164 LABORATORY EXERCISES IN BACTERIOLOGY. 



seventy-two hours are usually sufficient for the appearance of the colonies so that 

 they may be detected. Should gelatine have been selected as the medium of growth, 

 the confusion arising from liquefaction of the medium may require earlier counting, 

 or that recourse to agar preparations as media be had. 



Should the number of colonies present in the dish, plate, or tube be small, there 

 can, of course, be no difficulty in their enumeration ; but one must always be careful 

 not to overlook small colonies, and a magnifying glass should be used for their detec- 

 tion, and in order not to mistake bubbles or small dirt particles for colonies. If the 

 preparation be placed over a dark surface, as black paper or cloth, the colonies stand 

 out more clearly and are more easily recognized. 



If, however, there be great numbers of bacteria present, recourse must be had 

 to some device to aid the worker. Thus, a piece of black paper may be ruled with 

 white lines into squares having each side one centimeter in length, and this then 

 placed beneath a plate or dish culture, through which the lines are visible, dividing the 

 culture into square centimeters. One may, if time and inclination agree, proceed 

 to determine the number of colonies in each of these squares in regular system until 



FIG. 50. MODE OF COUNTING COLONIES IN AN ESMARCH TUBE. 



all have been included in the count. This is the safest procedure if there be not too 

 many colonies. Or, having counted the colonies in a number of squares in different 

 parts of the culture, a mean may be struck for one square centimeter. The square 

 surface of the culture being known, the calculation for the whole culture is easily 

 made. For counting the number of colonies in a Petri dish a black surface, marked 

 off by concentric circles, radial lines, and subdivisions, into divisions each one square 

 centimeter in area, may be found more convenient than the squarely ruled surface. 

 In counting the colonies in a rolled tube a strip of paper, in which an opening of one 

 square centimeter size has been cut, may be folded about the tube (Fig. 50) ; the colonies 

 showing in the opening counted, and the slip removed from place to place until ten 

 or twelve separate square centimeters have been gone over. From the inner diameter 

 of the tube may be calculated the circumferential measure of the film (diameter multi- 

 plied by 3.14159) ; this is multiplied by the extent of the film in the long axis of the 

 tube (exclusive of the curve of the bottom, which may be neglected to make correction 

 for the error arising from the fact that the inner circumference of the film is less than 

 that of the tube) and thus the square surface of the film obtained. The number of 



