INOCULATION OF CULTURE MEDIA 1 7 



6. Sterilize the needle as before. This step must never be 

 forgotten. 



7. Take the cotton-wool plug in the forceps, put it in the 

 flame, and singe all parts of its surface. Then plug the tube 

 while the wool is still burning'. Label it. 



To make a stab culture take the other gelatin tube and pro- 

 ceed as before until you get to step 5. When you have passed 

 the needle into the tube drive it steadily into the medium, 

 taking care not to deviate from the axis of the tube. Finish 

 the process as before. 



All this may seem involved. As a matter of fact it is very 

 simple, and need not take more than a minute to perform. 

 But every step must be carried out, and the whole process 

 must be learnt so thoroughly that it is performed automatically 

 whenever a culture is made. 



In many cases another method of inoculation is used for 

 inoculating surface growths on agar. It will be noticed that 

 a slope of this medium contains a small amount of fluid 

 ("water of condensation") which is squeezed out of the 

 medium as it cools. This may be utilized in a simple method 

 of "plating" out" to get isolated colonies of organisms from 

 material containing several species. Inoculate the water of 

 condensation with a loopful of the pus, sputum, etc., under 

 examination; shake the tube vigorously from side to side, so 

 as to mix the material in the fluid; let the fluid flow over all 

 parts of the surface of the medium; put the tube in an upright 

 position, and let the water of condensation drain to the 

 bottom ; sterilize your needle and take a loopful of this fluid, 

 and with it inoculate the water of condensation in a second 

 tube, mix, and flow over the surface as before. This may be 

 repeated on a third or fourth tube if you are dealing with 

 material very rich in organisms, and after incubation one or 

 other tube will be almost sure to show isolated colonies, from 

 which a pure culture can be obtained. 



When the ag'ar tubes are dried up and there is no water of 

 condensation, a few drops of broth may be added. 



INCUBATION OF CULTURES 



The limits between which bacteria can live are very wide; 

 some grow best at one temperature, others at another, the 



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