bacillary 

 forms, such as the diphtheria bacillus, division appears to occur asymmetrically, in 

 the sense that the daughter cells remain attached at one side of the cylinder, after 

 division has proceeded across the whole width of the organism from the opposite 

 side. In the early stages of the subsequent growth of the daughter cells, this 

 local attachment seems to act as a hinge, about which the elongating cells swing, 

 so that they come to lie at varying angles to one another, depending on the period 

 which elapses before division becomes complete. The groupings so formed, which 

 have been compared to Chinese letters, or to cuneiform characters, are very char- 

 acteristic of certain species. 



Colony Formation. — When bacteria are grown on solid media, and care is taken 

 to avoid too heavy an inoculation, the individual cells multipl}' and form isolated 

 colonies. The appearance of these colonies is, in many cases, highly characteristic 

 of the group or species to which a given bacterium belongs. We know little of the 

 internal structure of bacterial colonies, but that little suggests that the structural 

 dififerentiation is considerable (Legroux and Margrou 1920, Kavich-Birger and 

 Svinkina 1937), and it is possible that much light may be thrown on bacterial 

 morphology by further study along these lines. 



