148 EXCREMENTAL ORGANISMS 



organisms in milk was long ago suggested as a means of deter- 

 mining manurial pollution, but, after considerable work had 

 been done on the nature and significance of the streptococci 

 usually found in milk this test fell into general desuetude. It 

 was found that milk drawn under the best aseptic conditions 

 contained streptococci which found milk an excellent nidus for 

 reproduction and that it was practically impossible by simple 

 tests to distinguish these organisms from those derived from 

 manure. The examination of milk for Str. lacticus and Str. 

 pyogenes will be discussed later, but it may be stated here that 

 the identification of these organisms is far from being reliable 

 and that their significance is still an open question. 



For the estimation of streptococci, varying dilutions, as in 

 the enrichment method for B. coli, are inoculated into neutral 

 red dextrose broth tubes and incubated at S?"" C. for two days. 

 The sediment is then examined microscopically for long chains 

 by means of a hanging drop preparation and all doubtful cases 

 confirmed by stained smears. If desired, the streptococci may 

 be isolated in pure culture, and the morphological and bio- 

 chemical characteristics determined by spreading the diluted 

 sediment over ordinary nutrient agar or whey agar and fishing 

 off the isolated colonies after incubation. The properties of 

 Str. bovis, Str. equinus and Str. faecalis are given in Table LIX 

 on page 155. The criticism made above with regard to the 

 tube method for expressing a numeral value for B. coli applies 

 equally to this method for estimating streptococci. As prob- 

 ably only excessive numbers of fsecal streptococci have any 

 sanitary significance, the examination of a direct smear as in 

 the Breed method for estimating the total number of bacteria 

 or of a smear from a centrifugalised deposit, will give equally 

 good results with less expenditure of time and labour. 



