METHODS OF STUDYING BACTERIA 41 



Amongst bacterial products, certain extremely poisonous 

 substances, many of them of more or less unknown chemical 

 nature, are of extreme importance, especially in the case of 

 pathogenic or disease-producing organisms. The origin and 

 actions of these poisons are very varied. Thus they may be 

 ferments or enzymes secreted by the bacterium for the pur- 

 pose of attacking and digesting its food, they may be excreted 

 as waste-products of bacterial metabolism or cell-life, or they 

 may be derived from the disintegration of the food material 

 attacked by the organism. Ptomaines are poisonous alkaloid- 

 like substances formed in decomposing meat, fish, and the like. 

 They are not destroyed by cooking, and hence may cause one 

 form of "food-poisoning." Botulism (Latin: lotulus, a 

 sausage), or poisoning from eating infected sausages or similar 

 foods, often fatal in its effects, is of this nature, a specific 

 anaerobic organism, Bacillus botulinus, 

 being the bacterium which produces the 

 poison. 



|..x Toxins, on the other hand, are the 

 specific poisonous products of bacterial FIG. i4.Petri Plate or Dish 

 metabolism or cell-life and growth. They a glass box which can be 

 are sometimes retained within the bac- ?3&S3,E 

 terial bodies and are then called endo- a larger surface than can 

 toxins ; whilst others again are secreted e obtained in a test 

 into the surrounding media, whether in 



the living body or in the culture-tube. The best known and 

 most extensively studied examples of the latter the extra- 

 cellular toxins are those produced by the tetanus bacillus 

 and the diphtheria bacillus. These are soluble and diffusible, 

 and, if formed in the living body, they pass into the lymph 

 and blood and produce their serious results by poisoning the 

 tissues. Their exact chemical nature is still unknown. They 

 are very unstable, being easily destroyed experimentally by 

 heat or light, or by being kept for a long time. 



Isolation of Bacteria in Pure Culture. This is usually 

 attained by separating the bacteria in a given mixture, either 

 by smearing a minute quantity of it over the surface of a solid 

 culture-medium, or by diluting it with a sufficient amount of 

 melted nutrient jelly, which is then poured into a sterile Petri 

 dish (see Fig. 14), and allowed to solidify. After incubation 

 at a suitable temperature, if sufficient separation of individual 

 bacteria has been effected, scattered colonies, each consisting 

 of one variety of organism only, will be found, and these can 

 be individually picked off and subcultured in fresh culture- 

 tubes. 



