652 LABORATORY HINTS 



If acid and gas is produced it is presumptive evidence of Bacillus 

 coli. 



Take a loopful of liquid from the tube with the smallest amount of 

 water, and add it to a test-tube of 10 c.c. of sterile water. Shake and 

 inoculate a similar tube \\\{\\ one loopful of this. 



Repeat from the second to a third tube. 



Smear one loopful of each dilution over a gelatin or agar slope. 



One of these is sure to give discrete colonies. 



Subculture these into tubes of nutrient broth, and later examine for 

 motile organisms and do a Gram stain. 



From the broth, culture inoculations are made into the various 

 media necessary to identify the organism and the attributes of the 

 organism thus collected. 



If typical Bacillus coli have been found in the tube to which 10 c.c 

 of the w^ater had been added and not in that of 5 c.c, it would be stated 

 that Bacillus coli was present in 10 c.c, but absent in 5 c.c. 



Those waters showing no Bacillus coli in 50 c.c. are of a high 

 degree of purity. 



Spring waters should not show them in 50 c.c 



In upland surface waters their presence in 10 or 40 c.c. means 

 contamination, but not necessarily dangerous, as it may be from 

 animals grazing, and not pathogenic human excreta. 



If there are more Bacillus coli than 500 per litre the water is 

 suspicious. 



{d) To enumerate streptococci. 



As above, but use glucose broth or glucose neutral red broth, and 

 incubate at 37° C. for four hours, and then make a hanging drop 

 preparation. 



The primary Bacillus coli tubes can, of course, be examined for 

 them. 



Streptococci do not multiply after sampling, but Bacillus coli do. 



Their numbers should not exceed that permissible for Bacillus coli. 



{e) To detect the typhoid bacillus. 



This is difficult. Remember that as the incubation of typhoid 

 fever is twelve to fourteen days, the organisms ma}- have disappeared 

 from the water before the examination is started. Water containing 

 typhoid bacilli will contain large numbers of Bacillus coli also. Other 

 organisms may overgrow those of the typhoid bacilli. 



There are many wavs of concentrating the agar in the water, such 

 as filtering through a porcelain filter, adding a little alum, entangling 

 them in the aknninium hvdrate, centrifuging, &c. 



