BACTERIOLOGIC EXAMINATION OF SOIL 181 



Roux uses flasks which are drawn out at the neck into a thin 

 twisted capillary glass tube. After sterilization they should contain 

 a small amount of distilled water, which is heated over an open flame 

 to boiling and evaporated down to a small residue. The capillary 

 tube is then fused over a flame and the bottles cooled, when they will 

 be found to contain a vacuum. A heavy string is attached to the 

 closed capillary tube and the bottle enclosed in a metal capsule, 

 which is lowered into the water. When the apparatus is at the desired 

 depth the string is pulled, breaking the capillary tube and allowing 

 the water to rush into the vacuum in the flask. The apparatus is 

 then drawn to the surface. 



A trustworthy bacteriologic examination of water can only be made 

 if the plates are prepared on the spot where the samples are collected. 

 If water is removed to a distance, even when packed in ice, the 

 subsequent count of the colonies does not furnish an accurate result, 

 because some water bacteria multiply near the freezing point, while 

 others are killed by chilling. 



Inoculation of Culture Media. The culture media must be inocu- 

 lated with definite amounts of the water. For this purpose 1 c.c. 

 pipettes, subdivided into 0.1 and 0.01 c.c., are required. These 

 pipettes should be placed in glass tubes which are fused at one end 

 and closed with a cotton plug at the other. The upper ends of the 

 pipettes themselves must also be closed with cotton plugs, and they 

 and the tubes sterilized in the hot-air sterilizer. Water containing 

 few bacteria may be mixed with the culture media in quantities of 

 between 1.0 and 0.1 c.c. When there are many bacteria, as in the 

 case with water contaminated by sewage, the sample must be diluted 

 with sterile distilled water before being mixed with the culture media. 

 The sterile water is brought to the place of examination in volumetric 

 flasks in quantities of 25 c.c., 50 c.c., 100 c.c., and 250 c.c. One c.c. 

 of the suspected water is added to the sterile distilled water in the 

 volumetric flask. The latter is well shaken and 1 c.c. of the diluted 

 sample is added to the culture medium with a fresh sterile pipette. 

 The culture media used for the bacterial examination of water should 

 be of a very definite degree of alkalinity. It is necessary to 

 use both gelatin and agar plates. They should be kept at a tempera- 

 ture of 20 C. for eight days, since many water bacteria grow slowly 

 on artificial culture media. If water is examined with special 

 reference to certain pathogenic bacteria, special culture media and 

 incubator temperature are necessary. The most important pathogenic 

 bacteria occurring in water are the typhoid bacillus and the cholera 

 spirillum. 



Bacteriologic Examination of Milk. The bacteriology of milk will be 

 treated in subsequent chapters. 



Bacteriologic Examination of Soil. Qualitative Examination. As 

 already stated this is chiefly undertaken for the detection of patho- 

 genic bacteria like those of anthrax, tetanus, and malignant edema. 



