122 LABORATORY EXERCISES IN BACTERIOLOGY. 



SOURCE AND COLLECTION OF MATERIAL FOR INOCULATION. 



The wide range of parasitic and saprophytic occurrence of bacteria renders it 

 impossible to indicate any methodical procedure in their collection for inoculation, 

 nearly every case requiring some individual detail and particular precaution to insure 

 success. The physical character of the substance to be examined (whether gaseous, 

 liquid, or solid), its richness or poverty in bacteria, the simplicity or complexity of 

 the bacterial flora, and a variety of other considerations all influence the plan of pro- 

 cedure in a greater or less degree. 



In whatever manner the material be obtained or whatever its source, as soon 

 as it is removed from its natural surroundings and conditions, it is to be kept until 

 actual inoculation is accomplished in perfectly sterile containers, contact with every 

 unsterilized object affording opportunity for further contamination and invalidating 

 the results of the study. So, too, the least possible delay in inoculation should be 

 permitted after the acquirement of the infected material, especially if estimation of 

 the number of bacteria, as well as of their types, is to be included in the study; since 

 the chance for numerical increase or loss or even of loss of types in a comparatively 

 short period of time after their removal from their natural relations can easily be 

 verified. For example, a water containing organic contamination may, owing to a 

 prevailing low temperature, be infected by comparatively few bacteria when collected ; 

 yet, if kept for but a few hours at the higher temperature of the room, before inocu- 

 lation this number may enormously multiply and the result represent a condition 

 of contamination many times that actually existing in the water in nature. Moreover, 

 owing to different rate of growth, one type represented by few individuals in the original 

 sample may in the room temperature come to an exaggerated proportion as compared 

 with another; or the latter might actually be crowded out of existence by the an- 

 tagonistic influences of the first. 



In a general way it may be said that where the infected material is rich in bacteria 

 and its flora varied it is advisable to collect small quantities and to further isolate 

 the bacteria by dilution with sterile diluents, either before or during the process of 

 inoculation; and that, on the other hand, when a substance is poor in its bacterial 

 flora, in order to more certainly obtain all the types of its scattered organisms it is 

 essential that by filtration or some other method concentration of the organisms should 

 be accomplished before inoculation. Thus, sewage water, crowded with microbic 

 life, must for convenience and for reliability of results be diluted with sterile water 

 (or dilution inoculation performed), often to an extreme degree; while a pure potable 

 water should be passed in large amounts through a suitable filter so as to collect in 

 the latter its occasional, but perhaps variant, types in a convenient compass. The 

 organisms of the air are likewise usually collected by filtration on sterile filters. The 

 uncertainty of contact of the bacteria in a solid substance with the nutrient material 

 upon which the solid may have been inoculated usually makes it advisable that such 

 solids should as a preliminary measure be well diffused in sterile water, portions of 

 which are thereafter transferred to the medium, with the result of more certainly 

 and directly implanting the bacteria upon the nutrient. 



In obtaining the infected material from the living body, probably more than 

 under any other circumstances, the necessity for precaution to prevent contamination 

 by bacteria not concerned in the disease under investigation will be appreciated. 

 The presence of an enormous variety of natural but unimportant organisms in the 

 exposed parts of the body, the difficulties of manipulation, the uncertainties in selective 



