10 A MANUAL OF BACTERIOLOGY 



protoplasm frequently contains granules composed of 

 fatty or protein matter, pigment, and in some species of 

 sulphur ; occasionally certain granules stain blue with 

 iodine. In some species " metachromatic " granules occur, 

 chiefly at the poles ; these stain red or pink with many 

 blue dyes, e.g. methylene blue, are composed of nucleic 

 acid combined with an organic base and are to be regarded 

 as non-living reserve material (Dobell). 



In the past many have regarded the bacteria as enucleate 

 cells. This is probably incorrect, and Dobell finds that 

 all bacteria investigated possess a nucleus which may be 

 'in the form of discrete granules (chromidia), a filament of 

 variable configuration, one or more relatively large aggre- 

 gated masses of nuclear substance, or a system of irregularly 

 branched or bent short strands, rods, or networks, and 

 probably also in the vesicular form. The granules observed 

 by Rowland to take part in cell division (see below) and 

 staining with roseine are probably chromidia. 



The cell-membrane is usually invisible, but if the cell 

 is treated with salt -solution (2-5 per cent.) plasmolysis 

 takes place, the protoplasm shrinking away from the 

 membrane, which then becomes visible. It can also be 

 stained in vivo with very dilute solutions of roseine. The 

 cell-membrane sometimes becomes thickened, swollen, 

 and gelatinous on its outer surface, forming a layer or 

 so-called " capsule " around the organism. The clear 

 spaces frequently seen around bacteria in dried and stained 

 preparations, especially in those from blood and lymph, 

 are generally artifacts and not true capsules. In Clado- 

 ihrix and some other forms the cell-membrane becomes 

 hardened, leading to the production of a firm sheath. 

 When bacteria assume the resting stage groups of them 

 adhere together in a jelly-like matrix, forming what is 

 known as a " zooglcea." 



The chemical composition of bacteria varies much, not 



