. Pathogenic micro-organisms, including bacteria and Protozoa; a practical manual for students, physicians and health officers. mployed produce of course no gelatin is used exactly as agar, except that as the averageproduct does not congeal until cooled below 22° C. we have no fearof its cooling too rapidly. METHODS^ USED IN CULTIVATION OF BACTERIA. 71 In order not only to count the number of colonies and to obtaina characteristic growth, but also to prevent the inhibition of thegrowth of some and the fusing of others, it is desirable not to inoculatethe nutrient agar or gela
. Pathogenic micro-organisms, including bacteria and Protozoa; a practical manual for students, physicians and health officers. mployed produce of course no gelatin is used exactly as agar, except that as the averageproduct does not congeal until cooled below 22° C. we have no fearof its cooling too rapidly. METHODS^ USED IN CULTIVATION OF BACTERIA. 71 In order not only to count the number of colonies and to obtaina characteristic growth, but also to prevent the inhibition of thegrowth of some and the fusing of others, it is desirable not to inoculatethe nutrient agar or gelatin to be poured in one plate with too large anumber of bacteria. We therefore use the following dilution methodsin making culture plates of suspected material. Dilution Methods.—As it is impossible to know the number of bac-teria in any suspected fluid, it is usual to make a set of from two to fourdifferent plates, to each of which a different amount of material is added,so that some one of the series may have the required number of dilutions are made in bouillon or sterile distilled water. In the Fig. 38. Photograph of a large number of colonies developing in a layer of gelatin contained in a Petridish. Some colonies are only pinpoint in size; some as large as the end of a pencil. The colonieshere appear in their actual size. first tube we place an amount which we believe will surely containsufficient and probably too many bacteria. To the second tube weadd 10 per cent, of the amount added to the first, and to the third 10per cent, of the second, and to the fourth 10 per cent, of the third. Thus,if the first contained 60,000 bacteria the second would have 6000 (), the third 600, and the fourth 60 (Fig. 39). If, however, the firstcontained but sixty, the second would have about 6, and the remainingtwo would probably contain none at all. When there are many colon-ies present the dishes are covered by a glass plate (Fig. 40), ruled inlarger and smaller squa
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