40 BACTERIOLOGICAL METHODS 



as mold hyphse, mold spores, protozoa, diatoms, etc., besides 

 dirt particles, sand particles, starch granules, body cells, pus 

 cells, etc., etc., which would be lost or rather which would not 

 appear in the plating method. Furthermore, the counts can be 

 made with a great saving of time — in a few hours as against 24 

 to 48 hr., and longer, by the plate cultural method. It is true 

 that in many instances the direct method must be supplemented 

 by the cultural methods whgn, in the judgment of the analyst, 

 this becomes desirable or nja||Rary. 



Concentrates may alsoijff '^de by evaporation under reduced 

 pressure. With a little i« '^y a suitable equipment may be 

 constructed in the laborfi^ ^^^ The container of suitable ca- 

 pacity (i liter and more) is'^^onnected with an exhaust pump 

 which lowers the pressure suii&ciently to cause boiling at a tem- 

 perature not to exceed 37° C; 24 hr. is usually sufficient time 

 to evaporate the liquid to nearly dryness. After the evaporat- 

 ing process has continued for several hours, various enrichment 

 media may be added to the liquid to be evaporated, which will of 

 course aid the intended isolation and development of the de- 

 sired bacteria. If the enrichment medium is added from the 

 first, annoying bubbling and frothing may take place. This 

 method is especially useful in isolating the typhoid bacillus, the 

 paratyphoid group and the intestinal bacteria in general. 



b. Substances Which do not Require Concentration. — Badly 

 contaminated substances as sewage, milk from badly managed 

 dairying establishments, badly contaminated liquids of all kinds, 

 soups, broths, beer, wines and such products as tomato catsups, 

 jams, jellies, canned oysters, etc., may be examined directly 

 without the necessity of making concentrations, or of centrifugali- 

 zation, in order to make quantitative and certain qualitative 

 estimates. The substances of this class may be divided as follows, 

 based upon the approximate number of organisms per cc. as 

 determined by means of the spore and yeast counter described 

 under Fig. 5, and the Thoma-Zeiss hemacytometer. 



