GROWTH CHARACTERISTICS 119 



temperature, (2) to oxygen 1 These can be answered from some 

 of the following experiments : 



A. Growth on gelatin. (1) Stab culture. Note (a) rate of 

 growth ; (6) form of growth, (a) on surface, (/?) in substance ; (c) 

 presence or absence of liquefaction (d) colour ; (e) presence or 

 absence of gas formation and of characteristic smell ; (/) relation 

 to reaction of medium. (2) Streak culture. (3) Shake culture. 

 (4) Plate cultures. Note appearances of colonies (a) superficial, 

 (6) deep. (5) Growth in fluid gelatin at 37 C. 



B. Growth on agar at 37 C. (1) Stab. (2) Streak. Also 

 on glycerin agar, blood agar, etc. Appearances of colonies in 

 agar plates. 



C. Growth in bouillon, (a) character of growth, (b) smell, (c) 

 reaction. 



D. Growth on special media. (1) Solidified blood serum. 

 (2) Potatoes. (3) Lactose and other sugar media. Does fermenta- 

 tion occur and is gas formed 1 (4) Milk. Is it curdled or turned 

 sour ? (5) Litmus media. Note changes in colour. (6) Peptone 

 solution. Is indol formed ? 



E. What is the viability of organism on artificial media ? 

 3. Results of inoculation experiments on animals. 



By attention to such points as these a considerable knowledge 

 is attained regarding the bacterium, which will lead to its 

 identification. In the case of many well-known organisms, how- 

 ever, a few of the above points taken together will often be 

 sufficient for the recognition of the species, and experience 

 teaches what are the essential points as regards any individual 

 organism. In the course of the systematic description of the 

 pathogenic organisms, it will be found that all the above points 

 will be referred to, though not in every case. 



The methods by which the morphological and biological characteristics 

 of any growth may be observed have already been fully described. It 

 need only be pointed out here that in giving descriptions of bacteria the 

 greatest care must be taken to state every detail of investigation. Thus 

 in any description of microscopic appearances the age of the growth from 

 which the preparation was made, the medium employed, the temperature 

 at which development took place must be noted, along with the stain 

 which was used ; and with regard to the latter it is always preferable to 

 employ one of the well-known staining combinations, such as Loffler's 

 methylene-blue. Especial care is necessary in stating the size of a 

 bacterium. The apparent size often shows slight variations dependent 

 on the stain used and the growth conditions of the culture. Accurate 

 measurements of bacteria can only be made by preparing microphoto- 

 graphs of a definite magnification and measuring the sizes on the 

 negatives. From these the actual sizes can easily be calculated. In 

 describing bacterial cultures it must be borne in mind that the appearances 



