CHAPTER 2 



THE BIOLOGICAL CHARACTERISTICS OF BACTERIA: 

 MORPHOLOGY 



General Considerations. — With the exception of certain observations on the 

 finer structure of the bacterial cell, which will be referred to later, our knowledge 

 of bacterial morphology has been gained from the study of cells which have been 

 cultivated in the laboratory under artificial conditions. The morphology of 

 bacterial cells may be notably affected by the constitution of the medium on 

 which the bacteria are grown, the temperature of incubation, and many other 

 factors. In particular, the cells in a pure culture may show very striking changes 

 with age. It is customary to regard the forms found in young cultures as 

 typical of a given species, and the very different appearances, often met with 

 in old cultures, as due to the occurrence of degenerative changes. How far we 

 are justified in labelling all the morphologically atypical cells that we meet 

 with in old cultures as degeneration or involution forms, is a controversial ques- 

 tion which is discussed elsewhere. It must always be remembered, however, that 

 when a description is given of the morphology of any bacterial species, such a 

 description is supposed to apply to the cells found in a young, actively-growing 

 culture, on a medium which is favourable to the growth of that particular species, 

 and incubated at the optimum temperature, unless the contrary is specifically 

 stated. Those who are for any purpose describing the appearances met with in 

 preparations from bacterial cultures should always recollect that such descrip- 

 tions have little value unless the exact conditions of cultivation are carefully 

 noted. 



Apart from variations associated with age, variations in form, sometimes of 

 a very striking character, may occur in young cultures of a single bacterial species. 

 Different forms of cell may be present in a single culture ; or the cells may appear 

 to alter their form in successive subcultures ; or different strains of a single bacterial 

 species may show morphological differences, which persist in successive subcultures 

 carried on over a considerable period of time. A description of the morphology 

 of a given bacterial species should include the characters displayed by the modal 

 form, and the extent to which these characters vary. Variability of form is, in 

 itself, very characteristic of certain bacterial species ; while other species show 

 only minor differences in the shape and size of the bacterial cells. 



The Size of Bacterial Cells. — Ignoring for the moment the very large bacteria 

 which have been described by a few investigators, and those very minute organisms 

 which will pass through a porcelain filter — the filtrable viruses — it may be said that 

 the dimensions of most cultivable forms are of the order of low multiples or sub- 

 multiples of//, i.e. of 1/lOOOth of a millimetre. Among the spheroidal forms, the 



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