140 



LABORATORY EXERCISES IN BACTERIOLOGY. 



the developed colonies are counted ; from which number the total in the original quan- 

 tity of the water passed through the filter can readily be calculated. Special colonies 

 are removed from the medium, by means of the sterile needle, for isolation and further 

 study. 



If in the test examination the plate was crowded with growth, dilution of the 

 sample should be practised, sterile water being added as the diluent. The sample, a 

 small one, is collected in a sterilized glass bottle provided with glass stopper; and 

 should it have been standing for a short time in this, it should be well shaken so as to 

 evenly distribute the organisms contained, these having tended to settle to the bottom. 



Several dilutions are generally made (1:10, 1:100, 

 1 : 1000). From each of these dilutions one cubic 

 centimeter is transferred to a like number of tubes 

 of liquefied gelatine, diffused by agitation, and the 

 medium plated. As a control a small amount (0.1 

 cubic centimeter) of the original specimen is similarly 

 dealt with. The inoculated preparations are then put 

 aside at room temperature and the bacteria allowed 

 to grow. At the end of the third or fourth day 

 development will probably be complete (daily obser- 

 vation should, of course, have been made), the colonies 

 counted, the number corrected for the dilution, and 

 the mean result determined. 



In collecting samples from a cistern, pool, or 

 well, from which the test-specimens are to be taken 

 at different depths, it is customary to use glass- 

 stoppered bottles arranged in the following manner: 

 The bottle and stopper having been sterilized, the 

 operator carries it closed to the place of collection, 

 where a stout cord of sufficient length to reach the 

 depth desired is attached to the stopper and a like 

 one tied to the neck of the bottle; and a convenient 

 piece of rock or other weight is attached to the 

 bottom of the bottle. It is lowered to the desired 

 depth, the stopper withdrawn by a pull on the cord 

 attached to it, and the bottle allowed to be filled with 

 the water. It is now quickly pulled to the surface, 

 but steadily and without jerks, a little of the con- 

 tents poured over the stopper to rinse off the water 

 of the upper strata through which it was drawn, and 

 the stopper adjusted. It is probable that a little 



water from the overlying layers will have entered the bottle, but this amount may be 

 disregarded, the full condition of the bottle and the small size of the neck preventing 

 much displacement. To prevent this source of error special forms of collecting apparatus 

 are sold in which it is possible by mechanical device to replace the stopper while the 

 bottle is in the depths (Fig. 42). Samples are taken from the top, middle depth, and 

 bottom, and each used in making cultures as above described, dilutions being made 

 as above, if necessary. Either the mean of the results obtained or a statement of 

 the result for each depth should be indicated in the report of the analysis. 



It is to be remembered in all such procedures that the number of organisms arrived 



FIG. 42. BOTTLE IN FRAME, 

 ARRANGED FOR COLLECT- 

 ING WATER FROM DEFINITE 

 DEPTHS AND REPLACING 

 STOPPER AFTER ENTRANCE 

 OF WATER. 



