10 A MANUAL OF BACTERIOLOGY 



protoplasm frequently contains granules composed of 

 fatty or protein matter, pigment, arid 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 enu- 

 cleate 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 

 aggregated masses of nuclear substance, or a system of 

 irregularly branched or bent short strands, rods, or net- 

 works, and probably also in the vesicular form. The 

 granules observed by Rowland to take part in cell divi- 

 sion (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 

 Gladothrix 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 " zoogloea." 



