BACTERIOLOGICAL TECHNIC 89 



bacteria, excepting that after the washing and rinsing, the root, instead of 

 being crushed, is cut or broken across and the inoculation material is 

 taken from the inner tissue by means of a platinum needle or scalpel. 



E. Bacteria of the Air. — Air currents carry the germ-laden dust and dirt 

 particles. The number and kind of air bacteria depends upon environ- 

 ment, climatic conditions, moisture, sunlight, etc. The air currents are 

 the main factors in germ dissemination. Spores and dry (though not 

 dead) bacilli may be carried many miles. Air microbes are derived from 

 the soil surface and from the objects surrounded by the air. Bacteria 

 are exhaled with the breath (as in talking, sneezing, coughing) and 

 are carried and distributed from and by animals, plants and clothing. 



The air may carry organisms derived from the soil, from water and 

 from other substances contaminated by organisms. The dirt and dust 

 particles wafted about by air currents may have lodged upon them the 

 germs of tuberculosis, the pus formers, the streptococcus group, rarely 

 also the anthrax bacillus, the tetanus bacillus and the bacillus of malignant 

 oedema; beside the spores of higher fungi, yeast cells and even the larvae 

 of intestinal parasites, etc. 



Air microbes may be studied by exposing a Petri dish containing steril- 

 ized agar or gelatin, for two minutes or longer. The number of colonies 

 that will appear will depend upon the locality, season, air moisture, etc. 

 To determine the number of microbes in a given volume of air the Sedg- 

 wick-Tucker aerobioscope is used, though similarly constructed apparatus 

 may be made by any fairly skillful student. The aerobioscope consists of a 

 glass cyUnder as shown in the illustration. The open ends are plugged 

 with cotton. Granulated sugar is loosely packed into the narrow end 

 and all is then sterilized in a hot-air sterilizer (not over 120° C). Pass 

 a given quantity of air through the aerobioscope by attaching an aspirator 

 bottle to the narrow end and allowing a given volume of water to run out 

 of the bottle. The volume of air drawn through equals the volume of 

 water run from the bottle. Of course the cotton plug is removed from the 

 larger end of tube while the water is running. The bacilli and spores are 

 caught in the sugar, while the air passes through. Replace cotton plug 

 and shake the sugar into the larger end of tube. Remove cotton plug 

 again and pour in about 10 to 15 cc. of liquefied (40° C, not hot) gelatin. 

 Roll the tube held horizontally. The gelatin dissolves the sugar and mixes 

 with it. Roll on ice to hasten the hardening of the gelatin. Set aside in 

 incubator, at room temperature (20° C, about). The number of colonies 

 which appear indicates approximately the number of microbes in the 

 volume of air aspirated. Let us suppose that the number of colonies was 

 125, the volume of air aspirated 10 liters, from which we would get 1250 

 bacteria per cubic meter of air. 



