66 SEWAGE AND ITS PURIFICATION 



with the organism, gently shaken, so as not to produce air- 

 bubbles, and then allowed to solidify. If the organism produces 

 gas during its growth, the jelly will soon become impregnated 

 with small bubbles of the gas, which gradually increase in size 

 and number. B. coli communis, a non-liquefying bacterium 

 present in large numbers in sewage, gives the shake reaction 

 after six hours' incubation at 38° C. (See Fig. 4.) 



6. Roll Cultures can be employed in the place of gelatine 

 plates when it is required to start the cultures in situ, but they 

 must be kept cool, and are soon spoilt by the liquefying bacteria 

 of sewage. Quantities of about 10 c.c. of nutrient gelatine are 

 sterilized in wide test-tubes ; these are inoculated in the usual 

 manner, and a rubber cap is drawn over the cotton-wool plug. 

 The tube is then held horizontally in cold water, and rotated 

 with the fingers till an even layer of the gelatine has set round 

 the walls of the tube. 



7. Surface Plate Cultures. — A tube of gelatine is melted and 

 poured into a Petri dish ; the dish is then covered and set 

 aside in a cool place, so that the gelatine shall become quite 

 firm. One c.c. of the diluted sewage is then added, and rapidly 

 spread over the surface of the gelatine with a sterile bent glass 

 rod, and the cover replaced. After a few hours the nutrient 

 gelatine will have absorbed the water, depositing all the bacteria 

 on its surface, and thus preventing any colonies from starting 

 in the depth of the gelatine. In this manner, after some days' 

 incubation, the growths are all obtained as surface colonies, 

 and, consequently, are easy to examine and subculture, 

 whereas deep-seated colonies often remain mere dots, and in 

 many cases do not show any differences. (See Fig. 6.) 



In addition to the several methods of cultivation which I 

 have described, special reagents may be incorporated with the 

 nutrient media, such as litmus to demonstrate the production 

 of acidity or alkalinity, an iron salt for HgS, magenta to detect 

 any bleaching action, sugar to aid the production of gas, hydro- 

 chloric and carbolic acids to inhibit the growth of certain 

 bacteria. Also, a number of micro-organisms, including many 

 pathogenic forms, grow luxuriantly in a particular medium, 

 their development in other culture materials, if any, being poor 

 and not characteristic ; indeed, all attempts to cultivate some 

 have hitherto entirely failed, among which were the nitrifying 

 bacteria, until it was found that they required food material 

 practically free from fermentable organic matter. These 



