ESSENTIALS OF MILK BACTERIOLOGY 239 



Spores are found when drying is slow : if drying occurs rapidly, the 

 adult form goes into the quiescent condition directly. 



What are the conditions favorable to bacterial growth? Suit- 

 able food (which includes amongst other things a suitable con- 

 centration, and a neutral or nearly neutral reaction) ; suitable tem- 

 perature ; suitable atmosphere ; high humidity and darkness. 



Suitable Food. — In general, bacteria use as foods all waste 

 products of animal or vegetable life and the dead materials of the 

 animal or vegetable body; some bacteria, however, can grow in 

 wholly inorganic solutions and some refuse to grow except in 

 such solutions. The food must not be present in too great a 

 proportion to the water present (i. e., the concentration must be 

 low), and there must be only small amounts of free acid or alkali. 

 (See media used in bacteriology, p. 264.) Bacterial food, in suitable 

 condition as to concentration, reaction, and absence of anything 

 injurious to bacterial life, i. e., ready for bacterial use, is called 

 medium (plural, media). 



Suitable Temperature. — Most bacteria grow best at about 

 20 C. to 40 C. ; some prefer the neighborhood of 22 C. (room 

 temperature), some 37 C. (body temperature), others refuse to 

 grow except at or about one temperature, 22 or 37 C. as the case 

 may be. Species are known which flourish best at o° C. ; others 

 which flourish best at 6o° to 70 C. (hot springs — manure). The 

 laboratory incubator is usually regulated to run at 37" to 38 C. 



Suitable Atmosphere. — Some bacteria grow only in the pres- 

 ence of free oxygen (strict aerobes) ; some only in the absence 

 of oxygen (strict anaerobes) ; many grow equally well under 

 either condition; or grow under both conditions, preferring one, 

 however (facultative aerobes or anaerobes). Bacteria grown in the 

 laboratory without special precautions to exclude air must neces- 

 sarily be aerobic, strict or facultative. 



