. 



448 BACTERIA PATHOGENIC TO MAN 



study the location of the surrounding privies and sources of contamina- 

 tion. A number of observers, resting on the agglutination test, have 

 thought they have isolated typhoid bacilli from the soil and water, but 

 these investigators had not considered sufficiently the matter of group 

 agglutinins, and their results are not trustworthy. 



Bacteriological Examination of Air. 



Saprophytic bacteria are always present in considerable numbers in 

 the air except far out at sea or on high mountains. They are more 

 abundant where organic matter abounds, and in dry and windy weather. 

 Pathogenic bacteria, on the other hand, are only occasionally present 

 in the air. The practical results obtained from the examination of air 

 for pathogenic bacteria have been slight. We know that at times they 

 must be in the air, but unless we purposely increase their numbers 

 they are so few in the comparatively small amount of air which it is 

 practicable to examine that we rarely find them. Examination of dust, 

 however, in hospital wards and sick-rooms, in places where only air 

 infection was possible, have revealed tubercle bacilli and other patho- 

 genic bacteria. 



The simplest method of searching for the varieties of bacteria in the 

 air and their number in any place is to expose to the air for longer or 

 shorter periods nutrient agar spread upon the surface of the Petri dish. 

 After exposure the plates are put either in the incubator at 37 C. or 

 kept at room temperature. The more careful quantitative examination 

 is made by drawing a given quantity of air through tubes containing 

 sterile sand, which is kept in by pieces of metal gauze. When the 

 operation is completed the sand is poured into a tube containing melted 

 nutrient gelatin or nutrient agar, and after thoroughly shaking, the 

 mixture is poured into a Petri dish and the bacteria allowed to develop, 

 either at 37 or 20 C., according as the growth of the parasitic or sapro- 

 phytic varieties is desired. Instead of agar or gelatin ascitic broth or 

 animals may be inoculated. Such examinations are occasionally made 

 of the air of theatres, crowded streets in cities, etc. They give inter- 

 esting but hardly valuable results. 



Bacteriological Examination of the Soil. 



The subject from its agricultural side cannot be considered here. 

 The soil can be gathered in sterile, sharp-pointed, sheet-iron tubes. As 

 in water, we wish to learn the number of bacteria and the important 

 varieties of bacteria present. To estimate the number, small fractions 

 of a gram are taken. 



According to Houston uncultivated sandy soil averages 100,000 bac- 

 teria per gram, garden soil 1,500,000, and sewage-polluted 115,000,000. 

 The most important bacteria to be sought for are bacilli of the colon 

 group and streptococci. Both of these suggest fairly recent excremental 

 pollution. 



The period during which typhoid bacilli remain alive in soil is un- 



