80 A MANUAL OF BACTERIOLOGY 



these are very useful, as the culture medium may be kept 

 in them ready for use. 



In addition to the isolation of species from mixtures 

 and for diagnosis, plate cultures are also used to enumerate 

 organisms. Assuming that every colony arises from a 

 single organism, which is approximately the case, the 

 number of colonies represents the number of organisms 

 originally introduced into the gelatin, and if a known 

 weight or volume of the material inoculated be used, the 

 number of organisms in it can be calculated. For example, 

 in the bacteriological examination of water a measured 

 volume of the water is added to melted gelatin by means 

 of a sterilised pipette, and by counting the resulting 

 colonies the number of organisms originally present in 

 1 c.c. of the water can be estimated. 



Agar plate cultures may be prepared in a similar way. 

 The agar must, however, be brought to a temperature of 

 nearly boiling before it melts ; it is then allowed to cool 

 to nearly 45 C. and the tubes are inoculated in the same 

 manner as for a gelatin plate culture described above. 

 Unless the manipulations be carried out expeditiously the 

 agar will solidify, or the agar film in the Petri dish be 

 lumpy. 



Agar plates should usually be inverted during incuba- 

 tion, or the growth may become confluent owing to the 

 condensation water carrying the organisms all over the 

 film. 



The plate-culture method can be modified to suit 

 particular circumstances : for example, the melted gelatin 

 or agar, uninoculated, may be poured into the dishes and 

 allowed to solidify, and the film then inoculated by 

 streaking or painting with the material, or by pouring a 

 few drops of broth containing the organisms upon it. 

 This is practically the only way in which blood-serum can 

 be used, the sterile blood-serum being placed in the Petri 



