150 LABORATORY EXERCISES IN BACTERIOLOGY. 



may be kept) may be kept moist by placing a small open dish of water in the chamber 

 with the cultures, thus lowering the rate of evaporation from the inclosed media. 

 In potato dishes the purpose of the layer of moistened filter paper upon the bottom 

 of the dish is to prevent any rapid desiccation of the potato substance. The addition 

 of glycerine to agar or serum, aside from its special nutritive value, affords an influence 

 toward the retention of the moisture of the medium. 



3. Temperature. The influence of temperature on the growth of bacteria is an 

 important one, leading to the recognition of three classes of these organisms, according 

 as the temperature most favorable for their development is low (room temperature, 

 15-20 C.), about body temperature (35-38 C.), or high (50-55 C., or above), 

 known respectively as psychrophilic, mesophilic, and thermophilic bacteria. To the 

 first belong many of the saprophytes, as most of the common water bacteria; to the 

 second, most of the bacteria parasitic and pathogenic to animal life; to the third, 

 the putrefactive bacteria, many of the bacteria found in the ground, and others. In 

 relation to any one bacterium it is customary to speak of that lowest temperature 

 at which it first and feebly grows as its minimum temperature; that at which it best 

 develops, as the optimum temperature; and that highest temperature .at which it con- 

 tinues, although feebly, as its maximum temperature. The entire range from minimum 

 to maximum for most bacteria will be found to extend about thirty degrees, a few 

 having much wider range, and a number, particularly pathogenic varieties, a more 

 restricted one. Some forms are capable of development in ice or snow, others at 

 the highest heat of sun exposure ; whence it will be easily appreciated that the general 

 range for the whole group must extend from zero to as high as 60 or 70 C. For most 

 forms the optimum ranges from room temperature to body temperature. 



In their artificial cultivation the temperature sought to be provided is the optimum, 

 or that most favoring their growth, particularly for those easily disturbed by varia- 

 tions from this, as is the case with many of the pathogens. While slight differences 

 may not be sufficient to destroy the vitality of these forms, their effect (especially 

 if increased) is to interfere with the rate of development and often to occasion im- 

 portant alterations in the general functional activities of the germs. Sporulation 

 may take the place of the ordinary vegetative multiplication, variations in the shape 

 and size of the individual organisms may be induced, changes in the degree of virulence 

 of the pathogenic varieties may occur, as well as other modifications of their normal 

 manifestations. These changes are included under the term pleomorphism of bacteria, 

 which may be induced by important changes from any of the normal conditions of 

 life as well as by alteration of the culture temperature. Such pleomorphism does 

 not involve in any instance a change of specific type, cessation of growth and of life 

 occurring rather than absolute loss of type. Such variations may, however, persist 

 through generations under like modifications of life conditions. The anthrax organism, 

 for example, is thus lowered in its virulence by growth at a temperature of 40 C. 

 (optimum, 37 C.) in the manufacture of the vaccine commonly used in the immuniza- 

 tion of flocks and herds against the virulent disease. 



Usually two temperatures are available in the bacteriologic laboratory, that 

 of the room (15-20 C.), and the body temperature (37 C.), which is maintained 

 by means of an incubator and is commonly spoken of as incubator temperature. By 

 means of incubators any desired temperature may be obtained and maintained uniformly ; 

 and for special purposes in the larger laboratories there are usually provided incu- 

 bators in which temperatures are maintained at ranges below (30 C.) and above 

 (40-45 C.) that of the generally used warm chamber (37 C.). An incubator is essen- 



