ROLL CULTURE 113 



bacteriology. The principal characters to be noted are complete 

 morphology, staining characters, particularly with Gram's 

 method, colonial growth on agar and gelatine, potato, blood serum, 

 milk, sometimes inorganic salt solutions, the enzymic products 

 as indicated by fermentation of carbohydrates and solution of 

 proteins like milk curd and gelatine. With this last comes ammonia 

 and nitrite productions. The optimum temperature and media 

 and resistance to physical and chemical agencies must be taken 

 into consideration. For pathogenic bacteria we establish as far 

 as possible the relations with lower animals. This includes, of 

 course, the production of soluble toxins and endotoxins. 



The chemical activities of many bacteria are well displayed in 

 milk culture. Milk is run into tubes, and sterilized tincture of 

 litmus is often added to act as an indicator. Before using the milk, 

 it must be skimmed and free from all fat. 



The property of converting sugar into acids and gases is best 

 studied in fermentation tubes. 



Into sterile fermentation tubes bouillon containing sugar is run, 

 these are plugged and sterilized. They may be inoculated with 

 bacteria and if gas production occurs it is quickly manifested in the 

 closed arm. The component gases may be studied and the various 

 properties determined. This gas ratio is of use in identifying 

 various bacteria and differentiating them. The closed arm of the 

 tube being shut off from free air by the amount of bouillon in the 

 open arm is practically an anaerobic tube and is employed for this 

 purpose. Bacteria that grow in the closed arm are considered 

 anaerobes. By inoculating a gelatine tube with bacteria while it is 

 melted and then letting it solidify, previously shaking the tube 

 vigorously, gas formation will be speedily manifested by the presence 

 of bubbles. Acids are detected in cultures by the employment of 

 various indicators in the culture media. Litmus, lacmoid, and 

 neutral red are used for this purpose. By titrating bouillon of 

 previous known acidity with a decinormal soda solution, the amount 

 of acid produced by different bacteria can be estimated. 

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